Supply Chain - Mapping & Modelling - Modelling - CFD

Tunstall Reservoir is located approximately 5km to the north of Wolsingham, in County Durham. It is an impounding reservoir retained by an earth filled embankment with a central clay core. The dam is approximately 25m high and 300m long. The reservoir was previously used to feed a water treatment works located at the toe of the dam; which is now decommissioned. The reservoir is now used for amenity purposes and to provide environmental compensation flows to the downstream valley. Following a Statutory Inspection under Section 10 of the Reservoirs Act, a number of measures in the interest of safety were recommended by the Inspecting Engineer in order to ensure the safety of the dam. Section 10 of the Reservoirs Act

Following a Statutory Inspection under Section 10 of the Reservoirs Act, a number of measures in the interest of safety were recommended by the Inspecting Engineer in order to ensure the safety of the dam. These recommendations involved the following:

Recommissioning the existing siphon that used to feed the works and was no longer in use had to be intercepted and provided with a new route in order to provide additional drawdown capacity to the reservoir. Repairs to the lower part of the spillway where the existing blocks were damaged. Filling an existing tunnel within the dam structure. [caption id="attachment_18531" align="alignnone" width="1200"] (left) Condition of the spillway blocks prior to the works and (right) new concrete slab with stainless steel protection to the joint - Courtesy of Esh-Stantec[/caption] Siphon re-commissioning

Various options were considered at the concept stage in order to restore the use of an existing siphon that was abandoned in the early 2000s when the treatment works were decommissioned. The preferred option involved the following:

A siphon discharge chamber to break the siphonic effect. This was decided given the uncertainty about the existing system and the significant difference in levels in the downstream leg. Thus, the water would flow by gravity between the chamber and the outfall. This discharge chamber also functions as a thrust block to restrain the thrust acting on the new gate valve that has been installed immediately upstream of the chamber. The use of the existing settlement tanks to retain the water and aid to the settlement of suspended solids. A penstock chamber has been provided between the tank and the new discharge point to retain the water for a day before it is discharged into Waskerley Beck once the majority of the sediment has settled. Routing the new pipework away from the areas of the embankment that present the most valuable Waxcap habitats. Waxcap mushrooms are rare as they are restricted to grassed areas that are not grazed, and are locally protected in Durham. [caption id="attachment_18533" align="alignnone" width="1200"] Tunstall Reservoir dam - Courtesy of Esh-Stantec[/caption] Spillway repairs

The original Victorian spillway structure is formed by sandstone blocks bedded in concrete. However, the Inspecting Engineer identified that the majority of the blocks in the lower part of the spillway presented signs of wear and tear and needed replacement.

The proposed solution consists of four new reinforced concrete slabs that have been placed over the existing spillway, except the most upstream slab where blocks had to be removed to tie-in with the existing structure and avoid a step. An expansion joint was necessary in the centre of the new slabs. The flexible sealant in this joint has been covered by a stainless steel plate that has been designed to protect the sealant from the predicted high velocities expected in the flows and reduce the maintenance requirements.

A number of groundwater relief drains have also been incorporated into the design in order to capture the emerging groundwater at the lower part of the spillway and both reduce the hydraulic load on the new slabs and improve the long-term reliability of both the new concrete and the remaining sandstone blocks. These discharge to a new headwall in the bank downstream of the end of the spillway.

[caption id="attachment_18535" align="alignnone" width="1200"] Tunstall Reservoir spillway - Courtesy of Esh-Stantec[/caption] Tunnel infill works

When the dam embankment at Tunstall was completed and the filling of the reservoir began, significant leakage was observed through the eastern abutment. A concrete cut-off wall extending into the hillside was constructed in 1879 which significantly reduced the leakage rate. Nevertheless, as some leakage continued, a tunnel and shaft were constructed downstream of the cut-off wall, and an 18-inch cast iron diameter pipe was installed inside the tunnel to capture the leakage and convey it to the treatment works.

Following the Section 10 inspection of the dam in February 2021, a number of Measures in the Interest of Safety (MITIOS) were raised recommending works to address the leakage and the stability of the tunnel. One of these IOS measures recommended to fill the tunnel whilst providing some drainage, as a result of the concerns about its structural integrity and the potential risk to the integrity of the dam embankment and the spillway in the event of a collapse of the tunnel.

The project team scoped and contracted a ground investigation (GI) in Autumn 2022 to investigate the leakage paths into these tunnels.

Based on the findings of the GI and the review of the reservoir levels, it was concluded that the cut off wall is in good condition and that the main flow path goes around the concrete cut off wall. There is also a strong correlation between reservoir levels and the recorded flows, which decrease significantly when the levels are 6 m below the top water level (TWL). This was observed again during the construction works, as the water level was kept 6 m below the TWL for health and safety reasons while there was work ongoing in the tunnel and the spillway.

[caption id="attachment_18544" align="alignnone" width="1200"] (left) View in the unlined section of the tunnel (notice the beams across the soffit) and (right) view of pipe inlet and partial headwall (notice standing water on the invert) - Courtesy of Esh-Stantec and CC Informatics[/caption] Various options were considered during the Concept stage. The option of not filling the tunnel was disregarded as it would not eliminate the potential risk to the integrity of the dam embankment and spillway channel. Filling the entire extent of the tunnel was considered, however it was not preferred due to the significant constraints associated with it. These constraints include the difficult access and requirement to work in a confined space, potential land ownership issues, the presence of a SSSI immediately to the east of the abutment, and the increasing depth to the tunnel as it continues beyond the spillway. For all of these reasons, it was agreed with that filling the extent of tunnel beneath the dam and spillway would be sufficient as that would eliminate the risk to these structures.

Three possible fill materials were considered:

Cement grout was discounted due to concerns of potential pollution as a low viscosity mix would be likely to mix with groundwater and filter through the rock, causing a pollution incident. Foamed concrete was discounted as a result of the potential constraints to place it at the furthest point inside the tunnel, and the remote location of the site and access within the site would further complicate the timely delivery of concrete. Expanding geopolymer foam was selected as the preferred material as it reduced the risks associated with grout and foamed concrete; and it had successfully been used by Esh-Stantec in the delivery of other projects.

The expanding geopolymer foam was deemed to be more cost-effective than the alternatives as it offered significant savings to the installation process, because:

It avoided the risk of pollution and therefore any mitigation works to stop grout leakage; Simple installation from a mobile compound, avoiding the need for complex plant and equipment; Geopolymer mix materials delivered to site in barrels, allowing for easy storage and transport within the site. Installation from the surface via boreholes every 10m along the line of the tunnel. Lower carbon footprint that cement based alternatives. [caption id="attachment_18545" align="alignnone" width="1200"] (left) View of the geopolymer in the tunnel, and (right) view of the pea gravel stop end - Courtesy of Esh-Stantec and CC Informatics[/caption]

The filling was undertaken via a number of injection holes that were drilled from the surface at 10 metre intervals, and expanding geopolymer was injected via these injection points. The progress on site was as follows:

A pre-drilling survey took place to confirm the condition of the pipe in the tunnel ahead of the works. Eleven injection holes were drilled along the dam embankment and across the spillway. Prior to the injection, after the drilling had been completed, high resolution cameras were used in each injection hole and a drone survey was carried out to confirm that there was no damage caused to the pipe and that the expanding geopolymer would not enter the pipe. Pea gravel was introduced through the final injection point in order to create a mound that would stop the advance of the geopolymer and prevent it from blocking the pipe. The use of the gravel mound avoided the need to carry out works inside the tunnel with the associated risks of working in the confined space. Once the project team was satisfied that the drilling had not cause any damage to the pipe, the injection began, advancing at 10 metres per day on average. [caption id="attachment_18534" align="alignnone" width="1200"] Overview of the siphon diversion route in preparation for the works - Courtesy of Esh-Stantec[/caption] Tunstall Reservoir MITIOS: Supply chain - key participants Principal designer & contractor: Esh-Stantec Esh Group Stantec UK Geopolymer injection subcontractor: Geobear Ltd High resolution cameras & drone survey: CC Informatics Confined space entry specialist: MRS Training & Rescue Ground investigation: Ian Farmer Associates Topographical survey & laser scan of tunnel: Academy Geomatics Ltd Pipework modifications: UTS Engineering Steel fabrications: JHT Fabrications Ltd Conclusion

The contract has been delivered by Esh-Stantec and the recommendations in the interest of safety made by the Inspecting Engineer have now been addressed. The siphon will provide additional drawdown capacity to the reservoir in the event of an emergency, the damaged part of the spillway has been repaired to a satisfactory standard, and the potential risk of collapse of the tunnel has been removed.

Guildford Borough Council (GBC) are progressing with the development of approximately 41 hectares of brownfield land located along the west bank of the River Wey as it runs through central Guildford and north through Slyfield Industrial Estate. Guildford Sewage Treatment Works (STW) lies within this land and requires relocation in order to enable the development to proceed. This relocation forms a critical part of the Slyfield Area Regeneration Project, and will allow the delivery of up to 1,550 new homes, partially on land occupied by the existing STW, to enable local housing needs to be met. The new sewage treatment works will be built to the north of Slyfield Industrial Estate on land that has been used for quarrying, and in turn used as a landfill site, and is currently largely unused. In addition to the construction of a new works, relocation of the works to this location will require the construction of a new transfer tunnel to divert existing incoming flows to the new site, and the construction of a new outfall to the River Wey. The existing STW must also be decommissioned to enable use by the Weyside Urban Village (WUV) Development. Design flows The new Guildford STW is designed for a population equivalent (PE) of 125,300, with room to expand the site in the future. Design criteria Flow rate Dry weather flow (DWF) 322 l/s Average flow 368 l/s Flow to full treatment (FtFT) 746 l/s IPS peak capacity 2,250 l/s Location of new works The site was formerly quarried for sand and gravel during the 1960s and 1970s, with the resultant voids filled with waste materials progressively moving from west to east (i.e., towards the River Wey), which continued until the late 1980s. There is no record relating to the exact nature of the fill materials. The site was understood to have taken household and commercial wastes, but not special or difficult wastes. Following this, the site has remained vacant and undeveloped. [caption id="" align="alignnone" width="1200"] (left) Location of new sewage treatment works and tunnel route and (right) Slyfield Regeneration Project area - Courtesy of Adams Hendry[/caption] Transfer tunnel The STW transfer tunnel will consist of a 1.4km long, 1500mm internal diameter, 1780mm outer diameter tunnel, made of precast concrete pipe, which will be installed by pipejacking (microtunnelling) method. Six shafts are required to facilitate its construction and will serve as temporary launch and reception shafts. Network modelling by Arcadis verified the designed flow velocities remain self-cleansing and provide for the flow rates required. A geotechnical interpretive report has been prepared in review of the ground conditions for the scheme as whole, and to ascertain suitability for shaft sinking and tunnelling. The principal geological stratification for the site consists of Made Ground overlain principally with River Terrace Deposits, with sections of overlapping Alluvium, overlaying the solid geology of firm to very stiff clay of the London Clay Formation (LCF). The selected method of microtunnelling was chosen as it allows long pipeline sections to built, and its suitability to the ground conditions within LCF at Guildford, while limiting the exposure to personnel to underground works, against traditional methods. The microtunnelling is operated from a central control cabin at ground level, and only limited personnel access into the tunnel is required to service the microtunnel boring machine during construction. A ground movement assessment has been prepared and modelled using XDisp software by Oasys. The analysis constitutive model is based on the O’Riley & New method. The method predicts ground movements resulting from ground loss which produces strain of the soil within the influence zone, and projects the settlement in a gaussian distribution curve. Ground improvement & remediation The project team has consolidated decades of site investigation data into a unified format and 3D model. The process involves first combining data within OpenGround software and applying scripted translation to a common description - for example having all London Clay called ‘London Clay’, rather than ‘LC’ or ‘Clay’. [caption id="" align="alignnone" width="1200"] RIC 1 and 2 pass areas - Courtesy of BAM Enpure Ltd JV[/caption] This combined dataset is then used in Leapfrog 3D geological modelling software, which applies intelligent interpolations, and highlights anomalies that can be quickly assessed and removed. The resultant 3D model has then been used to enhance communication in the engineering AutoDesk BIM 360 federated models, and in the proprietary Arcadis SmartAtlas GIS viewer. Rapid Impact Compaction (RIC) is a method of imparting energy to the ground using a repeated but relatively short drop-weight, where a base plate (or “puck”) is in constant contact with the ground. RIC is preferred over a larger drop-weight technique due to its more progressive and less disturbing impact on the ground it is being used on. The method is also faster and uses smaller scale setups. A challenge for the RIC method was whether the energy it imparts is sufficient to densify the ground, versus other higher energy drop-weight methods. A trial was deemed necessary to demonstrate the suitability of this method. RIC is proposed to be applied over the majority of the site with an expected densification of landfill down to approximately 4.5m. Areas where assets will not be supported on driven full depth piles will undertake a second pass of the RIC method, reducing the long-term settlement of the landfill material. The predicted reduced surface will be levelled as required by the infill of depressions with imported stone or other suitable material. Improvement of the existing ground by reinforcement through installation of Vertical Rigid Inclusions (VRI) method across the site. Supporting of level sensitive and heavier elements will be on precast driven piles founded below the landfill material. The combination of methods will be designed to achieve Thames Water’s settlement performance values over 40 years, and remain within the allowable values of 50mm and 100mm. This will reduce the need for full depth driven piling which would otherwise be required. Reduction of infiltration to the landfill materials by ensuring impermeable surfaces drain to the treatment works and are released at the outfall away from the site. Gas and leachate well monitoring is provided along the perimeter of the site to monitor any impact and provide leachate removal points if required. [caption id="" align="alignnone" width="1200"] (left) Rapid impact compaction rig and (right) pile under test - Courtesy of BAM Enpure Ltd JV[/caption] Guildford STW Relocation Project: Supply chain - key participants Client: Thames Water Principal contractor & designer: BAM Enpure Ltd JV Client designer: Jacobs Civil designer: Arcadis Planning consultant: Adams Hendry Consulting Ltd Process & M&E: Enpure Ltd CFD analysis: The Fluid Group Physical modelling: Hydrotec Consultants Ltd Tunnel design: Joseph Gallagher Ltd Temporary works/shoring: MGF Ltd Precast structures: A-Consult Ltd Precast structures: FLI Precast Solutions Rapid impact compaction: Cofra Ltd Piling: Aarsleff Ground Engineering Ltd XDisp software: Oasys Controlled modulus columns: Vibro Menard Ltd Inlet works and PST odour covers: Power Plastics Ltd Inlet pumps: Grundfos Imported sludge Strainpress: Huber Technology Escalator screens: Longwood Engineering Company Ltd Grit removal plant: Jacopa PST scrapers: Colloide Engineering FST scrapers: Purac AB Odour control plant: Odour Services International Ltd FBDA grids and blowers: Xylem Water Solutions Sludge thickening & dewatering: Kent Stainless Ltd Cake Silo: Saxlund International Cloth pile filters: Eliquo Hydrok Ltd Polymer batching & dosing: Richard Alan Engineering Ferric plant: Colloide Engineering Tank logging system (WASP): JR Pridham Services Ltd Glass lined steel tanks: Balmoral Tanks Ltd Sludge tank mixing: Landia UK Ltd Pumps, mixers & flocculators: NOV Welfare building: M-AR Limited Guildford STW - the new sewage treatment works Inlet pumping station The new STW is being designed to maintain inflow from the external catchment ranging from dry weather to 1 in 100-year storm (plus climate change allowance) with future-proofing beyond the 2041 design horizon. [caption id="" align="alignnone" width="1200"] (left) inlet pumping station physical model and (right) inlet pump testing physical model - Courtesy of Hydrotec Consultants Ltd[/caption] The pump design has been developed to fulfil the project brief load requirements (future peak IPS capacity 2585l/s) and some flexibility either side of the requirements is available due to the standard capacity equipment design offered from the supply chain. The proposed arrangement now consists of a wet well 12.5m diameter, approximately 20m deep. A common inlet will be provided to dissipate any entrained air, and two selectable pumped sections with double isolation will be provided by wall mounted penstocks on full height walls. The pumping well will be constructed in two halves, hydraulically connected in normal operational mode however each half will be able to be isolated (using motorised penstocks) in accordance with specification requirements surrounding double isolation, to allow person entry into half of the wet well at a time. Each half of the wet well will have three pumps installed giving a capability to provide full flow to treatment from two pumps with a standby permanently installed. The pumping station was first modelled by computational fluid dynamics (CFD) to demonstrate the assumed flow patterns that will occur. The pumping station was then physically modelled to comply with the specification and process design requirements particularly with respect to distribution, mixing, settlement, turbulence, dispersion, and freeboard for the full range of flows that each structure is expected to receive. [caption id="" align="alignnone" width="1200"] Storm overflow physical models - Courtesy of Hydrotec Consultants Ltd[/caption] Inlet works The inlet works is being designed to receive all pumped flows from the inlet pumping station (including future peak IPS capacity 2585 l/s), inlet works drainage and screening liquor return flows. Flows will enter the inlet works and then separate across the four escalator screens inclined at an angle of 45°. Each screen will be provided with a local proprietary control panel. Each panel will incorporate a differential ultrasonic level control. An emergency bypass will be provided upstream of the screens to accommodate all four screens blinding at the same time. Three 1200mm wide duty screens will be required to screen flows from low flow up to 2585 l/s, screening 6mm in two directions. The inlet works and grit treatment plant will be covered with odour covers which will be connected to the inlet works and PST odour plant. The inlet works has been classified as a Zone 2 Area under the odour covers and all equipment beneath the odour covers will be ATEX rated. Storm tanks Storm flows from the inlet work storm separation chamber will flow into two storm tanks via a 1400mm above-ground pipe. Within the 1400mm pipe between the storm separation chamber and the storm tanks there will be a partial pipe MCERTs certified flow meter to measure the total flow into the storm tanks. During normal operation the blind tank will fill first, then the spill tank will fill. In the event that both tanks are full to capacity (and still in storm flows) the spill tank will overflow via a weir and the storm tank will discharge to river. [caption id="" align="alignnone" width="1200"] Elevated inlet works - Courtesy of BAM Enpure Ltd JV[/caption] The storm tanks contents will automatically return to the inlet pumping station via 700mm gravity pipe when the flow being pumped to the inlet works drops below a predetermined set point. The return flows will be controlled to approximately 0.5 DWF via an automated valve controlled by an electromagnetic flow meter located in the gravity main, returning to the common inlet well of the inlet pumping station. Storm tank cleaning will be a pumped system with the two pumps located external to the storm tanks. Storm tank cleaning will be initiated during the latter part of the storm tanks draining back to the inlet pumping station and will not operate when the tanks are filling. The pump discharge pipe work will include connections for flushing with washwater. Ferric storage & dosing Two 45m3 ferric storage tanks will be located in a concrete bund with the dosing skid packages and control system located within an adjacent kiosk. The bund will be capable of containing 110% of the one of the storage tanks plus 10% allowance for rainwater. A drive-in bunded area for chemical deliveries will be provided and an underground chemical interceptor will contain any local spillages, complete with a Castell interlocked 3-way valve to allow the bunded drainage area either to drain into the interceptor (during a chemical delivery) or to drain into the site drainage (normal operation). Chemical dosing points will be provided upstream of the PSTs, on the common outlet channel of the ASP, and upstream of the tertiary treatment plant. The dosing points will utilise sparge systems to ensure adequate distribution of chemicals into the channels. Odour control plant The odour plant is a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 8432 m3/hr (of which 4813 m3/hr is from the inlet works and 3619 m3/hr from the primary settlement tanks). Service water A potable water main supplies a service water break tank via a ballcock (float valve). The service water break tank has a working volume of approximately 30m3 which gives a nominal retention of approximately three hours at average throughput. Service water booster pumps draw water from the tank and transfer it to the service water main network where it feeds the polymer make-up system for all thickeners and dewatering belt presses. Primary settlement distribution chamber The distribution chamber has been CFD modelled to calculate the flow split to each downstream primary settlement tank from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. This required the inlet bellmouth to be turned down to improve flow splitting. [caption id="" align="alignnone" width="1200"] PST distribution chamber CFD models: (left) with four PSTs in service and (right) with six PSTs in service - Courtesy of The Fluid Group[/caption] Primary settlement Primary settlement consists of four 47.15m long by 10m wide tanks, with a depth ranging between 6.24m at the hopper end and 5.3m at the outlet length (equating to 1 in 50 floor slope). Each PST tank has three sludge hoppers at the inlet end. The outlet of the PST discharges into a common 1000mm wide outlet channel. From this outlet channel the 1000mm outlet pipe from the drop box combines with the 700mm RAS lines into an above-ground 1200mm pipe. Retractable odour covers will be provided across the PST tanks to allow for future inspection, cleaning and maintenance. The odour covers will be connected via ducting to the inlet works/primary settlement tank odour plant. All mechanical equipment installed below the covers will be constructed from 316 stainless steel or high performance plastics. The primary settlement tanks have been classified as a Zone 2 Area, and all electrical equipment beneath the odour covers will be ATEX rated. The design of the PST Tanks is based upon a maximum inflow of 3022m3/hr to all four in-service tanks, and a peak flow to each tank of 1007m3/hr with one PST out of service. In normal operation all four PST tanks will be in service. Within each PST tank a chain and flight scraper system will be provided which will utilise high-tech non-metallic materials as the main materials of construction. Scum from the primary tanks will gravitate via a rotating surface skimmer pipe to the above-ground primary settlement scum tank. The primary scum return pumps are progressive cavity pumps, operating in a duty standby configuration. The pumps are fixed speed pumps operating between a high and low level in the scum tank. The PST sludge hoppers will desludge based upon a SCADA-adjustable interval and duration. The primary desludge pumps are progressive cavity pumps, operating in a duty/standby configuration. The pumps are variable speed pumps operating to maintain a SCADA-adjustable flow set point. Once the effluent has flowed through each PST it will enter into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the ASP distribution chamber via a drop box and 1000mm above-ground pipe. [caption id="" align="alignnone" width="1200"] ASP overview - Courtesy of BAM Enpure Ltd JV[/caption] Activated sludge plant The ASP plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The activated sludge treatment tank consists of four ASP lanes. Each ASP lane has a selector tank followed by an anoxic zone and then the three aerobic zones. The main aeration lane will be fitted with aeration pipework and fine bubble diffusers. The aeration vessel is 80m long and 6.1m wide with a water depth of 7.05m. The total instantaneous maximum air demand of the ASP is 10,438 m3/h. The first third of the aerated fraction of each lane will contain 50% of the aeration capacity, the middle third 30%, and the final third 20%. Each aeration lane will have three passes, arranged as channels of 6.15m width. The air for the diffuser grids will be supplied from three ASP air blowers operating on a duty/assist/standby basis. The aeration system will be controlled by an ammonia based real time control (RTC) system utilising a feed forward ammonia load input and a feedback ammonia concentration. Once the effluent has flowed through each ASP lane it enters into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the FST distribution chamber via a drop box and 1200mm above-ground pipe. [caption id="" align="alignnone" width="1200"] Anoxic and aeration zones flows - Courtesy of BAM Enpure Ltd JV[/caption] Final settlement tanks The FST plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The four FSTs will consist of 32m diameter flat bottomed circular tanks with a side wall depth of 6.15m. The FST will separate solids from the flow via settlement, producing a clarified effluent which overflows a peripheral weir into a launder channel, before gravitating to the tertiary treatment distribution chamber via a 1000mm below-ground pipe. The combined clarified outlet flow from the four tanks will be monitored by a common flow meter, as well as suspended solids and phosphorus monitoring instrumentation, all provided on the common line to tertiary treatment. The design of the FSTs is based upon a maximum inflow to each tank of 1741m3/hr with one FST tank out of service. In normal operation all four FST tanks will be in service. Each tank will have a 700mm inlet pipe, which will flow into a central diffuser drum before entering the FST tank. Each tank features a 600mm outlet pipe fed via a peripheral launder channel, as well as a 400mm RAS gravity outlet pipe. Each of the four tanks features a full bridge rotating scraper system which will run continuously in normal operation. [caption id="" align="alignnone" width="1200"] 32m final settlement tanks & sludge return (RAS pumping station) - Courtesy of BAM Enpure Ltd JV[/caption] Sludge return (RAS/SAS pumping station) Sludge from each pair of FSTs will enter one of two RAS/SAS desludge chambers. The two chambers will be located side by side and will share a set of three duty/duty/standby RAS transfer pumps, which will recycle the settled sludge from both RAS/SAS desludge chambers to the activated sludge plant distribution chamber feed line. The variable speed centrifugal RAS transfer pumps will be capable of pumping 2200m3/hr at a maximum head of 8.9mhd. The pumps will operate between high and low level in the duty RAS/SAS desludging chamber, with both chambers provisioned. A pair of duty/standby SAS pumps is provided, which will facilitate wastage of surplus sludge, with each pump drawing from a single RAS/SAS desludge chamber. The fixed speed centrifugal pumps will transfer sludge to the SAS buffer tank. Control of the SAS pumps will be carried out either on a timer or based on the level in the SAS buffer tank, with the common discharge line featuring flow monitoring. Tertiary treatment Clarified effluent flow from the FSTs will pass to the tertiary ferric mixing and flocculation (post-precipitation) stage, where the option will exist to dose ferric sulphate solution into the flow in order to encourage flocculation of remaining solids. This stage will be split into two parallel streams, with flow from the FSTs initially entering via a single bellmouth inlet into a common inlet channel, before passing via twin overflow weirs into two rapid mix chambers, and subsequently overflowing into two flocculation chambers. The design of the rapid mixer chambers will ensure complete mixing (>95% CoV) of chemical in less than 4 seconds. Flow from the flocculation chambers will weir into a common outlet channel leading to a flow distribution chamber feeding the tertiary treatment stage. The distribution chamber has been CFD modelled to calculate the flow split to each downstream cloth pile filter from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. The tertiary treatment stage will comprise six cloth pile filters, which will operate with all filters in service - however the peak flow will be able to be treated with only five filters in service. The filters will be sized to accommodate a peak flow of 840 l/s, producing a final effluent of 20 mg/l TSS (95%ile) and 0.2mg/l TP (average). Bypass pipework is to be provided adjacent to the cloth pile filter structure, by which clarified effluent from the FSTs may bypass the tertiary ferric dosing/flocculation and cloth pile filter stage. Each of the six cloth pile filters will feature a central rotor drum onto which filter discs are attached. The rotor and discs will be fully submerged within the tank, with the discs supporting the physical cloth filter media. Flow entering the filters will flow from outside the discs through the cloth into the discs’ interior, with solids collecting on the external surface of the cloth pile, creating a headloss. As this headloss increases, the water level in the tank will rise and at a pre-set water level (as detected by an ultrasonic level sensor) a cleaning cycle will be initiated. [caption id="" align="alignnone" width="1200"] Tertiary treatment flocculation chambers: Eliquo Hydrok Mecana cloth pile filters - Courtesy of BAM Enpure Ltd JV[/caption] Filtered final effluent (FFE) from the inside of the filter disc and rotor drum assembly will pass forward through the unit via a low-level aperture to an outlet chamber, from which flow will weir into the common final effluent outfall chamber. An emergency overflow weir is to be provided, allowing unfiltered effluent from the filter tank (external to the discs) to weir over into the outlet chamber. Backwash liquors from the backwash pumps will exit the filters via a common manifold, draining by gravity to the grit plant liquor PS. Eight backwash pumps will be provided per machine, giving a total of 48 pumps across all six machines. The pumps will be submersible units placed within the tank, with each pump being dedicated to three filter discs. The backwash flow rate is rated for 10 l/s (subject to the installed pipework design/headloss). Each of the six filter tanks will see some solids settlement occurring (and potential accumulation of other debris), which will create a build-up of sludge in the bottom of the tank. This sludge will be periodically removed by a set of two duty/standby sludge pumps per machine (i.e. 12 sludge pumps installed in total). The discharge from each sludge pump will be connected to the common backwash manifold, leading to the grit plant liquor pumping station. Final effluent discharge Flows leaving the cloth pile filters will pass out from the main filter chamber via a low level aperture into a small outlet chamber, from which flows will weir over into the common outfall channel. The final effluent (FE) will flow under gravity through an FE flow measurement flume, with an ultrasonic instrument for measurement of final effluent flows discharged to outfall. The FE outfall channel will feature a pipe leading to the FE washwater system, with a further line branching off this pipe servings as the suction line for FE Meteorburst Analyser. On exiting the FE outfall channel, flows will enter a dedicated FE outfall main, which will drain by gravity, discharging to the river north of the STW site. [caption id="" align="alignnone" width="1200"] New Guildford STW: 3D model night-time overview - Courtesy of BAM Enpure Ltd JV[/caption] Guildford STW - Sludge Treatment Plant Sludge thickening equipment Primary sludge from the PSTs will be pumped via the primary sludge pumps to the primary sludge buffer tank, which will have a volume of 120m3 and will be sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby primary sludge thickener feed pumps will be provided to transfer sludge (at up to 7.0%DS) from the primary sludge buffer tank to the primary sludge gravity belt thickeners. Two primary sludge gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation, although the streams will be able to run as duty/assist if required. Each machine will be able to handle up to 51m3/h (771 kgDS/h) of primary sludge, at up to 7%DS achieving a solids capture rate of 95%. Surplus activated sludge (SAS) from the FSTs will be pumped via the SAS pumps to the surplus activated sludge (SAS) buffer tank, which will have a volume of 67m3 and has been sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby SAS thickener feed pumps will be provided, which will transfer sludge (at up to 2.0% DS) from the SAS buffer tank to the SAS gravity belt thickeners. Two SAS gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation but will be able to run as duty/assist if required. Each machine is sized to handle up to 55m3/h (332kgDS/h) of SAS, at up to 2%DS, achieving a solids capture rate of 95%. Sludge balancing/blending tanks Thickened primary sludge, surplus activated sludge, and screened sludge imports will be pumped to two thickened sludge blending tanks. The tanks will have a combined volume of 2150m3 (1075m3 each), and will be sized to provide a combined total of 4-days retention time prior to dewatering of the blended sludge. The tanks will operate in a duty/standby configuration, with either tank being available for selection as the duty unit for either filling or emptying. [caption id="" align="alignnone" width="1200"] Thickened sludge blending tanks - Courtesy of BAM Enpure Ltd JV[/caption] Each tank will feature separate inlet connections for the three types of sludge being received, with actuated valves and crossover lines allowing all sludges to be directed to either of the two tanks. The tanks will also feature outlet actuated valves, which will allow either tank to be isolated from the sludge belt press feed pumps downstream, thereby ensuring the pumps only draw from a single tank at any given time. A set of three duty/duty/standby sludge belt press feed pumps will be provided, which will transfer blended thickened and imported sludge from the thickened sludge blending tanks to the sludge belt presses. The pumps will be variable speed progressive cavity units, with a peak capacity of 30m3/h (i.e. 15m3/h per pump) at a maximum head of 48.5m, based on sludge of up to 7.0%DS. Dewatered cake leaving each belt press will enter a discharge cake hopper leading to the suction end of a sludge cake pump. Three pumps will be provided, with a single pump dedicated to a single machine; each pump will operate in conjunction with the associated belt press upstream. The pumps will be variable speed progressive cavity units, with a peak capacity of 7.4m3/h of sludge (up to 30%DS). The pumps will transfer the cake via individual discharge lines to the sludge cake silos, with manual discharge crossover facilities allowing sludge from the pumps to be directed to either of the two silos as required. Cake silo storage & discharge Dewatered sludge cake will be pumped to two truck-loading cake silos, with a combined storage capacity of 600m3, sized to provide 7-days of storage at average cake production. Each silo will feature a discharge screw conveyor (with a sliding frame ‘anti-bridging’ device) which will discharge the cake through a hydraulically operated slide gate valve. Odour control plant The odour plant will be a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 6629m3/hr, extracted from the various tanks and thickening/dewatering machines in the sludge plant area. New STW outfall Outfall route The outfall route from the new works site location was assessed thoroughly in the ITN 2 stage of the project’s life and from exiting towards the middle of the works site, proceeds north towards the River Wey. The route deviates initially to minimise any impact on significant trees that exist within the floodplain, and any impact on the band of vegetation around the perimeter of the site. Final effluent & stormwater pipeline The final effluent pipeline will be a 750mm diameter pipe. This will be situated above ground from the final effluent channel to the edge of the site, and will be supported by piles when within the treatment works site. The pipeline then proceeds below ground through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. A jetting/access point will be provided just before the pipeline dives below ground, giving the facility to clean this arrangement and flush from a more direct point than the pipes exit from the final effluent channel. Air relief may also be provided at this point, subject to detailed design. The storm overflow pipeline will be a 1050mm diameter pipe. This will be situated below ground from the storm tanks to a sampling manhole at the edge of the site; supported by piles when within the treatment works site. The pipeline will then proceed from the manhole through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. The sampling manhole will provide an ability to clean this arrangement. [caption id="" align="alignnone" width="1200"] (left) Outfall route plan - Courtesy of Bam Enpure Ltd JV, and (right) outfall low flow scenario CFD model - Courtesy of Arcadis[/caption] Outfall structure The outfall structure will consist of a reinforced concrete headwall, set back from the river channel, to be founded in the river terrace deposits and flared to direct flows downstream and minimise the impact of upstream debris from collecting. The headwall will have duckbill valves on the discharging pipes, and erosion control products utilised in the surrounding area to tie the headwall into the natural riverbank and bed. The erosion mattress employed following the headwall structure will provide a secondary point at which turbulent flows can dissipate energy without eroding the natural bed of the river. Where the headwall ties into the natural riverbank, the hard/soft interaction between headwall and natural soil will be reduced by the installation of Rock Rolls erosion control and willow stakes at the base, and the installation of a natural hydro-seeded CocoBN blanket or equivalent (to be seeded with local seed varieties) which will retain the slope shape until vegetation establishes and provides an effective natural erosion control. This will also provide a gradual slope to access down to the river. Programme The project obtained planning permission in October 2022, with construction works starting on site in July 2023. Detailed design of the civil, mechanical and electrical works are due for completion by December 2023. Construction of the works will be complete by August 2025, with commissioning due to commence in from April 2025 through to December 2025. Flows from the existing works will be diverted to the new works from December 2025 (10% flow) in a 7-stage commissioning process with 100% flows diverted to the new STW by March 2026. Following 100% flow diversion the existing STW will be de-commissioned ready for handover to Guildford Borough Council.

Ufton Nervet WTW is a groundwater treatment site located in the Kennet Valley Water Resource Zone, to the south of Reading and supplies an ‘island zone’ of approximately 12,000 properties. Groundwater is pumped through pressure filters for iron removal after which chlorine is dosed before flows enter the contact tank, which is a single cell semi-buried tank of concrete construction where the water comes into contact with chlorine to achieve disinfection. SO2 de-chlorination occurs before high-lift booster pumps pump the water into the water supply network. Thames Water contracted MMB to design and build the proposed run-to-waste (RTW) and contact tank (CT) as part of the AMP7 Capital Delivery Alliance Lot 1- Water non-infra Thames Valley framework. The problem The existing contact tank asset has been raised on the drinking water safety plan as a resilience issue since it is not possible to take it out of service for a periodic planned maintenance inspection. This should occur so risks to water quality and long-term structural integrity can be managed, without shutting down water production to maintain customer supplies. Since the existing contact tank is semi-buried, in the event of high turbidity, there is no permanent automated means to run-to-waste pre or post-contact tank. The only option available is a manual invention and over-pumping into an adjacent ditch. The project has to overcome the following constraints: The WTW feeds an ‘island zone’ network, meaning if the site becomes inoperable the customer potable network loses water and pressure in less than eight hours because there is no means to re-zone the network to feed it from another site. Ufton Nervet WTW is a small site with limited space which is surrounded by a watercourse ditch in poor condition. Areas of the site which do not contain existing assets have low to moderate categorised trees, or modified grassland in poor condition. The site is surrounded by agricultural land which is a floodplain so any new assets would need to be kept to a minimum to avoid having an impact on the floodplain storage, but still meet the planned operational and maintenance requirements expected of any asset. The site has restricted access via a single lane road meaning delivery of larger items would be challenging, but the contact tanks need to be large enough such that if one is taken out of service those remaining provide enough contact time between chlorine and water. All these constraints mean any spare space is at a premium and careful planning is needed to phase the work to keep outages and overall programme as short as possible. [caption id="" align="alignnone" width="1200"] Existing contact tank - Courtesy of MMB[/caption] Solution It is proposed to tie-in to the existing contact tank feed with DN500 pipework and isolation valves on each side of a new tee piece, which will enable the works to continue running whilst the new contact tanks are built offline, before the existing 200m3 tank is decommissioned and demolished at the end of the project. Four pressurised contact tank vessels will be installed in the field adjacent to the site. These are elevated out of the floodplain with an access gantry provided to allow operational staff to cross from the existing site to operate the tank isolation and maintain air valves. The contact tanks will be 21m long and 2.1m in diameter to meet the 15 mega litre/day (MLD) throughput and client asset standard contact time, even with one contact tank taken out of service for inspection to enable the site to remain operational. Chlorine will be dosed via a new point of application (POA) static mixer and an aeration gas dosing system will be housed in kiosks with associated sample pumps, which will replace the current cascade aerator on the existing contact tank. A balance tank will provide the hydraulic break between the pressurised contact tanks and existing high lift booster pumps, with an internal dividing wall to allow either cell to be taken offline. Two pairs of actuated valves and turbidity instruments will allow for a manually initiated automated system pre and post the contact tanks to be de-chlorinated before running to waste. The RTW system will comprise a dichlorination chamber, flow meter chamber, sample chamber, and headwall all sized for a maximum discharge rate of 120 l/s. A new site road will provide vehicle access to the contact tanks and a perimeter security fence installed around the extended site boundary. [caption id="" align="alignnone" width="1200"] 3D model showing the four pressurised contact tank vessels - Courtesy of MMB[/caption] Ufton Nervet WTW Contact Tank: Supply chain - key participants Because the project scope is complex and requires specialist input and early supplier engagement the following multi-disciplinary teams were involved in delivering the project. Client: Thames Water Principal contractor & designer: MMB Specialist design (planning, CFD, ecology): Mott MacDonald Civils/groundworks: Hercules Site Services Ltd Topo/GRP/Point Cloud: 40Seven Ltd Ground investigations: Dunelm Geotechnical & Environmental Contact tanks: Vessco Engineering Ltd Mechanical installation (pipework): JB Fabrications Mechanical installation (metalwork): FabTek Ltd Electrical installation: IMAC Ltd MCCs/systems integration/ICA: BGEN Ltd Kiosks & security covers: Morgan Marine Ltd Static mixers/aeration system: Statiflo International Ltd Valves: AVK UK Ltd Precast concrete: FLI Precast Solutions Legato® interlocking blocks: Elite Precast Concrete Ltd Instruments: ABB Ltd Instruments: IFM Electronic Instruments: Siemens Sample pumps: Grundfos Ltd Security systems: Reliance Ltd Security fencing: Allen Fencing Ltd Flood protection measures (borehole pump buildings): Flood Control International Overcoming the constraints Computational Fluid Dynamics (CFD): CFD modelling of the proposed contact tanks at Ufton Nervet WTW was undertaken to prove the performance of the proposed design. The performance of the contact tanks is measured in terms of the standard retention time t5 (the time taken for 5% from the point of the chlorine addition to measurement at sample point No. 4 after the contact tanks). [caption id="" align="alignnone" width="1200"] CFD modelling - Courtesy of MMD CFD Team (Spain)[/caption] The purpose of the model was to provide evidence that the water has a minimum t5 retention time of 20 minutes to comply with the standard if the raw groundwater is considered unstable, or 15 minutes if considered stable. A non-reactive tracer was added to the model at the primary dosing point and measured at the outlet through time to obtain its exit age distribution. The model was run as a worst-case scenario with one of the furthest of the four tanks taken out of service; meaning only three were modelled including interconnecting pipework. The model was also based on a 15 MLD design horizon which is conservative as it is 1.5 MLD beyond the abstraction limit and capability of the existing borehole pumps. It was agreed with the client that the groundwater was considered stable such that a 15-minute t5 was the target figure, and the model showed an 18.2-minute t5 with 21m long contact tanks. This offered a saving of approximately £230k by having shorter tanks because the CFD modelling showed that 25m long tanks would have been required for a 20-minute t5. This would have made delivery of the tanks down Ufton Lane even more challenging. Even so, Autotrack analysis was completed to confirm in theory there was a route to the site, which was viable for delivery of the 21m tanks, which the tank suppliers’ delivery management company confirmed was possible during a pre-delivery survey. This was considered to be the right balance of resilience and affordability because it is unlikely the unstable groundwater events would occur at peak design horizon demand whilst one of the contact tanks is out of service. [caption id="" align="alignnone" width="1200"] Exit age distribution of tracer measured at the outlet - Courtesy of MMD CFD Team (Spain)[/caption] Use of SYNCRO to hold a digital construction rehearsal to de-risk programme: A P6 programme has been produced which lists numerous construction activities and outages limited to eight hours due to the island zone constraint, and a 3D model has been produced to illustrate the physical space each asset needs when installed. As part of MMB’s Digital Delivery Plan SYNCRO (one of the digital tools utilised by MMB) was used to hold a construction rehearsal which led to the following benefits; a better understanding of the space constraints, engaged client stakeholders for programme buy-in, improved safety and a de-risked programme. By combining the P6 programme with the 3D model and reviewing it via SYNCHRO software allowed a better understanding of the concurrent activities and the ability to spot where there could have been potential space constraints. The digital rehearsal revealed that the design was proposing to install precast pipe support plinths and the access metalwork at the same time as the contact tanks were being installed, so SYNCHRO was used to update the P6 programme live to re-sequence the work. This rehearsal simulated the contract lift of the contact tanks and illustrated just how much space was needed to have a safe working area to complete a lift of a 21m long 10t tank on a 20m radius. This again led to a change in the construction sequence of the mechanical installation of pipework and access metalwork. Focusing on safety and being prepared to challenge programme where there looked to be too much concurrent working, should ensure a right first-time approach before committing to starting on site, reducing the risk to quality or safety and having to abortively re-plan works, which may end up increasing costs. [caption id="" align="alignnone" width="1200"] MMB SYNCHRO digital rehearsal - Courtesy of MMB[/caption] The digital rehearsal review coincided with a change in a key construction assurance client team member, so the above process sought their buy-in and confidence in the construction programme and occurred at a significate project milestone (contractual solution review) where the client can make a go/no-go decision on whether the project goes ahead into detailed design and construction. The review received positive feedback from the client that the session proved to be a valuable and more engaging way to bring them up to speed with the scheme and the construction programme. The project will revisit SYNCHO to plan in more detail at other key milestones like works outages, or whilst on site as a more visual way of completing 5-week-look-ahead planning sessions. MMB has a Digital Delivery Network which is a community of people who can support projects with tools like SYNCHRO and others which can be found on the MMB Digital Delivery Hub. The benefits of this digital rehearsal were more cost/risk mitigation rather than direct cost reduction on a project. The estimated consequence of not having realised the space constraints and programme concurrent working is 10-weeks lost time meaning £148k cost increase ‘saved’ (avoided). It is difficult to estimate the cost saved/benefit of having a happy engaged client stakeholder like a construction assurance engineer. This is assumed as saving/mitigated a potential two-week delay resulting in a approximately £30k saving. Providing maintainable assets vs keeping flood plain impact negligible: The existing site not only feeds an ‘island zone’ network; it also becomes an island during flood events and is surrounded by drainage ditches. Relocating the entire site would be uneconomical and too intrusive so expanding the treatment works into the adjacent field is the best whole life cost solution even if that field is a flood plain. This meant any new assets needed to have as small a footprint as possible to ensure it had negligible impact on the flood plain, and yet still needed to be assessable for the planned maintenance and main driver for the scheme; to be able to take one contact tank out of service for quality and structural inspections. [caption id="" align="alignnone" width="1200"] Flood plain location of contact tanks - Courtesy of MMB[/caption] The solution is to install the steel contact tanks on slim 2m tall concrete plinths, with the inlet/outlet stainless steel pipework supported off steel pipe supports founded on Legato blocks or strip foundations. Precast units were selected where practical and economical to speed up the install to reduce the risk of needing to poor concrete into a floodplain as part of the 10-month long construction programme. The outlet pipework was looped back to join the inlet pipework such that all the isolation and air valves can be operated from the same common access platform, which is 21m long by 6m wide and stands 2.8m above ground level, which saved approximately £400k in metalwork and civil plinths The platform then bridges over the ditch between the treatment works and adjacent field to provide operational access. To ensure all the new assets didn’t have a negative impact on the floodplain a flood model and flood risk assessment was completed based on the volume of new assets to be installed in the field, which showed negligible impact to the flood plain. The contact tanks can be accessed at ground level outside of flood events via a new Grasscrete access road, which removed the need for road drainage which saved £37k. Foundations would have the existing underlying alluvium layer of soft organic clay replaced to target the medium dense gravels layer with stone to allow for better drainage during periods of rainfall and reduce the risk of settlement. The alternative to the elevated assets would have been to bund the entire area which would have had significant cost, carbon and floodplain offset impact which was calculated using the Mott MacDonald Moata Carbon Portal. [caption id="" align="alignnone" width="1200"] Ufton Nervet WTW 3D models - Courtesy of MMB[/caption] Biodiversity net gain (BNG): Approximately 2m of the existing ditch will be disturbed to install the new headwall, so another section approximately 10m long will be enhanced from poor to moderate condition equating in a 13.3% BNG in river units. The 0.1ha of agricultural grassland will be enhanced from poor to moderate condition to achieve a net gain of 10% to account for that lost from installing the new assets. Approximately 88 urban trees are proposed to be planted and maintained to moderate condition along the 180m boundary fence line to achieve a net gain of approximately 40%. To achieve a net gain of 10% would mean only 22 small trees needing to be planted, but this would of only provided a visual screening along 25% of the fence line. Therefore a hedgerow will also be planted along approximately 125m of the inside of the fence line to provide further visual screening and offer at least a 27% net gain in BNG hedgerow units. Conclusion The project started in January 2022 so has been delivered through the COVID pandemic with a team remote-working via MS Teams but making use of Mott MacDonald’s practice network has enabled colleagues from across the UK, Spain, and India to work together for the first time. The project is anticipated to start on site in April 2023 and to be completed by January 2024.

September 2022 saw the successful completion of a new kilometre outfall pipeline in Arklow, Ireland. The lack of wastewater treatment facilities in Arklow resulted in untreated wastewater being discharged directly to the Avoca River. The design and construction of a new wastewater treatment plant and long sea outfall was required to improve the water quality of the Avoca River and bring benefits in terms of health, environmental integrity and facilitate economic and social development for Arklow town and its surrounds, which has been constrained by the lack of adequate wastewater treatment capacity. This paper focuses solely on the marine design/construction elements of the project as construction of the new treatment works and other associated structures are still ongoing. Background Arklow is a town in County Wicklow on the southeast coast of Ireland. There were no wastewater treatment facilities in Arklow town and as a result, untreated wastewater was being discharged directly into the Avoca River. This was not compliant with obligations under the European Union Urban Wastewater Treatment Directive. Uisce Éireann, therefore proposed a development to provide an effective wastewater collection network, treatment capacity and treated effluent outfall that would improve water quality in the Avoca River. Ward & Burke was appointed as the main contractor for delivering the Arklow Wastewater Treatment Plant (WwTP) project, with the design and construction of the associated long sea outfall sub-contracted to Van Oord Ireland Ltd. Royal HaskoningDHV was appointed by Van Oord as designer for the long sea outfall (LSO). The overall project required the design, construct and implementation of the following: A new, state of the art, wastewater treatment plant that has been designed to provide an ultimate treatment capacity for a population equivalent of up to 36,000 at the Old Wallboard Factory, North Quay, Ferrybank. Sewer pipelines (along the North and South Quays) to bring the untreated wastewater to the WwTP. A marine outfall pipeline to safely discharge the treated wastewater to the Irish Sea. Upgrade of the existing rock armour revetment adjoining the site. [caption id="" align="alignnone" width="1200"] Outfall pipe arrival from Norway by sea to the dock in Arklow for preparation and assembly - Courtesy of Van Oord Ireland[/caption] The proposed outfall pipeline was required to discharge the treated effluent from the wastewater treatment plant to receiving waters. Both marine and river outfall options were initially considered for the proposed development. However, hydrodynamic and water quality modelling demonstrated that the wastewater discharge standards required for a river outfall would be much more onerous than that of a marine outfall. For that reason, the river outfall was not considered further. Solution The detailed design of the new outfall system for the Arklow Wastewater Treatment Plant Project considered and finalised the new outfall pipe route, hydraulic design, preferred material and construction methodology, pipe weighting, backfill and diffuser arrangement. The outfall system comprised of a 630mm outer diameter pipeline (630mm OD SDR 17 PE100 pipe) and started at a connection point onshore, behind the revetment and extended offshore for approximately CH927m to the discharge location. The outfall discharge arrangement/diffuser configuration was located in approximately 13m of water depth from the Extreme Water Level (EWL) and consists of six risers (355mm O.D. SDR 17 PE100 pipe), four diffuser ports (160mm O.D. SDR 17 PE100 pipe) each with 24 return duckbill valves that horizontally discharge the treated effluent at a height of approximately 1m above the seabed level. The detailed design trench detail was developed to provide a minimum cover depth to the outfall pipeline of 1.5m following installation. This cover depth allowed for both seasonal and long-term fluctuations on the seabed levels and protection from potential damage due to dragging anchors. The trench was backfilled with selected as dug material, typically medium dense sands and gravels, that were side-cast during the trench excavation. Where the outfall crossed beneath the existing and proposed coastal revetment, the pipe wall structural thickness was checked and verified to prevent any high point loads from the revetment rock armour. [caption id="" align="alignnone" width="1200"] Outfall pipe floated along the quay wall in Arklow prior to preparation and assembly works - Courtesy of Pipelife Norge AS[/caption] The marine outfall was also designed to be stable in the ‘worst case’ design loads for current and wave loading conditions provided. It was therefore necessary to weight the pipeline with concrete collars and subsequently concrete kennels after installation (float and flood method), to provide a specific gravity of not less than 1.35 that could be achieved without any reliance on backfill material. This installation weight was provided with 1015kg (290kg/m) (in air) fixed weight concrete collars at 3.5m centres and to satisfy the permanent on bottom stability, 2292kg (654.86kg/m) (in air) concrete kennels were provided at 3.5m centres in between the fixed weight concrete collars. The collars and kennels were provided by Tracey Concrete. Constraints Arklow is a town with a population of 14,000 people and a small port which generates medium to high traffic within the port. This constituted working and delivery restrictions during the preparations of the offshore section of the outfall pipeline. In addition, the turbulent weather did not allow for preparing the outfall pipeline offshore, outside of the port vicinity. Additionally, a survey undertaken by Uisce Éireann showed an existing subsea cable in close proximity to the designed LSO route. The cable supplied power to a substation located South of the site location from the offshore Arklow Bank Wind Farm. The survey undertaken by Uisce Éireann confirmed the location of the power cable and showed that the cable route was outside the redline boundary for the works. However, the owner/operator of the subsea power cable was consulted to ensure that the new LSO route and location was compliant with the applicable exclusion zones/constraints for the subsea power cable. [caption id="" align="alignnone" width="1200"] Welding of risers for diffuser section of outfall pipe - Courtesy of Van Oord Ireland[/caption] Site investigation Site information was provided by Uisce Éireann from a ground investigation undertaken in 2017 at the proposed site location, as well as information from the Geological Survey Ireland (GSI) website borehole archive that included relevant land-based and overwater boreholes. The ground investigation indicated that the ground conditions generally comprised of a variable sequence of medium dense to coarse soils and gravel. The offshore investigation data indicated that there is a significant variation in the ground conditions between the boreholes which were spaced at approximately 150m intervals e.g., buried valleys, rock outcrops etc. Large boulders of metamorphic rock were also encountered in the boreholes. Whilst the depth of the boulders generally exceeded 3m below bed level, the presence of boulders at shallower depths could not be discounted and posed a risk for the construction of the outfall. The requirement for further site investigations were identified and these included magnetometry, sub-bottom profiling and unexploded ordinance (UXO) surveys to determine the mitigation measures required to be undertaken prior to the proposed works and construction of the pipeline. Arklow LSO : Supply chain - key participants Arklow WwT Project delivery contractor: Ward & Burke Long sea outfall design/build: Van Oord Ireland Ltd Van Oord designer: Royal HaskoningDHV HDPE pipe: Pipelife Norge AS Concrete collars & kennels: Tracey Concrete Tideflex S35 valves: MeasurIT Technologies [caption id="" align="alignnone" width="1200"] Excavation of trench at the onshore connection point using the backhoe dredger ‘Razende Bol’ - Courtesy of Van Oord Ireland[/caption] Marine works A total length of 927m of polyethylene pipe (630mm OD SDR 17 PE100) was manufactured by Pipelife Norge AS, in three lengths of 309m which were then towed from Norway by sea to the dock in Arklow, Ireland for preparation and assembly. Along the quay wall, concrete collars, each weighing 1 tonne were installed around the pipe strings at 3.5m centre to centre distances. The collars were lifted from land with special lifting equipment and the two halves of the collar were connected from the water using small workboats. Flanges were fitted with the assistance of a marine tug boat and the on board crane. During these operations the HDPE pipe was lifted out of the water. The lifting operations of the pipeline where analysed in detail to predict stresses in the material and the bending radius of the pipe. Using this analysis the lifting operations could be adjusted to prevent buckling or damaging the HDPE pipe. For the installation of six riser feet on the pipe, a spreader beam was used to lift the pipe 1m out of the water. The usage of the spreader beam followed from the analysis done for the lifting. Excavation of the trench was carried out using land-based excavators on the revetment and Van Oord’s backhoe dredger ‘Razende Bol’ for the marine trench. A minimum cover of 1.5m above the crown of the pipe was required for the outfall pipeline through its entire route. The excavated trench material was cast adjacent to the trench for re-use as backfill material upon completion of installation. [caption id="" align="alignnone" width="1200"] (left) Outfall pipe during sinking operation and (right) outfall pipe aligned prior to sinking operation - Courtesy of Van Oord Ireland[/caption] Installation After pipe preparations were completed and the trench was excavated, the pipe installation commenced. The three separate pipe sections were connected with flange connections next to the quay wall in Arklow in order to create one long pipe length of 927m. Bend restrictors to prevent over-bending at the flange-flange interface were also installed. The finished pipeline was manoeuvred out of the port using two tugs and multiple small work boats. The pipe was anchored overnight just outside the Arklow port. Before the sinking operation, the pipes were aligned and brought into the onshore connections point where pumps were set-up to flood and sink the pipe. The pipes were pulled into the shore using a pre-installed pull wire and an onshore drum winch. The polyethylene pipelines were installed using the ‘float and flood’ method starting from the fixed onshore end. During the sinking operation, the offshore end of the pipes were connected to a multicat which was moored alongside the backhoe dredger ‘Razende Bol’ for proper positioning. The controlled pumping of seawater into the onshore pipe end, initiated the sinking process which caused the pipe to form an S-curve of approximately 20m long between the seabed and seawater surface. Two tug boats (shoal busters) were used in order to position the pipe correctly at the location of sinking. As a contingency measure and to ensure the installation process was well controlled, a foam pig was pre-installed in order to create a seal between the pipe section filled with air and the section filled with water, in case the pipe had to be re-floated due to inclement weather. Once the pipe sections were completely installed in the trench, the profile was checked by a multibeam survey to verify the final pipe position. Post pipe installation, the foam pig was removed from the pipe. A Type II pressure test of 2.25 bar was performed in accordance with the procedure set out in IGN 4-01-03, to 1.5 x the maximum internal working pressure. [caption id="" align="alignnone" width="1200"] Outfall pipe concrete collars being fitted in water with special lifting equipment - Courtesy of Van Oord Ireland[/caption] Once the outfall pipeline had been sunk to the sea bottom and pressure tested, concrete kennels, each weighing 1 tonne were placed in between the concrete collars at 3.5m centre to centre spacings to provide additional permanent weighting. Additionally, six diffusers were installed from the backhoe dredger platform by divers. Each diffuser comprised of a flanged PE pipe, fittings and a riser with a four-port crosshead, each with a flange allowing for Tideflex S35 valves to be fitted directly. Included as part of the diffuser installation, was a removable access flange at the top of the riser to enable access inside the riser. After completing the diffuser and kennel installation, the pipe was backfilled over its full length to provide 1.5m cover. Rock armour protection was further provided extending approximately 7m around the diffuser section from its centre to protect the area around the diffuser from scour. Also provided was a steel protection frame for the diffuser, fitted with sacrificial anodes to allow a 50yr design life for protection from dropped objects, anchors, trawlers and fishing nets. Finally, on the offshore end a marker buoy and on the onshore end a Saint Andrews Cross were installed to mark the pipeline location. Due to the turbulent weather at the site location, including hold points in the working method, so that each installation step could be paused at planned intervals, allowed Van Oord to work around the weather. Additionally, working with local contractors who helped to adjust and adapt work methods and techniques within the limited amount of space, allowed the team to prepare and install the outfall system using specialised equipment and marine vessels, with minimal disruption to the environment and public. In particular, a bespoke rigging system was designed and fabricated using a local yard to assist with the efficient installation of the collars whilst working from the water. [caption id="" align="alignnone" width="1200"] Outfall pipe during sinking operation - Courtesy of Van Oord Ireland[/caption] Conclusions The successful completion of this project will allow treated effluent to be safely discharged from the new Arklow WwTP once operational and bring benefits in terms of health, environmental integrity and facilitate economic and social development for Arklow town and its surrounds, which had been constrained by the lack of adequate wastewater treatment capacity.

Guildford town and the surrounding residential areas are currently served by an existing sewage treatment works (STW) located adjacent to the River Wey, just north of the town centre. The land which this occupies has been identified as part of the Slyfield Area Regeneration Project (SARP), which includes the new Weyside Urban Village (WUV). This mixed-use regeneration project will include circa 1,500 new homes, new community facilities and 6,500m2 of commercial space to improve the local area. Thames Water Utilities Ltd (TWUL) is working in partnership with Guildford Borough Council (GBC) to move the STW to a new location, which is not suitable for housing.

The location chosen is on the north-east extent of the existing Slyfield Industrial Estate and is on an area previously used as a landfill site. In August 2023 Water Projects published a case study which provided insight into the extensive pre-construction design for civil and process engineering elements of the project. This article delves into the construction of the project and provides an update on the significant progress made to date on site.

Site historical use

The site for the new STW is approximately 70,000m2 and was previously used as a landfill site. It had originally been used as sand and gravel quarries and then subsequently used as landfill from the 1960s to the 1980s.

The landfill material is generally domestic and light industrial. It consists of black bag waste, timber, plastics, fabrics but contains fragments of asbestos and other materials.

The site was capped off with a granular layer in the early 1980s and has laid dormant ever since. The landfill depth varies across the site from between 2m and 6m thick. This then overlies an alluvium layer, sands and gravels and London Clay. The depth of the London Clay is approximately 10m below existing ground level.

[caption id="attachment_16131" align="alignnone" width="1200"] A screenshot of the 3D model used for design and construction - Courtesy of Bam Nuttall Ltd[/caption] Civil engineering: Basis of the design

Typically, major structures within a sewage treatment works are built below or partially below ground level to maximise gravitational energy. The new Guildford STW differs in that the strategy is to deliberately design and build above ground as far as practicable.

This is to minimise the disturbance to, and removal of the existing landfill thereby reducing the impact on the environment and the cost of disposal.

Undertakings

After a period of Early Contractor Involvement, Thames Water awarded a contract to BAM-Enpure Joint Venture (BEJV) in 2021 to design, build and commission this significant new works and once operational, to de-commission the existing works.

BEJV is a non-integrated joint venture, formed specifically to combine the excellence of experienced civil engineering contractor Bam Nuttall Ltd and the process engineering/MEICA specialism of Enpure Ltd to deliver a turnkey solution for Thames Water. The workscope split is as follows:

Bam Nuttall Ltd: Civil engineering workscope Principal contractor Civil engineering design Ground stabilisation & piling Structures Underground pipework Tunnels, shafts and connections Buildings Outfall pipeline & structure Roads, fencing & general civil engineering works [caption id="attachment_16143" align="alignnone" width="1200"] Construction of the high level inlet works structure - Courtesy of Bam Nuttall Ltd[/caption] Enpure Ltd: Process engineering/MEICA workscope Principal designer MEICA design Process design Mechanical works Electrical works Control systems Commissioning

Decommissioning of the existing STW will also be undertaken by BEJV, in addition to working collaboratively with TWUL operational and networks teams to integrate existing TWUL operational assets into the new sewage treatment works.

Site trials

BEJV undertook a series of site trials in 2021/2022. The purpose of this was to investigate the proposed ground improvement techniques further in order to validate proposals and inform detailed design. There are a combination of techniques which interface with each other in a complex manner.

The trials also gave important information about potential effects to both leachate levels and ground gas within the existing landfill to ensure there would be no impact on environment, ecology and human health.

Four trial areas were chosen, to provide a variety of conditions and in each of these areas a set of boreholes was established at increasing offsets away from the trial areas. These boreholes contained monitors for both leachate levels and ground gas volumes and were monitored 24/7. In addition, noise and vibration monitoring was also undertaken. [caption id="attachment_16140" align="alignnone" width="1200"] (left) Rapid impact compaction underway and (right) a selection of site plant in use - Courtesy of Bam Nuttall Ltd[/caption]

The three techniques tested, and subsequently used within the works were as follows.

1. Rapid impact compaction (RIC): RIC was undertaken to remove the risk of long-term settlement to the landfill. Whilst the main structures would be supported on piles, this was to remove the risk of long term ground settlement underneath and around the structures. RIC is a method of imparting energy to the ground using a repeated but relatively short drop-weight, where a base plate is in constant contact with the ground. It is undertaken on a pre-determined grid pattern. At Guildford a 9T hammer was chosen, fitted to a 35T base machine and working on a 2.5m x 2.5m grid. Each ‘pass’ of the technique used 40 blows at each point.

Within the trial areas the technique was tested with 1, 2 or 3 passes (ie, 40, 80 or 120 blows per location). Cone penetrometer tests were used to measure the extent of deep level ground compaction before and after the trials. The continuous monitoring from the boreholes demonstrated that at each ‘thump’ of the ground there was no discernible change in leachate levels or ground gas volumes, and that there was no dispersal of leachate happening throughout the landfill.

From the trials it was determined that all areas of the site that are being built on should receive either 1 or 2 passes of RIC for the permanent works.

[caption id="attachment_16135" align="alignnone" width="1200"] (left) Driving precast concrete piles and (right) vertical rigid inclusion installation - Courtesy of Bam Nuttall Ltd[/caption] 2. Precast driven piling: The main structures and pipelines are founded on precast driven piles. These are end-bearing into the London Clay, approximately 10m below ground level. Precast driven piles were chosen as the solution as the operation does not bring any landfill to the surface. Various size piles were driven to various depths and tested with both dynamic and static tests. Borehole monitoring showed that there was no discernible change to leachate and ground gas levels and that there was no dispersal of leachate happening throughout the landfill.

3. Vertical rigid inclusions (VRI): VRIs are mass concrete piles which are used for lightly-loaded structures and areas such as roads and hardstandings. They are a displacement pile, with the surrounding soil pushed sideways by an auger. As the auger is withdrawn the resulting void is filled with concrete up to ground level. They therefore work by increasing the density of the ground. A granular layer is then installed above to remove any ‘hard spots’. Again, this technique does not bring landfill material to the surface. It was trialled in two areas and found to be suitable for use, without causing any effects to leachate levels or ground gases. It is therefore being used as part of the permanent works over the whole site wherever there are not piled structures.

The trials of all three techniques, together with the subsequent site testing proved that the design principles were correct, and gave information on how to refine the solutions further. It also demonstrated that the techniques would not cause any adverse environmental effects.

[caption id="attachment_16142" align="alignnone" width="1200"] (foreground) Ferric mixing and flocculation chambers and (background) concrete bases for sludge tanks - Courtesy of Bam Nuttall Ltd[/caption] Guildford STW: Civils works supply chain - key participants Main contractor: Bam Nuttall Ltd Planning consultant: Adams Hendry Consulting Ltd Civil engineering design: Arcadis Group Ltd Boreholes & testing: Geo-Environmental Services Ltd Asbestos monitoring: Socotec Ltd Rapid impact compaction: Cofra Ltd (Boskalis Group) Vertical rigid inclusions: Foundation Piling Ltd Concrete piling: Aarsleff Ground Engineering Ltd Temporary works specialist: Mabey Hire Ltd Temporary works specialist: MGF Ltd Tunnels & shafts: Joseph Gallagher Ltd In situ concrete works: Kelly Formwork Ltd Precast concrete works: DJ Civils Ltd Precast concrete manufacturer: FLI Precast Solutions Precast concrete manufacturer: A-Consult Ltd Soil processing: Remediation Waste Management Services Concrete supplier: Heidelberg Materials Gas protection: PAGeo Contracting Ltd Crane supply: Delden Cranes Enabling works

Enabling works took place during Spring 2023 and consisted of site clearance, setting up the site offices and welfare and establishing fencing. The whole site was then covered with a 100mm thick layer of imported granular material to provide further protection against the risk of asbestos within the existing capping layer.

A series of 20 further boreholes were installed around the perimeter of the site to enable daily monitoring of both leachate and ground gas within the landfill and to ensure that none was being mobilised by our works. Mobile monitoring stations for noise, dust, vibration and asbestos were all set up, which are monitored continuously throughout the works. Over half a million data points have already been captured.

[caption id="attachment_16138" align="alignnone" width="1200"] Piles and blinding concrete for the four final settlement tanks and (left) temporary ground support for pipelaying into the centre of the tanks - Courtesy of Bam Nuttall Ltd[/caption] Guildford STW: MEICA works supply chain - key participants Main designer/contractor: Enpure Ltd CFD analysis: The Fluid Group Cake silo: Saxlund International PST scrapers & ferric dosing plant: Colloide Inlet works & PST odour covers: Power Plastics Ltd Grit remover plant: Jacopa Escalator screens: Longwood Engineering Imported sludge Strainpress: Huber Technology Sludge thickening & dewatering: Kent Stainless Ltd Odour control plant: Odour Services International Ltd FBDA grids & blowers: Xylem Water Solutions Cloth pile filters: Eliquo Hydrok Ltd Storm tank cleaning system: Sulzer Pumps Wastewater Ltd Tank logging system (WASP): JR Pridham Services Ltd Glass lined steel tanks: Balmoral Tanks Ltd Sludge tank mixing: Landia UK Ltd Transformers: Wilson Power Solutions Ltd Groundworks and foundations

The permanent works started in earnest in Summer 2023 and in August, rapid impact compaction was conducted over the site, with a combination of one or two passes, as informed by the results of the previous site trials. In total the ground was hit 360,000 times! Precast piles were then driven for the major structures and pipelines. These were all 250mm square piles and varied in length from 16m to 20m. A total number of 3,422 piles have been driven using three piling rigs.

Vertical rigid inclusion installation is currently underway, with a total of 1,700 out of 9,000 already completed. The works are sequenced to avoid clashes with other work areas, on what is a congested site. Following installation, each work area is left for the piles to cure for the following seven days, thus effectively sterilising large areas of the site at a time. This therefore requires detailed collaborative planning on all work fronts to ensure that the site can remain operational.

[caption id="attachment_16136" align="alignnone" width="1200"] Activated sludge plant precast walls erected, prior to pouring base slab and concrete stitches - Courtesy of Bam Nuttall Ltd[/caption] Concrete structures

Of the 59 concrete structures, some are simple reinforced bases, however ‘the big 5’ are significant structures:

Inlet works. Primary settlement tanks (PSTs). Activated sludge plant (ASPs). Final settlement tanks (FSTs). Tertiary treatment plant (TTP).

Due to the repetitive nature of their construction, BEJV chose to construct the PST and the ASP using precast concrete techniques, which are then stitched together on site. The four FSTs and two storm tanks are also being constructed using precast panels which are then post-tensioned.

The inlet works and TTP are complex, bespoke structures with very little repetition and so are more suited to traditional in situ concrete construction. The inlet works is a particularly complex structure, and is the tallest structure on site at 13m above ground level and being formed of five separate elements.

As of August 2024, the inlet works concrete structure is half complete and the TTP structure underway. The PST precast walls are mostly in place and stitched together, with roof beams now being installed. Works have also started on the ASP walls.

Pipework to the FSTs is underway, which will then allow the construction of the bases to follow on. To date (August 2024), 4,000m3 out 14,000m3 of concrete has been poured, and 205 out of 454 precast wall elements installed.

[caption id="attachment_16141" align="alignnone" width="1200"] (foreground) PST precast walls with concrete stitches and bases and (background) ASP precast walls - Courtesy of Bam Nuttall Ltd[/caption] Transfer tunnel

The flows to the existing STW enter at the southern end of the works. A new transfer tunnel has therefore been constructed to intercept this and transport the flows to the new STW. This tunnel is 1.4km long, 1.5m internal diameter and flows by gravity. It is formed at an invert depth of 10.6m to 15.7m below existing ground level, entirely within the London Clay.

To construct the tunnel a series of six shafts were sunk. The largest of these is 12.5m diameter and forms the inlet shaft in the new STW. The raw sewage will then be pumped from this to the inlet works, from where it will then flow by gravity through the new STW. The remaining five shafts were used to launch or receive the tunnel boring machine (TBM) and pipe-jack, as well as being used for auxiliary connections to sewers in the existing network which don’t get picked up by the main connection.

The tunnel was constructed using the pipe-jacked method using a tunnel boring machine. This was named ‘Daphne’ in a competition by Bisley Church of England School. Daphne Jackson was the first female physics professor in the UK and taught and researched in Guildford. The tunnel consists of four straight sections, one curved section and one short hand-dug section to connect to the existing network.

Current progress is that all of the shafts have been sunk, the main tunnel is complete and fit out the shafts has commenced.

[caption id="attachment_16139" align="alignnone" width="1200"] Safdar Ali, Andrew Dawes and Izhar Ahmad from the BAM team at the completion of the tunnel - Courtesy of Bam Nuttall Ltd[/caption] Outlet

The outlet for the new STW consists of a 750mm diameter pipe from the TTP and a 1050mm diameter pipe which acts as a storm overflow during extreme weather events. These pipes are laid across a floodplain to the River Wey, a distance of approximately 330m before discharging via a bespoke headwall into the River Wey. This has been designed in conjunction with Environment Agency to minimise erosion in what is classified as a salmon river.

Due to Environment Agency and Local Authority permitting requirements as well as practical requirements of working on a floodplain with underlying running sand, these works are practically restricted to a short duration in the summer. This is being undertaken during Summer 2024 and the works are on target for completion within this period.

Below-ground pipework

The majority of underground assets in the new works are being constructed within the existing landfill. This brings plenty of challenges in both design and construction. Assets must be designed to withstand both differential settlement and potential degradation from existing contaminants in the land fill material.

A variety of ductile iron, carbon steel, and concrete pipes are being used with various protection methods. The work is being self- delivered by the BAM workforce and the installation of below-ground pipework is underway with a focus on where it interfaces with the new structures. This requires extensive temporary works solutions to provide a safe working environment for the workforce within the band of landfill material.

Steel sheet piles are being used to provide temporary ground support and excavated landfill material is being sorted in a material handling area on site. This is being done to reduce the amount of hazardous material being sent off site, halving the amount of excavation and disposal required using traditional open cut methods.

[caption id="attachment_16133" align="alignnone" width="1200"] Threading buried process pipework between new structures requires extensive temporary works - Courtesy of Bam Nuttall Ltd[/caption] Integration of existing assets

As well as the main connection taking flows form the existing works to the new, there are three other existing assets which are to be integrated into the new system, at various points along the length of the new tunnel. This involves extensive research and planning in an open and collaborative approach with TWUL networks teams.

Bowers Farm Main was successfully diverted in November 2023, and has been constructed such that the final connection and commissioning to the new system will be a more straightforward operation. Planning works are currently underway on both Stokes Road syphons and Abbotswood syphons in order to construct similar scenarios for future connection and commissioning.

The main connection to the existing Guildford Main at the head of the new scheme entails connecting into an existing 10m deep concrete and brick built chamber, which carries live flows 24/7. The site team have undertaken several successful drawdowns in conjunction with TWUL to ascertain the dimensions and condition of the chamber in order to plan the final breakthrough works in great detail. This design work is currently underway and the new tunnel hand drive has been stopped a few metres short of the final breakthrough and shored up, prior to the final connection to the existing network.

[caption id="attachment_16164" align="alignnone" width="1200"] (left) Concrete bases for the tertiary treatment plant, (top right) cross-sectional view through the primary settlement tank, showing the piles and gas dispersal layer beneath the base, and (bottom right) precast wall panels are joined with an in situ concrete stitch - Courtesy of Bam Nuttall Ltd[/caption] Future works

Construction works of the ‘Big Build’ are continuing at pace and the BEJV Team are now focusing on ensuring a safe, effective and efficient integration of civils and MEICA elements. This will start in late 2024 and continue throughout 2025.

Following successful completion of commissioning of the new sewage treatment works in 2026, the existing works will be de-commissioned making way for the next stage of the Weyside Urban Village, providing homes and leaving a true legacy for local people and businesses.

The River Lyd Pumping Station is consented to abstract up to 40 Mld from its intake source at Lifton, Devon during November through to March, pumping raw water approximately 7km to Roadford Reservoir. The pumping station was last used on a full operational basis during the mid-1990s and the original equipment had remained in a shutdown and non-operational state up until the point of the upgrade works. The existing electrical switchgear was deemed to be life-expired and beyond the state of any possible refurbishment. The primary driver for the scheme and the scope of works was to upgrade and re-commission the pumping station for operational use at full output design capacity. During the course of the capital works project, a temporary works scheme was implemented in order to maximise operational use of the available pump sets while the capital build was in progress. Background The pumping station is configured with six Pleuger-type QN borehole pumpsets supplied by Flowserve Ltd and which had been previously refurbished by the manufacturer in the intervening period, and the pumps were maintained in storage at Roadford Reservoir. Four pumps are sized at 5 Mld and two at 10 Mld, providing for a combination of flows up to 40 Mld. The discharge from the common pump manifold is to the 900mm diameter rising main to Roadford Reservoir operating with a system pressure at 11 bar. At the works, two surge anticipation valves protect the pipeline in the event of failures and resulting critical surge conditions. Downstream of the common pump discharge manifold, and within the section of pipework that forms the washout connection at the river headwall, are two pressure transducers which have been installed to provide high pressure alarms and initiate pump stop in the event of an adverse operating condition and to afford protection to both pumps and also the rising main. From the common discharge, manifold flows leave the site through the existing 900mm rising main and are measured by a new Siemens electromagnetic flow meter installed as part of the refurbishment works. Eel screens Prior to the upgrade and re-commissioning works of the pumping station, a separate contract was undertaken by Galliford Try in 2021 to replace the river eel screens. The scope of works in this respect was to make the connections between each of the eel screens designed and manufactured by Andritz Ltd and a new air-burst cleaning system. The air-burst compressed air system is a self contained package plant designed and built by Quantum Engineering Developments Ltd with its own local control panel and PLC which control the cyclical cleaning of each of the eel screens. Local alarms are initiated at the LCP and key alarms are relayed back to the main plant MCC as required. Duty/standby screw type air compressors are installed, supplying a single air receiver with operating pressure at 9 bar. There are six air lines that connect with its respective screen and the control of air is via electro-pneumatically operated valves installed at the air compressor skid. Flexible 50Nb air hoses are installed in ducts and connect each piped outlet with a connection to the eel screen. Within the pump chamber a piped manifold of manual isolating valves and air hoses has been installed to provide for the connection to each eel screen. [caption id="" align="alignnone" width="1200"] Airburst package plant - Courtesy of Galliford Try[/caption] HV & LV electrical works The scope of HV electrical works required the re-establishment of the HV supply to the site and these works have been undertaken by National Grid to provide a new 1250 kVA supply to their existing substation on the site and inclusive of new interconnecting HV cabling from their primary substation. A new transformer of required capacity has been designed and manufactured by Wilson Power Solutions Ltd under supply to Enerveo who have completed the site HV installation. The LV side of the transformer is connected to a new form 4A Type 3 MCC which has been designed and manufactured by Galliford Try. The build incorporates the six soft starts for each of the existing 190kW and 110kW pumpsets. The incomer is rated at 2000A and busbar 2500A. Phase failure, surge protection, and transformer protection are incorporated within the build. There are common controls section, distribution board feeders, 110V and 24V supplies. Plant control and surveillance is managed by a Schneider Electric PLC and HMI which are built within the C&I section. Water quality monitoring A containerised suite of raw water quality instruments has been installed in accordance with the requirements set out in South West Water’s Technical Standard for Intake Protection & Raw Water Monitoring. The respective quality parameters measured are colour, turbidity, ph/temperature, dissolved oxygen, and ammonia. The sample is drawn from the River Lyd by a pair of duty/standby Grundfos JP sample pumps and which are connected to the river headwall by means of dedicated ducted 25Nb flexible sample pipes. Actuated GF ball valves within the piped discharge local to the pumps allow for the sequencing of the duty/standby changeover, air scour sequence and back wash of the sample lines during the changeover cycle. Flow monitoring to the instruments is by means of inductive type flow meters. The control, monitoring, alarm initiation and operator interface is via a local control panel complete with HMI and PLC. River Lyd Pumping Station: Supply chain: key participants Principal contractor & designer: Galliford Try Pumpsets: Flowserve Ltd Flow meters: Siemens PLCs & HMIs & switchgear: Schneider Electric Eel screens: Andritz Ltd Air-burst compressed air system: Quantum Engineering Developments Ltd Generators: Aggreko UK HV transformer: Wilson Power Solutions Ltd HV installation: Enerveo JP sample pumps: Grundfos Software development & automation: Galliford Try Rising main: Kier Group Instrumentation: IFM Electronic Instrumentation: Partech Instruments Capital works delivery Before any works could be undertaken, significant tree and overgrown vegetation clearance had to be carried out at the entrance to the pumping station and along the lengths of the access paths and tracks that connect with the top of the site and the existing HV substation. GPR, topographical and ecological surveys were undertaken to establish conditions. There then followed a programme of dismantling works to remove the original GRP kiosk housing the 1990s MCC and transformer. The programme of civil works provided for the construction of reinforced concrete bases for the new 1250kVA transformer and air-burst package plant respectively. Installed on these bases are new GRP enclosures that provide the necessary protection for this equipment. A third GRP kiosk has been installed as the enclosure for the new MCC which utilised the original constructed base slab and trench albeit extended for the larger switchgear build. Elevated concrete piers have been constructed for the container housing the raw water quality instruments and with due consideration to the known risk of flooding at the site. The new enclosures required the submission and approval of a planning application. New ducts and drawpits have been installed to provide for the routing of pipework, HV and LV cables. The scope of mechanical installation works principally involved the re-installation of the refurbished Pleuger pumpsets and the associated pipework connections to the existing delivery manifold and rising main; the installation of the new airburst eel screening compressor skid and its hook up requiring a new air distribution system within the pumping chamber to supply each of the individual eel screens. The scope of the electrical installation works has been principally the uprating of the original 1000kVA supply to 1250kVA. This has required two new HV supply cables to be installed from the primary substation to the River Lyd site. A new HV transformer from Wilson Power Solutions Ltd and switchgear from Schneider  Electric has been installed to make the HV connection. The new Form 4 MCC has been designed and built as the replacement switchgear incorporating the most up to date electronic components and compliance with SWW Technical Standards. The software development and automation works has been carried out by Galliford Try. [caption id="" align="alignnone" width="1200"] (left) River water sampling container on flood prevention supports, (middle) Aggreko UK duty, primary assist and secondary assist generators, and (right) Generator flood prevention as design by the design team - Courtesy of Galliford Try[/caption] Temporary works During the period of the capital works build, and with continued falling level in Roadford Reservoir, Galliford Try was asked to put in place a temporary system of operation for the six installed pump sets using standby generators and temporary switchgear. Working with Kier Group, Galliford Try took charge of the works at the pumping station and Kier Group the rising main to Roadford Reservoir, responsible for checking, filling, flushing and pressure testing the pipeline; mindful that the system was last used in 1996. The original remit was to have the temporary plant installed and connected to allow the existing six pumps to operate by the 1 November 2022. All parties achieved their objectives, however South West Water was unable to obtain approval from the Environment Agency (abstraction permit) in the same timescale. This did however provide more time to allow Galliford Try’s capital works team to incorporate process elements of their capital build into the temporary works (flow meter replacement, air-burst compressed air system, telemetry and online operational water quality instruments). This has provided South West Water and the Environment Agency with a more robust pumping arrangement and water quality monitoring regime. Although the pumps and temporary works operation is scheduled to the end of March 2023, there is the possibility of an extension until the end of May 2023 which is being discussed between South West Water and the Environment Agency. Completion At the time of writing (April 2023) the capital works completion has been achieved insofar as it can be until such time as the temporary works operation has finished and the equipment removed off site. Following on from this, final landscaping will be completed along with those works required to complete all permanent connections to enable the pumping station to come back into operation from the beginning of November 2023.

Northumbrian Water’s Ovingham Raw Water Pumping Station (RWPS) is an abstraction station located on the north bank of the River Tyne in South Northumberland between the towns of Wylam and Ovingham. The station provides raw water from the River Tyne to Horsley Water Treatment Works (WTW) and the Great Southern Reservoir at Whittle Dene WTW. Treated water is then pumped to Birney Hill Reservoir and supplies the City of Newcastle upon Tyne and Tyneside with a population in excess of 800,000. Background There are two delivery mains to Horsley WTW (eastern and western mains), and the pumping station currently contains six pumps (four fixed speed Pleuger pumps and two pumps on variable speed drives). In the recent years these pumps have suffered multiple failures which puts at risk the supply of raw water to the WTW. A feasibility report completed in March 2016 stated that the root cause of historical mechanical failures of the existing pumps is a large volume of silt being carried over into the pump bays when the river is in flood. There are also operational and maintenance issues around the current high voltage inverters and drives. Northumbrian Water Group (NWG) appointed Mott MacDonald Bentley (MMB) to investigate viable options for pump replacement in 2018 and the pump options returned by suppliers were for submersible pumps, submersible borehole pumps, axially split casing pumps, and vertical turbine pumps. MMB and NWG agreed that vertical turbine pumps (VTPs) were the most appropriate pump option for the site and the pumping station was designed to the following duty requirements: Deliver 110.7 MLD (1280 l/s) with a standby pump available. Deliver 154.7 MLD (1791 l/s) with all pumps running, including the standby pumps. Be capable of a minimum station flow of 40 MLD (462 l/s). Be capable of feeding Horsley and Whittle Dene simultaneously. Provide the full range of flows between these figures, whether the river is at LWL, HWL, or anywhere in between, and irrespective of whether delivering up the east main, the west main or both. Be most efficient at 106 MLD (1226 l/s). Following a tender process Bedford Pumps Ltd were chosen to supply the new VTPs. [caption id="" align="alignnone" width="1200"] Ovingham RWPS - Courtesy of MMB[/caption] Scope of the works The replacement of two existing variable speed pumps with a duty of 384 l/s with two Bedford Pumps VTPs c/w 680kW IP 68 motors capable of pumping 452 l/s and minimum solids handling capacity of 25mm to improve the pumping and solids handling capability of the station. The installation of two floor-mounted high efficiency 3.3kV 680kW drives air cooled variable speed drives and a new PLC control system. Associated heating, ventilation and air conditioning (HVAC), steel support beams, platforms and electrical/instrumentation works. Ovingham RWPS: Supply chain - key participants Client: Northumbrian Water Construction delivery partners: MMB Hydraulic modelling: Mott MacDonald Topographic surveys: Atlantic Geomatics Geotechnical & environment Investigation: Dunelm Geotechnical & Environmental Ltd Vertical turbine pumps: Bedford Pumps Ltd Electrical installation: Intelect (UK) Ltd Local control panels: Blackburn Starling Ltd Systems integration: IDEC Technical Services VSD design & installation: Quantum Controls VSD construction: Schneider Electric Metalwork: JHT Fabrications Kiosks: Morgan Marine Surge system integration: Quantum Engineering Developments Ltd HV Cabling: Integrated Utility Services Pump refurbishment: Fluid Sealing & Engineering Ltd [caption id="" align="alignnone" width="1200"] Hydraulic model schematic extract from Ovingham RWPS Refurbishment Technical Note - Courtesy of MMB[/caption] Detailed design and engineering challenges Hydraulic analysis: A hydraulic model of the delivery mains (eastern and western mains) was built in Deltares WANDA v4.5 to calculate the steady state and transient pressures for a number of different operating scenarios. The maximum operation flows are 165 MLD (eastern main) and 180 MLD (western main). The subsequent technical note recommended the following: Ten existing air valves should be refurbished and four new air valves should be installed to the eastern and western main (completed). The 35.98m3 surge vessel on the eastern main should be replaced (future works). A surge vessel should be installed on the western main (completed). Fast acting double plate non-return valves should be installed on the pump outlet pipes (future works). CFD modelling: Computational fluid dynamics (CFD) was carried out to assess the most suitable wet well location to install the VTPs to minimise potential adverse hydraulic effects that could be induced by the joint operation of the VTP and Pleuger pumps. The modelling report recommended that the VTPs should be installed in wet wells No. 3 and 4. [caption id="" align="alignnone" width="1200"] Extract from Ovingham RWPS CFD modelling report - Courtesy of MMB[/caption] Cooling water: The motors installed on the new pumps needed to be IP68 due to the high likelihood that the RWPS would flood up to roof level, therefore fan-cooled motors were not suitable. The motors were supplied with a helical design cooling water jacket which was extensively tested in the motor supplier’s works. A number of options were considered to provide cooling water to the VTPs: Automatic changeover duplex strainers using rising main pressure. Potable water closed loop or ‘once through’ systems. Duty/standby automatic flushing filters. Duty automatic flushing filter with static filter standby. Due to the lack of spare low voltage capacity within the station and the lack of a nearby potable water supply Option 2 was ruled out. Option 3 was discounted due to lack of space but will probably be installed in the future when MMB complete the design of new eel screens at the site. Option 4 was discounted due to the amount of maintenance intervention. Option 1 was therefore designed and installed with automatic switch over between baskets on high pressure drop to enable a basket to be cleaned while the pump is in service. A pressure reducing valve was required to reduce the pressure from 10 bar down to 5 bar and this fitting has experienced pressure drops due to leaf debris passing through the inlet screens and so the layout will be modified. Electrical and instrumentation works: This element of the works included: Design, supply and installation of two off high efficiency, air cooled 3.3kV 680kW variable speed drives. This involved the de-commissioning and removal of the existing drives. Design and implementation of modifications to the existing 3.3kV Switchboard to allow the new VSDs to interface with the control circuit within the switchboard. Upgraded ventilation to the MV switch room to ensure the new VSDs operate at a suitable temperature. Bespoke local instrumentation/monitoring panels for the new pumps. Rather than adopt the standard panel offered by Bedford Pumps, bespoke panels were designed to integrate both the pump instrumentation and external instrumentation. A new PLC and remote I/O panel were installed to accommodate all of the new I/O associated with the systems being installed. Fibre optic cabling was employed to reduce long lengths of traditional cabling being installed. Systems Integration at site level and full SCADA integration at Horsley to provide the client with local and remote operation. [caption id="" align="alignnone" width="1200"] Vertical turbine pumps from Bedford Pumps - Courtesy of MMB[/caption] Heating ventilation air conditioning (HVAC): Heat gain within the building due to pumps, MCCs, screen motors etc. was modelled to ensure that during the summer the temperature within the building did not exceed 40° C when the external temperature was 30°C and during the winter the temperature within the building exceeded 5°C when the external air temperature was -5°C. Side wall mounted LPCB SR4 security rated aluminium louvres with a 30% free area were specified to the east and west of the building along with a 370W fan to extract warm air. Noise assessment: A pre-works noise survey was carried out to give a base line. The subsequent report found that plant noise breaking out of the station was low and so recommended that the design target noise levels for the replacement plant should be to achieve the same output noise level. Achieving these noise levels would ensure the adjacent properties would not be detrimentally affected. Construction Site works commenced March 2021 and ran through until October 2022. The client insisted that as the pumping station was a key operational asset it must stay operational throughout the works and so a big challenge was to complete the works whilst providing critical raw water supplies up to Horsley WTW. The pumping station which is situated in a rural location in Northumberland brought its own logistical challenges. Site staff had to consider that any plant used for lifting operations could be demobilized and moved from site prior to a flood event at short notice. Access to the pumps was via six large openings in the roof and covered by heavy concrete slabs. All lifts were done ‘blind’ and planned to precision to ensure everything ran smoothly with some complex lifts carried out. An adjacent HV building supplied power to the pumping station and required the installation of new VSDs. An environmental survey confirmed the presence of bats and so site staff ensured a qualified ecologist was on hand to make sure the bats and their roosting spots were not disturbed. The works were monitored and carried out during selective working hours. Following the successful completion of the works the ecologist confirmed the bats had not been affected. [caption id="" align="alignnone" width="1200"] Ovingham RWPS east elevation - Courtesy of MMB[/caption] Customer NWG are very keen on community projects and ‘leaving a legacy’. MMB carried out works to refurbish park benches at Wylam Village and to refurbish the sports pavilion at Ovingham as a thank you to the local community for any disruption during the works. Looking ahead MMB were asked to design and construct a new surge vessel for the western main. This work was completed in September 2022 alongside the VTP works. New travelling band screens compliant with The Eels (England and Wales) Regulations 2009 are also required and MMB are currently at the design stage with a view to starting on site May 2023. New non return valves (NRVs) and replacement of the eastern main surge vessel is also expected to start in 2023.

The Southampton Shellfish Scheme comprises the upgrade to four wastewater treatment works and one combined storm overflow (CSO) discharging effluents to the Solent. The Water Industry National Environment Programme (WINEP) No Deterioration Shellfish Drivers (ND_SW) have dual targets: (i) to reduce the number of intermittent storm discharges to 10 significant spills or less per year (as a 10-year average) for the agglomeration of assets impacting Southampton Shellfish Water, and (ii) for the final effluent discharges, achieve a 6.6 log reduction in E. coli between crude sewage and the boundary of Southampton Shellfish Water. Additionally, WINEP U_IMP5 drivers resulting in an increase in flow to full treatment (FFT) were applicable at some of the sites, therefore design and construction for all drivers were combined under the same project. Site Pre-U_IMP5 initial calculated volumes (m3) Post Post U_IMP5 impact to agglomeration (June 2021) (m3) Final Volumes (m3) Millbrook WwTW Yes Yes Yes Slowhill Copse WwTW Yes Yes Yes Woolston WwTW Yes No No * Ashlett Creek WwTW Yes No Yes Ensign Park CSO Yes No No Table 1: Southampton Shellfish Scheme drivers * N/A, but FFT is being increased as a mitigation for ND_SW driver Southampton Shellfish Scheme sites

The technical solutions considered during design included increasing the storm tank capacity and the hydraulic capacity of the wastewater treatment works (WwTW) to reduce the intermittent storm discharges, with UV irradiation or membranes being considered to achieve the disinfection requirements.

[caption id="attachment_14597" align="alignnone" width="1200"] UV irradiation system at Slowhill Copse WwTW (December 2023) - Courtesy of Southern Water[/caption]

Details of the five sites are as follows:

Millbrook WwTW treats for a population equivalent (PE) of 147,000 with biological treatment provided using a 4-stage Bardenpho Activated Sludge Process (ASP). Slowhill Copse WwTW treats for a PE of 77,000 PE and provides biological treatment via a Modified Ludzack-Ettinger (MLE) configured ASP. Woolston WwTW treats for a PE of 70,000 and has an MLE configured membrane bio-reactor (MBR). Ashlett Creek WwTW treats for a PE of 15,000 and provides biological treatment via a sequential batch reactor (SBR). Ensign Park is a combined sewer overflow (CSO). Modelling & optimisation of storm volumes

The project drivers required a reduction of spill frequency at storm overflows for all the catchments that discharge into Southampton Shellfish Water. This impacted five separate wastewater treatment works catchments.

Early in the design, the Southern Water (SW) project team identified an efficiency opportunity of combining the FFT U_IMP5 (increase in flow to full treatment) Driver with the Shellfish Driver, which had a significant impact on reducing the volume of the storm storage tanks. SW held regular meetings and agreed the approach with the Environment Agency (EA) including alignment of Regulatory output dates to the end of the AMP period.

The assessment of storm storage is based on the agglomeration approach outlined in the EA PR19 shellfish guidance and methodology. The definition of a significant spill was agreed with the EA at the conceptual stage of the scheme. This assessment treated all the discharges as a single agglomeration.

[caption id="attachment_14601" align="alignnone" width="1200"] UV irradiation system at Millbrook (May 2023) - Courtesy of Southern Water[/caption]

In summary, the integrated combined driver approach provided the following benefits:

Avoided abortive investment: Storm tanks for ND_SW being oversized (increased FFT from U_IMP5 reducing storm flows). UV irradiation system and associated feed pumping for ND_SW being undersized (increased FFT from U_IMP5). Reduced carbon impact: By avoiding assets no longer required. By avoiding an inefficient 2-stage construction approach. Avoided potential abortive third party land acquisition and development. Reduced construction, traffic movements and the impact to the local community, particularly within the urban areas. Avoided unnecessary impact to the environment and ecology. Avoided potential abortive works for 3rd party infrastructure (for example, power upgrades).

To develop the solutions required at each wastewater treatment works, the sewer network performance for the five catchments was designed using a 10-year rainfall dataset. The observed spill frequencies across the catchment had to be agglomerated to ensure that 10 significant spills or less per year was achieved over a long-term (10 year) average.

This meant the modelling was more complex and took a lot longer than a usual model build and verification exercise because of the need to assess all five catchments together.

Following subject matter guidance from across the industry, the project team used an innovative approach of using an extended rainfall series duration and a single rain gauge. In addition, the team targeted the storage volumes at sites where there was available capacity to arrive at the most efficient storage volumes that were significantly lower than the original estimates, all while complying with good industry practice. In summary, this included:

Extending the rainfall series for a longer period than 10 years to be more representative of long-term weather patterns, addressing extremely ‘wet’ periods within the original 10-year series. Using a single rainfall profile as this coastal rain gauge is more representative of the rainfall falling on the main urbanised sections of the catchments. An extended rainfall series to help address the annual frequency, extremeness and year-on-year variability of rainfall patterns and particularly wet or dry periods. [caption id="attachment_14602" align="alignnone" width="1200"] Storm Shaft in Ashlett Creek (June 2024) - Courtesy of Southern Water[/caption] Optimising storm storage volumes

The project team considered the best location for increasing the capacity of storm storage considering the cost/benefit for reducing significant storm spills. For example, at Millbrook WwTW redundant storm tanks were utilised that provided a volume of 3,140m3. The increase in volume at Millbrook enabled the storage volumes to be optimised across the agglomeration of sites with volumes being offset against other more constrained sites.

Southampton Shellfish Scheme: Supply chain - key participants Client: Southern Water Delivery partners & outline/detailed design: GTB Galliford Try Binnies UV design & coastal & network modelling: Stantec UK Coastal & network modelling: Southern Water Civils & pipework: Knights Brown Civils & pipework: Impact Formwork Storm shafts & tunnelling: HB Tunnelling Piling works (Woolston WwTW): Rock & Alluvium Piling works (Woolston WwTW): Keller Foundations Storm tank remedials: Concrete Repairs Ltd Mechanical installation: Bridges Electrical Engineers Ltd Mechanical installation: Steelway Electrical installation: Field Systems Design Electrical installation: Loxton Electrical ICA: Total Automation & Power (TAP) MCCs: MCS Control Systems GRP kiosks: Industrial GRP Ltd Penstocks: Ham Baker UV: Trojan Technologies Membranes: Koch (KMS) [caption id="attachment_14599" align="alignnone" width="1200"] Refurbishment of redundant storm tanks at Millbrook to provide 3,400m3 capacity - Courtesy of Southern Water[/caption] Infiltration - optioneering To optimise the significant storage volumes at the works, sustainable catchment opportunities were considered for the large, urbanised Millbrook and Woolston catchments by the removal of specific areas of groundwater infiltration and surface water from foul sewers. The optioneering was undertaken using an optimisation software package to target impermeable areas (roads and roofs) connected to the network and working through simulations (up to 20,000) to assess parts of the catchment where connected roads and roofs could be removed from the foul sewer. The benefit was not as expected when considering the agglomerations due to spills from the other assets. In addition, the timescale to achieve long-term reduction from impermeable area removed or groundwater infiltration would require a multiple AMP period. Future opportunities using the optimisation techniques will be considered for similar CSO and storm spill frequency schemes to achieve sustainable catchment opportunities, by the removal of specific areas of groundwater infiltration and surface water from foul sewers. Woolston WwTW - increasing FFT

The initial significant storm volumes at Woolston were extremely challenging given the location of the site within an urban environment and lack of available space. The project team identified an opportunity of increasing the FFT at Woolston, which provided a significant reduction in storage. The scope involved utilising hydraulic headroom within the works, and through the installation of additional MBR capacity and associated pump upgrades, enabled a 20% increase in FFT at Woolston WwTW (427 l/s to 534 l/s).

[caption id="attachment_14603" align="alignnone" width="1200"] Construction of the shaft at Woolston - Courtesy of Southern Water[/caption]

The programme to deliver the increase in FFT at Woolston was short and provided an early reduction in E. coli concentration discharging to the Solent. This early delivery reduced 20% of the total E. coli concentration across the agglomeration of sites and was fundamental in agreeing new regulatory dates for the SW_ND (storm storage) drivers. The combination of the integrated driver approach to reducing spills, the innovative modelling and optimisation of assets reduced storage volumes by circa 46,000m3.

The final volumes are as shown in Table 2:

Site Initial calculated volumes (m3) (pre-U_IMP5) Post Post U_IMP5 impact to agglomeration (June 2021) (m3) Final Volumes (m3) Millbrook WwTW 3,684 200 3,400 Slowhill Copse WwTW 13,678 2,000 2,000 Ashlett Creek WwTW 4,176 1,650 1,650 Woolston WwTW 27,422 13,120 5,000 Ensign Park CSO 3,806 280 280 Blechynden Terrace CSO 5,000 0 * 0 * Newtown CSO 562 0 * 0 * Table 2: Southampton Shellfish Scheme storm storage volumes * Reduction driven by reduced spills in the agglomeration New final effluent disinfection at Slowhill & Millbrook WwTWs (AMP7 & AMP8):

In response to a number of drivers, the EA has recently revised its approach to design and permitting of UV irradiation systems for the disinfection of final effluent. The aim is to move away from the use of generic assumptions, to follow international best practise with an approach which employs the use of a site-specific data in conjunction with performance validated UV irradiation equipment. This revised approach enables development of a significantly better understanding of performance and limitations on a site-specific basis.

Understanding the variation and impact of key wastewater characteristics on parameters such as UV transmittance and the UV dose-response relationship enables selection of a representative design envelope and minimum target UV dose. This enables the capital solution to be optimised, minimises power usage, operational costs and carbon impact while securing environmental performance.

[caption id="attachment_14609" align="alignnone" width="1200"] UV irradiation system at Millbrook (June 2024) post commissioning/FST under construction and (inset) UV irradiation system at Slowhill Copse WwTW - Courtesy of Southern Water[/caption]

The steps undertaken for the development of the design parameters and permit conditions for the UV irradiation systems at Slowhill and Millbrook WwTWs can be summarised as follows:

Coastal/estuary modelling was used to determine the dilution/dispersion factor available from the effluent discharge location to the Shellfish Water boundary. The boundary is due to extend closer to the discharge points in AMP8 (i.e. reducing the available dilution of the final effluent discharges), therefore disinfection targets were evaluated for both AMP7 and AMP8 during design. This enabled the overall system (channels, MCC and power requirements) to be designed with the ability to install the equipment required in AMP7 and accommodate further banks to meet the lower E. coli targets required to avoid abortive and/or extensive additional construction in AMP8. A programme of sampling and analysis was carried out (from April 2020 to May 2021) to collect data on the variation of microbial and sanitary characteristics of the final effluent. The results were compared to the parameters available from historical site data in order to ensure that the sampling was representative of the expected range. Variation in UV transmittance was recorded using online instruments for assessment with flow data to understand their relationship. The UV dose-response relationship for E. coli for the final effluent was developed using collimated beam testwork to determine the target UV dose required to achieve the disinfection targets for permitting for both AMP7 and AMP8. The data was used to develop a range of design parameters that were then used in conjunction with UV suppliers’ performance validation information to determine a range of UV irradiation system configurations. The options were evaluated considering the different levels of resilience, redundancy and risk of non-compliance with the EA’s proposed new permit conditions.

The new UV irradiation systems have been in operation at both sites since Autumn 2023 and are consistently achieving concentrations of E. coli in the final effluent discharges that will maintain the concentration at the Shellfish Water Boundary below the post PR19 target value of 5 cfu/100ml as a geomean.

[caption id="attachment_14600" align="alignnone" width="1200"] (left) Millbrook UV pumping station and (right) MBR cassette capacity being increased in Woolston WwTW - Courtesy of Southern Water[/caption] Woolston WwTW - working with care

Due to the limited availability of operational land in Woolston, the new storm shaft is being constructed beneath a car park and green area own by Southampton City Council (SCC), the area also had a Multi-Use Games Area (MUGA). The project team and the representatives of the Council engaged in extensive discussions prior to submission of the planning application which ensured a positive outcome for both parties.

From those conversations the project team learnt that the Council had worked to create a low nutrient coastal grass environment in the zone affected by the project. The topsoil had been amended and sown with the current species in previous seasons. Therefore, the agreed approach was to retain the topsoil stored on site and reinstate it at the end of the project, enabling the green area to grow and cover the old disused carpark.

The first activity after site mobilisation was relocating the reptiles that naturally inhabit coastal grass biomes. All areas with long grass that were part of the construction compound had the grass cut in two passes to encourage reptiles to move to nearby unaffected zones. Additionally, the top 100mm of soil was hand excavated with an ecologist carefully moving those reptiles still on site.

SW, working closely with the local Council, has ensured that the pre-existing facilities not only are kept despite the new structures being built, but that they are also improved. The number of parking bays and EV charging points will be maintained with improved access arrangements to avoid unwanted camping vans. The MUGA will be improved so the excessive slope that the old court had is removed and the pedestrian access to an adjacent park is protected with bollards, as previously vans used to park using part of the footpath.

[caption id="attachment_14596" align="alignnone" width="1200"] Landscaping over Woolston storm shaft - Courtesy of Southern Water[/caption] Progress to date, successes and future phases

The storm tank at Millbrook WwTW and the increase in flow to treatment at Woolston WwTW were both commissioned in July 2022 while the projects to install UV irradiation at Slowhill and Millbrook WwTWs were commissioned in Autumn 2023. In combination, these improvements represent a reduction of 72,500 cfu/s of E. coli being discharged to the Southampton Shellfish Waters, which is more than 60% of the total E. coli concentration reduction when all the projects are completed.

Results from sampling carried out after commissioning have demonstrated that both UV irradiation systems at Slowhill and Millbrook have reliably achieved the target efficacy, with concentrations of E. coli in the effluent post disinfection regularly below 200 cfu/100ml. Additionally, concentrations of E. coli in the final effluent discharge from Woolston have been consistently below 35 cfu/100ml since July 2019, following commissioning of the MBR.

The next key milestones are 31 December 2024, when the new storm tank at Ashlett Creek WwTW will be commissioned, and 31 March 2025, when all the remaining storage and increase in flow to treatment drivers will be completed.

With approximately 60km of new 800/900mm pipeline required to replace the existing potable water mains, some of which are over 150 years old, the Tees and Central Strategic Pipeline will directly improve water quality, network resilience and operability within the Tees Network. The pipeline will serve approximately 50,000 properties and link a number of strategic assets serving the Barnard Castle, Darlington and Teesside conurbations, providing significant resilience to the overall system by 2028. The scheme will provide strategic connections between Lartington Water Treatment Works (WTW) and the existing service reservoirs (SRs) at Whorley, Shildon, Beaumont Hill and Long Newton. The works are being phased over two AMP cycles with Phase 1 currently under construction and Phase 2 in the definition stage. The scheme forms part of NWG’s wider Tees Network Strategy with further improvements to follow. Project organisation Northumbrian Water Group’s (NWG) framework technical design and design and construction delivery partners, Mott MacDonald, Stantec UK, Mott MacDonald Bentley and Esh-Stantec, are working together as part of a collaborative team called IDeA (Integrated Delivery Alliance) to deliver a major infrastructure project spanning across Darlington and County Durham. A key focus throughout the multi-disciplinary project has been effective collaboration which has been aided by involvement from specialist companies such as Lichfields and Bell Ingram. Figure 1 (below) shows the project organogram for this project. The delivery methodology, design, and construction principles have been developed jointly by all parties in the IDeA team. There have also been opportunities for carbon reduction by using low carbon water and a carefully considered material selection methodology. [caption id="" align="alignnone" width="1200"] Figure 1: Project team organogram - Courtesy of IDeA[/caption] Project scope In addition to the new pipeline, the scheme also involves the decommissioning and abandonment of over 80km of ageing pipeline as well as other assets including pipe bridges and booster pumping stations. To facilitate the abandonment of these assets, existing local networks will ultimately be connected to a combination of new or re-purposed pipelines, ensuring approximately 16,000 properties between central Darlington and Winston village to the west are among those that will benefit from the investment. Carbon reduction Carbon quantification has been an important part of the scheme in order to align with the transition towards a net zero carbon economy, and every attempt has been made to ensure the design solutions are not only hydraulically sound, but that carbon as well as total expenditure is reduced. NWG’s customers in this area are predominantly supplied from two water treatment works, depending upon prevailing supply and demand conditions: Lartington WTW: Source water is delivered under gravity into the water treatment works from upland impounding reservoirs and supplied into the potable water transmission network also via gravity. Broken Scar WTW: Source water is pumped from the River Tees into the works, treated and further pumped into the potable water transmission network. Potable water supplied from Lartington WTW is therefore more energy efficient and the construction of the new pipeline allows NWG to maximise this more cost-efficient water by delivering it into key locations of the potable water supply network in lieu of water from Broken Scar WTW, which may have passed through up to three water pumping stations. Typically, this will mean energy savings on the production and distribution of up to 30 Ml/d of potable water. [caption id="" align="alignnone" width="1200"] Figure 2: Overall pipeline schematic - Courtesy of IDeA[/caption] Design collaboration To deliver technical excellence, the project team has worked closely with NWG’s network operatives, controllers, and analysts to help them enhance their network control and to provide future planning for resilience. Extensive hydraulic modelling and scenario planning has been undertaken to ensure the network is robust, can meet requirements to support other areas of the system to the north and east of the new strategic network, as well as meet future aspirations for enhancements in the downstream network. Peer reviews of all design aspects is a continuous process on this project, utilising technical expertise from all parties. Design reviews are scheduled weekly to engage with NWG’s wider project team as well as internally within the IDeA team, meaning there is early challenge of key design aspects and collaboration to innovate and use best practice on technical issues. IDeA have been appointed as Principal Designer to ensure that CDM 2015 (Construction Design & Management Regulations 2015) is adhered too. IDeA has used the reach-back into their two parent organisations to maximise innovation, technical best practice, and the use of digital tools to assist in optimisation of the alignment, carbon assessment and delivery of the common data environment (CDE). ArcGIS online ArcGIS has been used as a CDE for this project to ensure the project team has a single source of information for making and recording design decisions. This software application was selected due its visual, interactive and geospatial environment. Geo-environmental constraints mapping, aerial photogrammetry, route alignments, utilities, local development information, hazards and design decisions are all recorded within the CDE allowing various data sets to be displayed across several geospatial maps. Alignment adjustments made within the CDE are immediately shared with the rest of the project team ensuring the team has access to information that is up to date, accurate and relevant. This CDE was also used to host the data for tenderers for Phase 1 to ensure they were provided with a common platform of information, see Figure 3 (below). [caption id="" align="alignnone" width="1200"] Figure 3: Phase 1 ArcGIS online tender package - Courtesy of IDeA[/caption] Stakeholder engagement Engaging and communicating with stakeholders at the earliest opportunity has been critical to moving the project forward. IDeA and NWG’s communications team have worked together to ensure a wide range of stakeholders have had early sight of the project including local MPs, affected councils, Environment Agency and local wildlife trusts, with letter drops and planning consultation drop-ins. IDeA has worked alongside Bell Ingram and NWG’s Estates team, to deliver early consultation and land entry notices to over 150 different landowners and their tenants impacted by the pipeline route and the significant associated site investigation works. These have included environmental/ecological surveys, ground investigation, geophysical surveys, and archaeological trial trenching. This collaboration is continuing throughout the concept design and definition stage of Phase 2 of the project. The outputs from the CDE have enhanced stakeholder engagement, allowing stakeholders to understand the scale of the project and the full constraints requiring the chosen alignment. The CDE has also been used to assist with the creation and management of land entry notices across the full scheme. Health, safety and environment There has been an extensive hazard logging process during the project concept and definition phases, using data sources such as NWG’s GIS system and from utility companies. Hazards in the vicinity of proposed works have been logged within the CDE, assigned a risk significance, and then fed into design decisions across the scheme; this process has been made more collaborative and visible by using CDE and easier to pass on into the detailed design and construction phases, see Figure 4 (below). [caption id="" align="alignnone" width="1200"] Figure 4: Hazards recorded and managed within a geospatial CDE - Courtesy of IDeA[/caption] Planning and environmental impact assessment Due to the scale of the works, planning permission is required for both phases of the main pipeline alongside an Environmental Impact Assessment (EIA). This work has required collaboration between the design teams, construction experts, the environmental and geotechnical teams and NWG’s planning consultants, Lichfields. The CDE has been used to collate the information for the EIA and has been instrumental in providing a visual platform to assist the team in defining the red line boundary to meet the requirements for the construction of the works, including working areas, access points, site laydown area and compounds. Planning permission has been granted for Phase 1 and is in process for Phase 2. The planning documentation for Phase 2 is anticipated to be ready for submission in Q1 2024. Design principles and key design aspects To optimise the pipeline route, a number of guiding design principles have been utilised including keeping the route as short as possible, keeping a uniform gradient fall in the vertical alignment, maintaining the pipeline below the hydraulic grade-line and favouring curvature over formed bends to reduce headloss. Geo-environmental impacts have been minimised by avoiding areas of significant ecological or archaeological importance and avoiding high risk ground conditions and potential ground or groundwater contamination. The route alignment has also been developed carefully considering landowner and customer impacts. Early engagement of material suppliers, ground investigation teams, early contractor engagement, and working closely with the environmental team has allowed IDeA to optimise the pipeline design and flag any potential hazards so design adjustments can be made with relative ease. Lessons learnt from Phase 1 are being used for efficiency and best practice in design and specifications for Phase 2. Steel was selected as the pipeline material via material selection matrix with input from suppliers, installers, world renowned technical specialists, NWG operational experience, as well as geotechnical, carbon and civil teams. [caption id="" align="alignnone" width="1200"] Delivery of steel pipes to the site and (inset) pipes arriving in the UK - Courtesy of FT Pipeline Systems[/caption] Tees & Central Strategic Pipeline: Supply chain - key participants Design - pipeline & tunnels: Mott MacDonald Design - pipeline & strategic connections: Stantec UK Planning consultants: Lichfields EIA: Mott MacDonald Land management consultant: Bell Ingram Land drainage design: Land Drainage Consultancy Ltd Design & construction: Farrans Construction Lead designer for Farrans: Atkins Tunnel design & construction subcontractors: Joseph Gallagher Ltd Pipe laying & associated welding: Grant Plant Hire Ltd Slide rail shoring: National Trench Safety UK Steel pipes: FT Pipeline Systems Pipe fittings: George Green (Keighley) Ltd Shaft segments: Macrete (Ireland) Ltd Tunnel precast pipes: FP McCann Strategic crossings and connections Strategic crossings and connections have all been identified, developed, reviewed, and enhanced with input from stakeholders from both NWG and IDeA through formalised design sprints, meetings and workshops. Resilience has been considered at every strategic crossing taking consideration of future maintenance access and redundancy of the system as a whole. Crossings and tunnels There are various major road crossings including the A1(M), three river crossings and two railway crossings. To determine the most suitable method of strategic crossing, all significant environmental, geotechnical and civil constraints have been carefully considered, with options being the subject of stakeholder review meetings. As a result of these discussions, tunnels have been proposed for one major road crossing, one river crossing and both railway crossings. The tunnels are all proposed at 1.8m diameter and will contain twin pipes of 700mm diameter. Lengths range from 60m to 250m and depths from 8m to 50m. The remaining major road crossings will be open cut. The two river crossings are to be traversed by pipe bridges both approximately 25m in length. [caption id="" align="alignnone" width="1200"] Tunnel shaft at the River Tees crossing - Courtesy of Mott MacDonald[/caption] Strategic connections into service reservoirs Five service reservoir strategic connections are required to deliver this project, one at the WTW and the others to SRs, with the end result giving enhanced resilience and operability to the pipeline and network as a whole. Significant consideration and collaboration with NWG stakeholders has been undertaken for each connection to de-risk construction operations and avoid interruption to supply to a significant number of customers. New flow meters, turbidity monitoring and control valves will be installed on these connections to provide further monitoring and control, enabling more flexibility in control of the system and improve water quality and efficient movement of supply throughout the network, maximising operating efficiencies. Reconnecting district metered areas Due to the extensive abandonment of pipework required, 20 district metered areas (DMAs) (approximately 16,000 properties) require re-connection to an alternative water supply. The DMA works use a combination of re-purposing existing mains, laying new mains and abandoning pumped supplies in favour of pressure-managed gravity solutions. DMA enabling works were discussed as part of the early engagement with NWG including flow reversal, flushing and conditioning. A key priority in these discussions has been the commissioning and decommissioning sequence of individual elements of the reconnection works to ensure minimum customer and operational impact. Asset abandonment and decommissioning As for the reconnection works, the abandonment works could have significant customer impact without proper consideration of sequencing and significant collaboration with NWG operational staff. The abandonment works will be sequenced to suit the commissioning of the pipeline in Phase 1, the Phase 2 pipeline and the reconnection works in Darlington and the surrounding area. The abandonment works will require a number of interfaces with the existing network requiring double isolation, thrust restraints, asset removal and trunk main drain-down and de-chlorination. Consideration is also being made to the reduction in resilience in the hydraulic network once abandonment has taken place, with contingencies being developed so that reconnected DMAs can still be reliably supplied. [caption id="" align="alignnone" width="1200"] FP McCann jacking pipes - Courtesy of Mott MacDonald[/caption] Conclusion At the time of writing (July 2023), Phase 1 of the project is under construction by Farrans Construction and the pipeline is expected to be in service by March 2025. The Phase 2 planning submission is anticipated to be made in Q1 2024, with detailed design and construction works starting in 2025. Lessons learnt from Phase 1 during the detailed design and construction phases will be used for efficiency and best practice in Phase 2. The collaborative environment created by the project team and use of ArcGIS as the CDE have enabled this project with multiple stakeholder interfaces to be delivered to efficiently and effectively. Where possible, the IDeA team involved in Phase 1 of the scheme have continued to support NWG in detailed design and construction and continued into development of the Phase 2 concept and design to retain project experience and carry lessons learnt forward throughout the project’s lifespan.