Supply Chain - Site Preparation - Demolition

As part of the AMP7 programme of work, Severn Trent raised schemes at both Lower Gornal and Roundhill STWs, with the aim of meeting reduced phosphorus and ammonia permits. Roundhill STW had tightening consents of 0.25 mg/l (down from 1 mg/l) for phosphorus and 4 mg/l (down from 5 mg/l) for ammonia, whilst Lower Gornal STW’s consents were tightening to 0.3 mg/l phosphorus (from 2 mg/l) and 1.5 mg/l ammonia (from 10 mg/l). The proposed solution from Severn Trent’s outline design was to close the site at Lower Gornal and replace it with a terminal pumping station which would transfer all non-storm flows to Roundhill STW; the full flow to treatment at Lower Gornal is 253 l/s. Early contractor involvement

Mott MacDonald Bentley (MMB), had previously been asked to price the scheme, but an agreement on target cost was not met. Challenges with the variance to the original affordability targets set by Severn Trent were the principal issue, and led to Severn Trent wishing to take a different approach to reduce costs associated with the works.

Severn Trent invited a team of operational, design and commercial staff from MMB to complete an early contractor involvement (ECI) for the scheme with a view to improving on the unfavourable cost position.

After a collaborative challenge between MMB and Severn Trent, MMB was able to review scheme drivers with a view to what could be done to keep Lower Gornal open and avoid a costly and disruptive pipeline installation to Roundhill.

MMB had completed similar schemes to Lower Gornal in the past at Trescott STW and Little Aston STW, so was able to quickly produce a proposal incorporating a new moving bed biofilm reactor (MBBR) and tertiary solids removal (TSR) plant to meet the tightened consents.

Retaining the works at Lower Gornal also mitigated the significant operational expenditure of running a new transfer pipeline and associated pumping equipment, maximising output from the existing assets.

The ECI has been considered very successful by all parties involved. With the removal of the requirement to transfer flows from Lower Gornal to Roundhill and the subsequent reduction in scope requirements for the scheme at Roundhill itself, Severn Trent saved more than £20m from the original cost build-up, bringing the delivery of the two schemes as close as possible to the original affordability target.

[caption id="attachment_15988" align="alignnone" width="1200"] Demolition of biofilter bank - Courtesy of MMB[/caption] Demolition

The new scheme at Lower Gornal required the demolition of an existing bank of four biofilters to allow space for the construction of the new assets. This was the only solution for the new plant due to historic mining activity surrounding the site making any other locations unfeasible. The existing biofilters where demolished by specialist sub-contractor Bagnall Construction Ltd to base slab level. Building the new plant on top of the old bases reduced the risk of differential settlement in a historic mining area.

Lower Gornal STW: Supply chain - key participants Main designer & contractor: Mott MacDonald Bentley Topographical survey: 40Seven Ground investigation: Dunelm Geotechnical & Environmental Ltd Demolition of biofilters: Bagnall Construction Ltd Demolition of PAC dosing: Ron Hull Group Formwork reinforced concrete works: TCL Structures UK Recirculation shaft & augerbore: Active Tunnelling Ltd Mechanical installation & fabrication: Franklyn Yates Engineering Ltd Electrical installation: Installation Maintenance & Controls Ltd (IMAC) Tertiary ammonia removal plant: FLI Water Blowers: Aerzen Machines Tertiary solids removal plant: Eliquo Hydrok Ltd Ferric & alkalinty dosing: Colloide Engineering Systems Motor control center: BGEN Ltd Biofilter drives: Drive Systems Ltd TSR backwash & washwater tanks: Balmoral Tanks Ltd MCC kiosk: Morgan Marine Lifting equipment: T Allen Engineering Services Ltd Access metalwork: Ant Access International Trace heating & lagging: Waste Water Manufacturing Landscaping: Holt’s Tree Care Ltd Surfacing: Tarmac [caption id="attachment_15984" align="alignnone" width="1200"] Construction progress (August 2023) - Courtesy of MMB[/caption] Chemical dosing

The process design incorporated ferric dosing into the primary settlement tanks’ (PST) inlet channel followed by an alkalinity dose. Both the 60m3 alkalinity and ferric rigs were provided by Colloide Engineering Systems. A crude dose is applied prior to entering the PSTs and is mixed by the turbulence of the main channel flow. The ferric sulphate reacts with the phosphate in the effluent to create an iron-phosphate sludge, which is settled out in the PSTs.

A tertiary dose is applied upstream of the TSR plant for the purposes of removing any residual phosphate in the effluent. The iron phosphate sludge from this plant is returned upstream of the PSTs and is co-settled with the crude-dosed flow.

A sodium hydroxide dosing plant has been provided in place of a decommissioned PAC dosing plant. Sodium hydroxide is dosed upstream of the PST distribution chamber (but downstream of the crude ferric dose) to correct the pH of the effluent entering the main treatment works.

By correcting the pH of the effluent through the main treatment works, the nitrification of the biofilters, and the MBBR plant is maintained at an optimal level, as well as ensuring that the tertiary ferric sulphate dose is effective.

Moving bed biofilm reactor (MBBR)

The flow is initially distributed equally over four existing primary settlement tanks before dosing, then travelling through the existing biofilters, onto six existing humus tanks, then through the MBBR and TSR.

[caption id="attachment_15982" align="alignnone" width="1200"] MBBR tanks completed (June 2024) - Courtesy of MMB[/caption] The MBBR plant was provided by FLI Water comprises two MBBR tanks, which operate in a lead/lag configuration. Flow is pumped to the MBBR plant from the tertiary feed pumping station, and passes into either MBBR Tank 1, or MBBR Tank 2; dependant on which tank is selected as the lead. Every day the tanks rotate the lead. This will be inhibited if the flow to the plant is too high (hydraulic safety), or if the ammonia monitored in between the MBBR tanks is too high (to protect the permit, if there is too much ammonia contained within the tank that would become the lag tank).

Each tank is filled with a plastic media which provides a surface for ammonia-reducing bacteria to grow upon. Flow is aerated by duty/standby blowers; which provides mixing for the media and aeration to each tank, thereby removing ammonia from the effluent.

Flow passes through the lead MBBR tank into the lag MBBR tank, before entering the TSR plant provided by Eliquo Hydrok Ltd. The flowrate and the orthophosphate concentration are measured at the inlet.

Tertiary solids removal (TSR)

Ferric sulphate is dosed in a static mixer at the inlet of the TSR plant and dosed flow enters the two flocculation tanks where it is mixed by four mixers. When mixed, the ferric sulphate binds with the phosphate in the flow forming an iron-phosphate sludge suspended in the flow.

This flocculated flow passes to three Mecana cloth filters, which operate in parallel. Flow must pass through the cloths in order to continue forward. The cloths block the passage of the iron phosphate sludge, retaining it within the Mecana tanks. As the cloths become blinded with sludge, the level within the respective tank will rise, and when the level is high, a backwash sequence will be initiated. Sludge retained within the TSR Sludge backwash buffer tank is returned to upstream of the PST’s at a constant rate.

Flow existing the TSR plant flows through a buffer tank, which retains a volume of water to be used as washwater. Excess flow will overflow this tank, and flow to the normal final effluent (FE) route, and then to the watercourse.

[caption id="attachment_15987" align="alignnone" width="1200"] TSR plant (June 2024) - Courtesy of MMB[/caption] Pumping stations

A new enhanced recirculation pumping station has been provided, which replaces the existing recirculation system. The TSR feed pumping station is hydraulically linked to the new enhanced recirculation pumping station, which is hydraulically prioritised.

This was installed using a top-down construction method by Active Tunnelling to a depth of 6.5m below ground level and a 28m augerbore was completed from the new shaft to make the connection into the existing TSR pumping station inlet pipework.

The augerbore method of construction allowed the new pipe to pass under existing live services, this prevented having to excavate around existing services and avoid any works shutdowns.

Site network

Recirculation flow is returned to a point upstream of the PST distribution chamber as the flow is circulated through the PSTs, as well as the biofilters, this dilutes the strength of the ammonia concentration in the PSTs. This more constant flow allows for a more consistent load of ammonia to the MBBR, making the MBBR more resilient to variations in ammonia load, and reducing the magnitude of load spikes to the MBBR Plant.

The site washwater network has been re-fed from a new washwater pumping station and new hosereel points have been provided to the new tertiary treatment plant. This effluent is taken from the outlet of the TSR Plant, entering a buffer tank which provides washwater at all times, allowing for a cleaner, and more reliable washwater network on the site.

[caption id="attachment_15986" align="alignnone" width="1200"] Recirculation pumping station (June 2024) - Courtesy of MMB[/caption] Collaborative working

Weekly on-site collaborative planning sessions with the design and site team, including subcontractors and the client has driven pace in the design and construction programme. This has ensured client buy in throughout the programme eliminating any blockers to progress. All works have been completed without any interruption to the day-to-day running of the plant and the existing permit has never been put at risk. The pace of delivery has meant the scheme has been completed early.

A procurement-led programme and effective liaison with the supply chain meant that all long-lead items for the scheme were delivered ahead of schedule. MMB took a novel approach to the procurement of control panels, working closely with BGEN Ltd to negotiate information provision cut off points in the contract. This alleviated all the issues that COVID caused with the procurement of control panel chips and ensured timely delivery.

MMB employed the carbon hierarchy approach throughout the scheme. This culminated in the reuse of an existing civil bund for the new alkalinity rig. The most noteworthy carbon saving was delivered by reusing 2000m3 of biofilter media, as engineered fill during the demolition works. This was graded and reused to provide the foundations for the new structures using the CLAIR:E process for sustainable land reuse.

[caption id="attachment_15985" align="alignnone" width="1200"] Completed plant (June 2024) - Courtesy of MMB[/caption] Project status

At the time of writing, September 2024, Lower Gornal is undergoing 28-day process performance testing and is currently comfortably meeting the contractual permit. The scheme is due for completion three months ahead of schedule and below the agreed target price. It has demonstrated what can be achieved with true collaboration between client and contractor. The scheme demonstrates MMB’s expertise in design and construction of MBBR and TSR plants.

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.

Llyn Bran is located in Denbighshire, north-east Wales and was a natural lake that was raised in 1899 by a concrete dam. The reservoir was used to supply water to the North Wales Hospital in Denbigh. The hospital closed in 1995 and Llyn Bran has not been used for drinking water since. As part of a separate discontinuance scheme at Llyn Anafon, Llyn Bran was identified as a ‘compensatory site’ for the loss of habitat at Anafon. One element of turning Bran into a compensation site was the removal of the dam, as well as other ecological enhancements. Through careful restoration work, Llyn Bran will eventually become a Special Area of Conservation. The reason for Llyn Bran discontinuance Llyn Bran Reservoir is required as a ‘compensatory site’ for the loss of habitat area that will be caused by discontinuing the dam at Llyn Anafon Reservoir, as required by the Habitats Directive. Llyn Anafon Reservoir was originally a natural lake which was raised by 1.4m in 1931 through the construction of an embankment dam. The use of this site for local water supply ended in 2002. A 2016 S10 inspection of Llyn Anafon resulted in a MITIOS (Measures in the Interests of Safety) to “develop a long-term solution to the poor and deteriorating condition of the dam”. In January 2019, it was agreed with relevant regulators that to provide a long-term solution, the reservoir would need to be decommissioned, with the dam structure removed and the lake lowered by 1.4m. [caption id="" align="alignnone" width="1200"] Llyn Bran before discontinuance - Courtesy of Welsh Water[/caption] Llyn Anafon is nationally and internationally protected for specific conservation features. This means prior to any project taking place, a full Habitats Regulation Assessment (HRA) had to be undertaken. The outcome of this assessment was that it was not possible to avoid adverse impacts to protected features and as such, the only route to proceed with the project was under the “Imperative Reasons of Overriding Public Interest (IROPI)” (Article 6.4 of the Habitats Directive) on the grounds of public safety. The Anafon Dam Discontinuance Scheme is the first IROPI case of its kind in the UK and has recently been approved by Welsh Government. To satisfy the IROPI case, Welsh Water was required to secure ‘compensatory measures’ for the loss of habitat at Anafon. Using a set of criteria agreed with National Resources Wales (NRW), Llyn Bran was identified as the most suitable site to provide the necessary compensation. A key element of the work was to ‘restore and maintain hydrological functioning’ i.e., removal of the dam to create a natural lake and restore connectivity to the downstream watercourse. Although the primary reason for discontinuing Llyn Bran was for compensation, it also had the added benefit of removing a dam which was no longer needed for operational purposes. [caption id="" align="alignnone" width="1200"] Llyn Bran dam before discontinuance - Courtesy of Welsh Water[/caption] Scope of works Create temporary construction access using bog mats. Draw down the reservoir by means of siphon pipes and pumps. Remove silt from the area immediately upstream of the dam. Demolish the entire dam and remove arisings for disposal off-site. Restore the watercourse channel downstream of the retained natural lake, incorporating silt management and retention measures. Create new water vole habitat along the shore areas of the retained natural lake using coir matting. Demolish outbuildings and associated structures with all building waste removed for disposal off-site. Remove exposed sections of pipework. Buried pipework to be sealed and filled by grouting. Install new roadside fencing along the north end of the reservoir. Install rip-rap protection around the upstream end of the B4501 culvert. [caption id="" align="alignnone" width="1200"] Ground conditions - Courtesy of Welsh Water[/caption] As with all discontinuance schemes the various surveys, permits, consents and negotiations in advance of the physical works were particularly complex, taking more than two years to complete. An overview of the consents and constraints required is below: Impoundment Licence and SSSI consent from Natural Resources Wales (NRW). Permitted Development Certificate from Denbighshire CC. Ordinary Watercourse Consent from Denbighshire CC. Protected species licence required for trapping and temporarily removing water voles (this was needed before permitted development could be granted). Annex 1 Habitats present around lake margins. Reservoir drawdown rate limited to 120 l/s. HRA level 1 & 2 required. Peat bog deposits along route of temporary access track. Bat surveys required prior to demolition of valve house & boat house. Challenges & constraints Site access With no formal access track to the dam, and the surrounding land being designated as Annex 1 Habitat due to the presence of peat, access was one of the main challenges. With 140 peat depths probes taken along the route of the proposed access track, and working closely with NRW, a methodology was developed for installing over 280m of bogmats to the dam without the need to excavate and remove the top layer of peat. This minimised the potential damage to the peat and made site reinstatement far easier. [caption id="" align="alignnone" width="1200"] (left) Bog mat access track and (right) amphibious excavator used to load barge with coir matting - Courtesy of Welsh Water[/caption] Peat The extent of peat encountered on site was far greater than expected once construction began. This resulted in an entire change in the contractors methodology for the coir matting. It was initially planned to use Bog Master excavator and tracked dumper to access the western shore of the reservoir, however it quickly became clear this was not practical. Thanks to the contractor being proactive, the methodology was quickly changed to using an amphibious excavator and barge, to transport the coir matting across the reservoir. Water voles Water voles were identified on site during the early site survey stages, meaning a licence was required to temporarily remove the voles during construction. Six water voles were captured by specialists and were transferred to a facility in Kent, where they were looked after while the work took place. A condition of the licence was that there must be 5-days with no water voles captured before it could be concluded that all voles were removed from site. Rather unluckily, there were two occasions where a vole was captured at the end of the fifth day, meaning the process took far longer than expected and the licence had to be extended. The voles are due to return home to Llyn Bran this summer. [caption id="" align="alignnone" width="1200"] Water voles were captured and transferred to a facility in Kent for the duration of the works - Courtesy of Welsh Water[/caption] Compensation flow A condition of the NRW impoundment licence was for a continuous compensation flow of at least 20 l/s for the duration of the works. This meant pumping to the watercourse, which had to be continually monitored. It was hoped to use a cloud-based telematics tool that makes it easy to monitor the pumps off-site but unfortunately, due to lack of signal, this did not work meaning William Hughes Civil Engineering needed to attend site over the weekends to check on the compensation flows. The summer of 2022 proved to be exceptionally dry, which crated a challenge in meeting compensation flows whist also not allowing the remaining lake to become too low. Flood risk The existing reservoir provided significant flood attenuation meaning the discontinuance increased the flood risk to a downstream highway culvert. Initially, there were concerns from the highways department that the culvert would need to be upsized, which would have added significant costs to the project. Following a detailed flood risk and stability assessment, Welsh Water were able to prove that the increased flood risk would not adversely affect the culvert and only minimal slope protection work around the culvert was required. [caption id="" align="alignnone" width="1200"] Amphibious excavator forming the new channel connecting to the downstream watercourse - Courtesy of Welsh Water[/caption] Llyn Bran Reservoir Discontinuance Project Construction timeline: 16 May 2022 - 4 November 2022 Project cost: £1.7m Drivers: Dam safety | Habitats Directive | Reducing costs | Ecological compensation measures Supply chain - key participants Principal Contractor: William Hughes Civil Engineering Design Partner: Stillwater Associates Environmental Consultants: Ricardo Energy & Environment Ecology/water vole licensing: Wildwood Ecology Ltd Silt management: Salix River and Wetland Services Amphibious excavator hire: Land and Water Plant Opportunities & Savings Forestry culvert At the design stages there were concerns about the condition of a nearby downstream forestry culvert and its ability to accommodate the reservoir outflows during emptying. Expertise within Welsh Water was utilised by requesting a CCTV survey of the culvert via the wastewater team. The team provided a prompt and efficient service and enabled us to confirm the capacity and condition of the culvert. [caption id="" align="alignnone" width="1200"] Cores being drilled through concrete dam to allow any extreme storm flows to pass through - Courtesy of Welsh Water[/caption] Dam safety mitigation During detailed design, the Dam Safety Team suggested coring holes though the concrete dam once the reservoir was drawn down to the required level. This would mitigate the risk of the reservoir refilling during severe weather, before the dam was fully demolished. Fortunately, on this occasion the weather was favourable and the cores were not needed. This approach will be considered for any future concrete dam discontinuances. Waste removal It was agreed with NRW that the stone from the haul road and compound could be transferred to a nearby quarry, just down the road from site. This was done under a waste transfer notice, saving costs. The arisings from the dam were removed off site. The dam was demolished far easier and quicker than expected, making the removal of concrete straightforward. [caption id="" align="alignnone" width="1200"] Dam demolition in progress - Courtesy of Welsh Water[/caption] Coir matting Due to the silt being drier than anticipated, there was spare rolls of coir matting left over. Rather than this being wasted, it was moved to nearby Alwen WTW where it is being stored before being used for silt management on the upcoming Cilcain Discontinuance Scheme which is due to start this spring. The future for Llyn Bran The discontinuance will give significant biodiversity opportunities in relation to the marginal areas around the retained natural lake and with the restored connectivity to the watercourse. The retained lake will also be used as a receptor site for aquatic plants translocated from Llyn Anafon. Long term ecological monitoring of the lake is required to ensure the compensatory criteria are met. This includes water quality and macrophyte monitoring as well as botanical surveys to check the natural regeneration of exposed shore habitats. A condition of the water vole licence is that activity is monitored until 2034. Through careful management and monitoring of the site, it is hoped that Llyn Bran will become recognised as a Special Area of Conservation.

Bourn Reservoir is an above-ground reservoir complex located approximately five miles outside Cambridge, England. Only one tank was still operational after the other two were removed from service due to natural degradation. A new tank was required to meet demand from large-scale developments in the supply area. A review was conducted to ascertain the total water volume required, revealing the need for an additional 8 ML of storage on top of the capacity of the remaining tank by the summer of 2022. Stonbury was contracted to demolish the disused circular prestressed tank in 2020, whilst the other remained in service, and in 2021, Stonbury commenced the construction of a new concrete tank next to the operational reservoir. Scope of works A summary of the works undertaken is as follows: Disconnection and demolition of the old Reservoir No 2. Ground investigation works and tree removal. Groundworks. Design of new concrete tank. Construction of new tank, drainage and connecting pipework. Installation of MEICA elements. Site description The Bourn Reservoir site is a small, spatially-constrained compound that originally contained Reservoirs 2 and 3. Land owned by the farmer adjacent to the site was utilised for the site compound which housed welfare units, a storage area, and turning space for vehicles and machinery. The land was returned after the programme was complete. [caption id="" align="alignnone" width="1200"] View of tank under construction with newly painted exterior walls - Courtesy of Stonbury[/caption] Project team The project was delivered for the client, Cambridge Water Company, by direct-delivery water sector specialist contractor, Stonbury, as principal contractor and principal designer, and Mason Clark Associates as their designer. Bourn Reservoir: Supply chain - key participants Client: Cambridge Water Company Principal contractor: Stonbury Designer: Mason Clark Associates Subcontractor: GM UK Ltd MEICA installation: IWS Mechanical & Electrical Ecological appraisal: Middlemarch Environmental Ltd Concrete delivery: Hanson UK Concrete pump hire: Camfaud Concrete Pumps Ltd Crane hire: Emerson Cranes Handrail & staircase supplier: L&G Engineering Ltd Internal steps & ladders supplier: Chemglass Access doors & hatches: Technocover Ltd Project challenges The build possessed several unique challenges due to constraints on time and space. The planning application review period was extended for further environmental assessment as part of the Tree Retention Scheme, which introduced significant time constraints to the programme. Proactive action was taken to minimise delay, and a revised foundation plan was submitted accounting for the proximity of tree roots and including provision of a perimeter root barrier trench for additional protection. Planning permission was granted in October 2021, leaving under a year to complete the project by summer 2022. Innovative pre-construction planning was required to overcome the issues posed by the constrained size of the operational area. As the site footprint dictated a tank shape that was non-optimal for water flows, a computational fluid dynamics (CFD) analysis was completed to identify a ‘best practice’ design to reduce dead spots and protect water quality. As the new tank footprint was partially on virgin ground and partially on ground where the previous structure had stood, there was a risk of differential settlement. To mitigate this risk, a tailored ground investigation was conducted to obtain data for a predictive settlement analysis to inform the engineered fill design and ensure a robust outcome. [caption id="" align="alignnone" width="1200"] Tank under construction, showing precast concrete slabs being maneuvered into place by a crane - Courtesy of Stonbury[/caption] Design Mason Clark Associates was responsible for the civil and structural design of: The reinforced concrete twin-compartment reservoir, excluding the detailed design of precast roof slab panels. The reinforced concrete valve house. The surface, underdrainage, and foul water drainage systems. The external steel roof access staircase. The client stipulated that each cell would require the following features: Opposed high-level inlet and a low-level outlet, connected to the existing network. Automated control valves. Combined metered washout/overflow pipe. Overflows with sufficient capacity to take design inflow whilst maintaining sufficient freeboard to the soffit of the roof. Washouts, sized suitably to enable the drain down of each cell over a 24 to 48 hour period without causing flooding to surrounding watercourses. Sufficient ventilation to ensure that each cell is not over-pressurised during filling or a vacuum created during drawdown. Access doors and hatches to meet LCPB 4 rating and SEMD requirements. All necessary ductwork for telemetry and security cabling. Mains alterations to accommodate the new reservoir. Arrangements for taking compliance samples. The proposed tank was to have an approximate footprint of 850m2 and operate with a top water level of 79.2mAOD (ground level is approximately 71.4mAOD). The inlet was to be connected to the pumping main, whilst the outlet connected to the same main as Bourn Reservoir 3 so that they would operate hydraulically together. The placement of inlet and outlet valves, baffle walls and weir boxes were designed in accordance with results from the CFD analysis. [caption id="" align="alignnone" width="1200"] Demolition of the disused circular tank prior to construction - Courtesy of Stonbury[/caption] Addressing the drive to build assets with lower embodied carbon and to reduce the length of the programme, the design incorporated the use of precast concrete roof slabs in addition to conventional in-situ pour concrete. This was preferred to a fully precast solution due to the risk of perennial ingress issues at precast joints. This would help to reduce embodied carbon by approximately 10% compared with a traditional build and make the 48-week programme possible. The structural elements of the reservoir and pipework were to have a minimum design life of 50+ years, and 20+ years for all other elements. All structural and geotechnical design was in accordance with the relevant section of the Eurocodes, and all concrete elements were designed with a water-tightness classification of 1. All work was designed to be carried out in accordance with the latest CESWI, WIMES and Cambridge Water Company general engineering specifications and to meet DWI requirements for potable water storage units and the current SEMD guidance. Environmental management A Project Site Environmental Assessment sought to register and mitigate site-specific environmental impacts such as noise, land and watercourse pollution, waste materials and damage to local ecology. Identified environmental risks were controlled through measures detailed in Risk Assessments and Method Statements. A preliminary ecological appraisal was undertaken during the pre-construction phase to determine the presence of dedicated nature conservation sites and protected species in proximity to the site. The site was not within or near an SSSI. No notable plant species were identified but some notable animal species were. Recommendations were provided based on the ecological mitigation hierarchy to avoid, mitigate, and compensate during and after the programme. [caption id="" align="alignnone" width="1200"] Construction workers building reinforcement cage for walls - Courtesy of Stonbury[/caption] Health & Safety A detailed H&S file was prepared for the client. During construction, special site-specific health and safety considerations were planned, managed and monitored using a variety of means, including: Site-suitable PPE. Approved RAMS. Vibration, noise and CoSHH assessments. Daily Point of Work Risk Assessments (PoWRAs). Suitable barriers and safety signage. Obstructions or trip hazards protocol. Agreed working hours and strict adherence to quiet periods. Asbestos protocol. Demolition In 2020, existing services into Reservoir 2 were disconnected and removed in preparation for tank demolition. Due to the spatial constraints of the site, demolition was carefully planned with an appropriate methodology to mitigate risk to the live operational tank. An excavator was employed to demolish the roof, with an attached claw to cut the concrete walls into small vertical sections. All broken material was pushed into the middle of the tank during demolition to protect the adjacent asset. Once demolished, material was removed from site. [caption id="" align="alignnone" width="1200"] The site for the new reservoir after demolition of the previous tank - Courtesy of Stonbury[/caption] Site preparation A site compound was constructed for offices and welfare facilities. This area also had space for limited parking, temporary materials storage and a turning area for construction vehicles. Heras fencing was erected on the boundary of the compound area and construction warning notices were posted at the site entrance gates. To prepare the area for the reservoir, the locations of services were pre-determined using a combination of existing service drawings, site knowledge and CAT scanning using a signal generator followed by hand-excavated trial holes. Base construction The site for the new tank was excavated, removing spoil and saving a small stockpile for backfilling. Old balancing pipework was removed, and the existing pipes were capped so that they could be reconnected upon project completion. Underdrains were installed. Fill-to-base was completed in accordance with the design informed by the predictive settlement analysis. The area was levelled, filled with type 1 material to the specified formation level and rolled. A 50mm concrete blinding base was poured from a dumper and levelled by hand. Concrete square bars were laid by hand to support the reinforcement steel placed directly on the blinding slab and tied, ensuring correct laps in accordance with the design. Timber formwork was levelled using a laser level placed on each end. Once this was braced, the floor was completed in four separate concrete pours. The sump was poured at the same time using a concrete pump and vibrated constantly as the concrete entered the reinforcement. [caption id="" align="alignnone" width="1200"] Formwork panels - (left) outside view during construction of the upper wall, and (right) inside view during construction of walls and columns – Courtesy of Stonbury[/caption] Wall construction A panel system with film-faced plywood was built for the wall and column formwork according to drawings. Completed shutters were lifted into place using an excavator for the 3m high lower walls and a tower crane for the 7.8m high upper walls. Reinforcement cages were fabricated on the ground and lifted into position with a tower crane to minimise working at height. Inner shutters were held in place with push-pull props drilled and fixed to the base slab. Outer shutters were fixed using Dywidag bolts and tie bars on the outer edge. Timber stop ends were fixed to the ends of the shutters. The concrete was poured in sections and vibrated as filling proceeded, employing both external formwork vibrators and 10m long internal vibrators to achieve the required concrete finish. Shutters were lifted, cleaned and re-positioned for each 5m wide pour section. Once all sections were full, laser levels were used to check levels and tops were floated off. Pipework At this stage, trenches were excavated for overflow and outlet pipework. Outlet pipework was installed first and connected to the existing valves. A flow meter was installed in line and a chamber was built around it. Pipes were capped prior to installation, chlorinated and bagged off to prevent contamination. Once installed, pipes were pressure-tested and the trenches were filled with stone and backfilled in layers. The flow meter chamber was excavated using a 5-tonne excavator and concreted with formwork and vibration. A Technocover lid was drilled and fixed to the cover slab and a galvanised ladder was affixed to gain access to the flow meter. Ducting was installed for the control panels and mains power. Once the main walls were complete, internal access and pipework were installed. [caption id="" align="alignnone" width="1200"] Construction workers guiding precast concrete roof slabs into place - Courtesy of Stonbury[/caption] Roof construction To construct the roof, 30 steel beams were lifted onto the reinforcement cage using a tower crane with two operatives in scissor lifts to guide them. The tower crane was then used to manoeuver 86 precast concrete panels (150mm) to create the roof slab. The panels were attached to the crane with a four-legged chain and lifted into position, guided by operatives in scissor lifts. Once all the panels had been placed on the roof, the joints were in-filled with concrete ready to take a 150mm structural concrete screed and Lytag levelling screed, laid with a fall of 1:80 front to back to allow water to run off from the roof. The outer walls were washed, spray-primed and painted with anti-carbonation green camouflage paint to protect the concrete from weathering and reduce visual impact from the road. A waterproof liquid seamless coating system was applied to the tank roof with anti-slip walkways. Finally, a permanent exterior access staircase and perimeter handrailing were installed. Leak tests The reservoir was filled with water and underwent a seven-day drop test to ensure there were no leaks. Once successfully passed, the tank was drained for a final inspection, clean and chlorination. Outside, attenuation drainage was installed and security fencing was erected. MEICA elements were installed by specialist contractor IWS Mechanical & Electrical. Their scope included a full ICA upgrade of the site, reservoir level monitoring, electronic security and electrical distribution for the site. [caption id="" align="alignnone" width="1200"] (left) Construction worker using the newly installed exterior staircase, and (right) the completed tank undergoing an integrity inspection - Courtesy of Stonbury[/caption] Post-construction Earlier CFD modelling and ground investigations had indicated the requirement to fill both cells immediately upon commissioning and keep them simultaneously in use for the first six months of operation to ensure ground was consolidated following the build. As a consequence, the inspection of the internal dividing wall could not be completed during the asset handover. To provide assurance to the client, Stonbury arranged to undertake a further inspection after six months, which confirmed integrity. Conclusion The project was completed according to programme without delays or health and safety incidents. The new tank was connected to the network in October 2022 and will increase water security within Cambridgeshire for decades to come.