Supply Chain - Mapping & Modelling - Coastal & Estuary Modelling

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.

The Ards North Long Sea Outfall, constructed at Ballyferris on the east coast of the Ards Peninsula, has an asset design life of 60 years and can convey flows of up to 4200 m³/day of treated effluent from the Ards North WwTW to the pre-determined deep water discharge point in the Irish Sea. Legislative compliance was demonstrated by computational and hydrodynamic effluent dispersion modelling and the termination point for the outfall was at a pre-agreed location 500m offshore. The treated effluent is dispersed and diluted through a multiple port diffuser in 4m depth of seawater.

Ards North WwTW

The Ards North WwTW catchment is seasonal with significant summer peaks in flow generated by holiday parks. In addition, the initial contract documents required a long sea outfall to be designed for storm tank overflows. This resulted in a significant range of design flows and challenges to achieve an asset standard compliant hydraulic design. Flows ranged from:

1. Minimum flow rate (Winter DWF): 13 l/s
2. FFT Ards North WwTW: 48.22 l/s
3. Maximum flow rate (Formula A at WwTW): 74 l/s

Given the range of flows, the exemplar design included a pressurised outfall fed from a header tank and final effluent pumping station. [caption id="" align="alignnone" width="1200"] Tow and sink operation - Courtesy of NI Water[/caption]

Exemplar design

Farrans Construction along with RPS Consulting Engineers were appointed to develop the exemplar design for construction. The completed design evolved during the design period with initial designs providing capacity for the full range of flows. On award, RPS completed a pipeline route assessment using Civil 3D ground models to review the option of providing a gravity solution capable of delivering the full range of design flows through the outfall to the diffuser against 1:100-year storm conditions with an appropriate allowance for sea level rise to 2030. RPS Water Environment & Marine completed an assessment of the tidal range and sea level rise to develop a maximum tidal discharge level. Admiralty Tide Tables for the secondary ports of Donaghadee and Portavogie were used, and linear interpolation of the Highest Astronomical Tide (HAT) and Mean Low Water Spring  (MLWS) levels provided tidal levels. Sea level rise due to climate change was assessed using derived data through UKCIP18, providing a tidal range of +3.64 to -1.91m AOD Belfast which was adopted for the design process.

Hydraulic analysis

Ground modelling and initial hydraulic analysis confirmed that any gravity solution would require significant depths of excavation on the land-based section, and it was concluded that a pressurised system using a header tank was the preferred solution reducing the depth of dig, associated costs, health and safety risks during construction, and whole life operation and maintenance of the asset. Potential pipeline routes were identified and appraised to establish the best option considering lands access, road opening, future maintenance to fittings and furniture, ground conditions and depth of dig. The preferred option was a route across agricultural land, local roads before passing through a caravan holiday park to the high-water mark where the land and marine-based sections of the outfall would join. [caption id="" align="alignnone" width="1200"] Aligning pipeline over pre-dredged trench - Courtesy of NI Water[/caption] A range of pipeline diameters and materials were considered to achieve optimum velocities, a balance between pipe diameter and TWL in the proposed header tank. PE pipes were selected for the outfall with varying SR ratings used to offer material cost savings using only higher rated pipes for the marine sections to take account of installation (towing and sinking) stresses on the system. Specialist hydraulic modelling was then completed by Hydraulic Analysis Limited (HAL) to provide confidence in the performance of the system against all combinations of design flows and tidal levels. The analysis confirmed the pipe diameter selection, siting of air and scour valves and provided specific specifications for pipeline gradients, bends and air valves installation to ensure stable free surface flow conditions were achieved within the mains gravity driven sections.

Renegotiated WOC

During the detailed design stage, a renegotiation of the Water Order Consent (WOC) allowed for discharge of storm flows from the Ards North WwTW direct to an adjacent watercourse. This significantly reduced the maximum design flow for the long sea outfall and on reassessment allowed the removal of the effluent pumping station and header tank offering significant operational savings for NI Water. [caption id="" align="alignnone" width="1200"] Barge mobilisation from Bangor Marina - Courtesy of Farrans[/caption] The final solution was therefore to discharge treated effluent directly from the sample chamber to a gravity outfall which acts in part as a pressurised system. This amendment to the design approach was verified by HAL and removal of the pumping station and header tank was agreed by all prior to finalising the construction details including:

• Design of pipeline bedding and surround for both land and marine-based works.
• Land-based pipelines were designed as traditional open-cut installations with bedding and surround specified to meet the requirements of BS EN 1295-1:2019 Structural Design of Buried Pipelines.
• Air, scour and hatchbox chamber details for the land-based section. Given the variation in summer and winter dry weather flows, the inclusion of hatch boxes was provided for maintenance and to aid cleaning of the outfall.
• Marine section bedding and surround. The marine section was considered as two sections namely:
  • Inter-tidal zone where the pipeline was bedded in suitable granular bedding and surround and installed with collars and kennels for flotation resistance and stability. The pipeline was protected with 2 layers of rock armour.
  • Offshore zone where the pipeline was installed in a deeper pre-dredged trench with an imported granular bed and surround and restrained against flotation using kennels at 3m centres. This section of the pipeline was backfilled with as-dug excavated rock.
• The outfall was completed with a collar and kennel arrangement and a 4-port crosshead diffuser, fabricated from 400mm OD PE pipework and fitted with four 150mm diameter Tideflex non-return valves. The diffuser arrangement is protected by a galvanised steel frame with sacrificial anodes.

Design for construction complied with the requirements of the NI Water Asset Standards and Specifications, CESWI, Relevant CIRIA Guidance and other Industry accepted standards. [caption id="" align="alignnone" width="1200"] The outfall was installed off the coast at Ballyferris adjacent to a caravan park - Courtesy of NI Water[/caption]

Undertakings

Farrans Construction was appointed as principal designer and contractor for the works, including an early contractor involvement stage consisting of scope and design development, stakeholder engagements, statutory approvals and ground investigations. Farrans appointed Marine Specialists Ltd, a specialist subcontractor, to assist in completing these works.

Ards North LSO: Supply chain - key participants

• Client: NI Water
• Principal designer/contractor: Farrans Construction
• Project management/technical support: Tetra Tech
• Exemplar design: RPS Consulting Engineers
• Hydraulic modelling: Hydraulic Analysis Ltd
• Specialist marine subcontractor: Marine Specialists Ltd
• PE pipe & fittings: Associated Pipeline Products
• Precast concrete collars: Ardmore Precast

[caption id="" align="alignnone" width="1200"] (left) Onshore pipeline assembly - Courtesy of Farrans, and (right) installation of concrete kennels - Courtesy of NI Water[/caption]

Installation

The primary method of installation was by open-cut trench and rock breaking technique with contingency arrangements in place for encountering ‘unbreakable’ materials, consisting of a drill, blast, dig methodology. The works were completed in the following phases:

1. Phase 1: Works comprised shore-based operations to prove the trench to depth along the intertidal zone, consisting of rock breaking, excavation and temporary backfilling of the trench. This phase was designed to mitigate the risk of delay due to adverse weather conditions and capitalised on large spring tides by utilising land-based equipment during the winter months. It would also minimise the disruption to the public by reducing the duration of pipeline installation and final connection on the beach.
2. Phase 2: This involved the marine-based operations, rock breaking and excavation of the trench proven to depth with the use of a barge-based 60t 360° track excavator with a 14m reach, Topcon 3D GPS system, 4t breaker and ripper tooth. The excavator operated from a pontoon barge 26.8m x 12.2m with a deck capacity of 360t.
3. Phase 3:  This phase included a combination of both shore and marine-based operations. The trenching extended seaward in accordance with the limiting conditions for weather, visibility, sea state, wind and gusts. The position, alignment and depth of the trench were confirmed by the 3D GPS systems and regular bathymetric surveys.

The pipeline

The pipe lengths (supplied by Associated Pipeline Products) were butt fusion welded, prefabricated to a string length of 500m, fitted with concrete collars (ballast weights) and tested. At the ends of the pipe string, a pupped stub was welded complete with stainless steel backing rings. A blank flange was bolted to the stub/backing ring to allow for the entrapment of air within the pipeline allowing the pipeline to float when placed in the water. Shore-based excavation work was completed at low tide to re-excavate the trench along the intertidal zone previously proven in phase 1 of the works in preparation for pipe tow-and-sink operation. Once the trench was proven to depth and prepared for pipeline installation, the pipe string was gradually moved toward the water using land-based excavators. [caption id="" align="alignnone" width="1200"] Re-excavating the proven trench - Courtesy of Farrans[/caption] At high tide, the pipe was floated, towed and positioned above the pre-dredged trench. Connection was made to the pre-installed land section of the outfall and the pipe was filled with water allowing a gradual sinking and pipe placement along the trench. Backfilling operations commenced once the pipe was surveyed and verified. Professionally qualified divers installed the prefabricated diffuser head. Live underwater video surveys were conducted to monitor the installation of all equipment. The execution of these activities adhered to stringent health and safety protocols, strict environmental considerations, and project timelines, ensuring the successful completion of the project on time and within budget.

Community liaison

Safety was at the heart of the project and protecting the public while works were ongoing on the beach, in close proximity to a holiday park was paramount. NI Water and Farrans undertook a range of initiatives to ensure that local holidaymakers, residents and the wider public were aware of the planned installation, including leaflet drops, beach signage, posters and importantly a children’s safety art competition. [caption id="" align="alignnone" width="1200"] NI Water and Farrans held a joint safety initiative for children in caravan parks - Courtesy of NI Water[/caption]

Successful delivery

The installation of the long sea outfall pipe was a complex and challenging element of the wider £18m Ards North Wastewater Improvement Project and required extensive planning, communications and collaboration with a wide range of stakeholders. With a wealth of expertise and experience on board and as a result of meticulous planning, strong teamwork and robust health and safety measures, the new long sea outfall was successfully installed in summer 2022, well in advance of commissioning of the new WwTW in spring 2023. This critical piece of infrastructure, along with the new Ards North WwTW and both new and upgraded pumping stations in the catchment, will support long-term economic growth in local development and tourism and will benefit all those living, working, visiting or investing in this scenic part of the Ards Peninsula.

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:

1. Millbrook WwTW treats for a population equivalent (PE) of 147,000 with biological treatment provided using a 4-stage Bardenpho Activated Sludge Process (ASP).
2. Slowhill Copse WwTW treats for a PE of 77,000 PE and provides biological treatment via a Modified Ludzack-Ettinger (MLE) configured ASP.
3. Woolston WwTW treats for a PE of 70,000 and has an MLE configured membrane bio-reactor (MBR).
4. Ashlett Creek WwTW treats for a PE of 15,000 and provides biological treatment via a sequential batch reactor (SBR).
5. 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:

1. 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.
2. 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.
3. 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

1. 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.
2. 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.
3. 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.
4. 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:

1. 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.
2. 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.
3. 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.