Supply Chain - Sewerage - Pipe Bridges

Yorkshire Water’s Esholt Sewage Treatment Works is located northwest of Leeds and serves a population of approximately 400,000. The STW treats the majority of sewage flows from Bradford and parts of northern Leeds, with a flow to full treatment of 3200 l/s. The project driver is part of the Water Industry National Environmental Programme (WINEP), which aims to reduce overall phosphorus concentrations in the final effluent to 0.5mg/l, thereby improving water quality in the River Aire. To achieve this new consent, YWS contracted Mott MacDonald Bentley (MMB) to undertake the detailed design, construction and commissioning of the work. The project scope included a new coagulant dosing system and tertiary treatment to ensure the new phosphorus consent was met. Coagulant dosing system

The dosing system involved the implementation of a 40m3 external ferric sulphate dosing tank with a chemical dosing system, with the point of application prior to the primary settlement tanks. MMB utilised the YWS standard product which is a DfMA solution in partnership with framework supplier, Northern Pump Suppliers Ltd (NPS).

The ferric storage tank, the dosing kiosk (which came pre-assembled with all the mechanical equipment and control panel), and the point of application cabinets were all manufactured off-site and could be dropped into place in delivery.

This reduced multiple different interfaces and enabled the site team to build all the other elements ready for the delivery. The ferric dosing system was placed strategically at the bottom of an existing hydro-turbine to utilise the turbulence caused by the screw in operation which provided optimal mixing for the chemical; no additional mixing was required.

[caption id="attachment_14176" align="alignnone" width="1200"] Primary ferric point of application cabinets - Courtesy of MMB[/caption] Challenging designs & re-use of existing structures

Due to the additional ferric being added into the sewage, the site required tertiary solids removal (TSR) to capture any remaining particles before going out to river, ensuring compliance with iron consent limits. Esholt has an existing TSR which consists of 12 deep-bed sand filters cells (DBSF). The DBSF works by allowing effluent to flow down through the bed, with residual solids being captured by the granular material in the bed. In 2015, the works was damaged by an extreme flood, and subsequently the DBSF has remained out of service ever since.

To avoid the need to install new TSRs, the proposed scope included the refurbishment of the abandoned DBSF. Using this ‘build less’ approach saved on capital costs and reduced embodied carbon emissions, but did introduce its own challenges.

Following the previous abandonment, part of the dirty backwash system had been re-purposed for alternative use, meaning that a new solution for returning the backwash flows was required. To address this, a new pumping station was proposed to regulate flows back into the process. Due to the poor ground conditions and high-water table, the design required the pumping station to be of wet caisson construction. The pumping station featured a 280m-long rising main, directing flows back to the existing humus tanks. The pumping station itself was planned to be 12m in diameter and 7m deep, located adjacent to the DBSF.

During design development, MMB reviewed the need for the proposed storage requirement, agreeing a slightly different operating regime for the pumps and DBSF backwash control. As a result, the team identified an opportunity to re-purpose an existing dry well structure nearby. By adapting this structure, they could house the new return pumps without the need to construct an entirely new pumping station.

[caption id="attachment_14174" align="alignnone" width="1200"] Deep bed sand filter, post refurbishment - Courtesy of MMB[/caption] Laser scanning

To verify this opportunity, and to ensure that the detail design could be completed with minimal changes, the team utilised GeoSlam Horizon. This handheld laser scanning technology took half a day to survey the existing dry well, and enabled MMB’s digital delivery technician to model the output in Revit. The model was then exported to virtual reality (VR) in which the project team held collaborative review sessions to confirm the design and ensure that the proposal would be compliant against the clients engineering standards. This also aiding access lifting and maintenance reviews with the client to ensure the refurbishment included aligning the design to the new client engineering specification.

By challenging the design requirements, MMB took out the need for a new pump station by re-purposing the existing structure. This removed the difficult caisson construction and by doing so achieved substantial costs savings for this element of work, and enhanced safety conditions for the construction team. This approach also significantly reduced the embodied carbon footprint. The team utilised the Mott MacDonald Moata Carbon Portal to undertake rapid calculations to understand the benefits of the proposed change, showing a capital carbon saving of ~248 tCO2e.

Rising main

The 280m rising main that directed dirty backwash flows back into the existing humus tanks was originally proposed below ground. The pipe was routed along the Industrial Emissions Directive (IED) boundary and the initial assessment concluded that the pipe required dual containment to mitigate the risk of any pollution.

[caption id="attachment_14173" align="alignnone" width="1200"] Dirty backwash rising main - Courtesy of MMB[/caption]

MMB worked collaboratively with the client to better understand the risk and consequence of a failure, as the media contained within was backwash water and not sludge, which is typically with focus under IED. In addition to this, a review was undertaken on programme and cost impact, as well as the risks during the construction phase.

The outcome of this review was an agreement with the Environment Agency to undertake a more pragmatic approach. The carbon saving calculated on the Moata Carbon Portal tool is estimated to be ~37t CO2eq moving from a dual contained below-ground pipe, to an above-ground single skin ductile iron pipe.

MCC

The existing MCC in this area had reached the end of its asset life and was insufficient in terms of capacity. To meet the power and control requirements of the refurbished sand filters, the new backwash arrangement, and associated equipment, a replacement was essential. However, the location of the existing MCC posed challenges, in that it could not accommodate the new MCC while complying with the latest Engineering Specification.

To achieve the upgrade, MMB proposed a hybrid approach to the design. Instead of installing the entire new panel in a separate GRP kiosk raised above the flood zone on a steel frame, the team opted for a more efficient solution. They integrated sections of the new MCC within the existing building, ensuring compliance with current specifications, minimising the length of the MCC which required a new structure. This reduced not only the kiosk size but all the periphery infrastructure including civils works, raised steel frame and security fencing.

[caption id="attachment_14178" align="alignnone" width="1200"] Side view of primary ferric dosing compound - Courtesy of MMB[/caption] Esholt STW: Supply chain - key participants Designer & delivery contractor: Mott MacDonald Bentley Dosing system subcontractor: Northern Pump Suppliers Ltd Chemical dosing trays subcontractor: APT Marine Engineering Ltd 3D printing subcontractor: Hyperion Robotics UK Ltd Electrical installation: Circle Control & Design Systems Ltd DBSF mechanical refurbishment: De Nora Water Technologies UK MCC platform subcontractor: AJ Engineering & Construction Services Ltd Thinwall stainless steel pipework: GTS Nitrogen Services Ltd Systems integration: Actemium Civil engineering/site engineers: Day Civils Ltd Access platforms subcontractor: GT Fabrications Security fencing subcontractor: Burn Fencing Ltd MCC security kiosks subcontractor: Technocover Ltd Electrical panels: Technical Control Systems Pumps: Xylem Water Solutions DI pipework: Electrosteel Castings (UK) Ltd 3D concrete printing

To take the electrical ducts around the site and between the two panels for power and control cables, multiple large scale draw-pits were required at changes of direction. These draw-pits, typically measuring 2m2 by 2m deep, typically involve in situ concrete casting with 0.25m-thick walls. Their construction can be carbon-intensive, time-consuming due to intricate formwork, and pose safety risks for the construction team working in excavations over extended periods.

After meeting at a Yorkshire Water-led Net Zero event, MMB began forming a strategic relationship with Hyperion Robotics. Hyperion create 3D printed structures with a cementitious grout, printing only what the structure actually requires to be structurally integral. Early discussions used the proposed draw-pits as a test case of how their innovative 3D printing technology could support the water industry’s need for innovation and a drive towards Net Zero.

[caption id="attachment_14177" align="alignnone" width="1200"] Delivery of 3D printed draw-pits - Courtesy of Mott MacDonald Bentley[/caption]

The process involved working with Hyperion to create a bespoke design, to allow incorporation of the specific electrical duct penetrations and angles, and sizing of the chambers. This process reduced the materials required by 40%. Collaborative reviews were held with MMB, Hyperion, Tarmac and Yorkshire Water to ensure adequate buy in to this new technology.

3D concrete printing involved printing cementitious grout in layers, by using this technology the time taken to print the 2m high 1.9mØ draw-pits required at Esholt was only two hours. This DfMA approach also had huge safety and programme benefits as the time required working within excavations was significantly reduced since the site team could directly lift the draw-pit into the excavation from the wagon.

The technology provides improved quality control and ensures precise application to meet the exact dimensions in a controlled environment. The 3D-printed design achieves a 40% reduction in embodied carbon, a 10% cost reduction, and a 50% improvement in programme for drawpit construction, compared to conventional in situ.

Progress

Esholt STW is on track to meet its compliance date of 0.5 mg/l of phosphorus by December 2024. The scheme has been a success by challenging the design to get the best value for the client but to leave a positive impact on the environment. MMB have kept in touch with the continuously growing construction industry, trialing new technology including utilising laser scans, VR and 3D printing.

Esholt is a prime example of how challenging design and reusing existing structures where possible reduces the impact the construction has and accelerates progress towards a net-zero built environment.

The WINEP Durham Transfers Project was designed to improve the quality of the Blackdene Burn, a watercourse located in the north Durham area. During AMP7, sewage treatment works (STW) sites within Northumbrian Water Group (NWG) ownership were identified for improvements in phosphorus concentrations within final effluent discharge to the environment. The scope of this project is to address the improvements at both Plawsworth STW and Pity Me STW, which currently discharge to the Blackdene Burn. Background

Esh-Stantec was appointed by NWG to progress the proposed solution; a phased approach whereby, during AMP7, final effluent from both STWs will be transferred to a discharge point on the River Wear via transfer pumping stations. The transfer of flows will be facilitated via new rising mains and gravity sewerage which will connect immediately upstream of the existing outfall from Brasside STW. In line with the phased approach, planned upgrades to Brasside STW in AMP8 will increase its treatment capacity and allow for the eventual abandonment of Plawsworth and Pity Me STWs along with chemical dosing to meet planned phosphorus limits discharging to the River Wear.

The designed solution allows for projected growth across the Plawsworth and Pity Me catchments (including the completion of the significant Sniperley development) with an AMP8 solution transferring all raw flows from Plawsworth and Pity Me to the upgraded Brasside STW for treatment, with final effluent from Brasside STW ultimately discharging to the River Wear.

The objective of this appointment is to provide the assets required to enable to the transfer of flows from Pity Me and Plawsworth STWs in the AMP7 and AMP8 scenarios. It is anticipated that the project will be completed in December 2024.

[caption id="attachment_18551" align="alignnone" width="1200"] Piling operations by Francis & Lewis International Ltd, for the construction of the pipe bridge substructure - Courtesy of Esh-Stantec[/caption] WINEP Durham Transfer: Supply chain - key participants Client: Northumbrian Water Principal designer & contractor: Esh-Stantec Esh Group Stantec UK Structural consultant: James Christopher Consulting Ltd M&E, including supply & installation of generators, MCCs & pumps to wet wells: Retroflo Ltd Directional drilling of 3km of rising main & installation of all AV chambers: Ken Rodney Construction Ltd Auger boring under the East Coast Mainline, coordinating with Network Rail, & supply/installation of track monitoring: Terra Solutions Ltd Design, manufacture, & installation of pipe bridge: Francis & Lewis International Ltd Design & sinking of segmental shafts: Active Tunnelling Ltd Temporary works/shoring: MGF Ltd New transfer stations

The first stage of the scheme involves the transfer of fully treated, final effluent to the River Wear, meaning that Plawsworth and Pity Me STWs remain fully operational throughout the construction of all assets which will become live prior to the upgrade of Brasside STW.

Plawsworth Transfer Station: The new transfer assets at Plawsworth STW were carefully designed and sequenced around the limited working room in the site and the requirement to maintain treatment operations during construction. The Plawsworth Transfer Pumping Station comprises:

A 4m diameter, 6m deep wet well shaft (sunk as a caisson). MCC and associated kiosk. Generator and associated kiosk. Above-ground valve arrangement. Flow meter chamber and all associated interconnecting pipework and ducting.

In AMP7, final effluent flows are intercepted and directed into the new wet well before being pumped to the River Wear. Due to the transfer of final effluent, a reduced emergency storage volume has been provided within the new wet well.

In AMP8, following upgrades to Brasside STW, raw flows will be intercepted upstream of the inlet works and diverted to the new SPS for transfer to Brasside via a new 250mm rising main. As part of the conversion from a treatment works to a terminal pumping station, the existing humus tank will be re-purposed to provide additional emergency storage in line with EA guidelines for the transfer of raw sewage flows and minimise emergency overflow spills to the environment.

In addition to emergency storage, a permanent standby generator has been incorporated into the design which will automatically cut in if there is a loss of power to the site.

The pumping station was designed as a three pump station (duty/assist/standby) to transfer ultimate AMP8 flows up to 32 l/s, however, in the AMP7 scenario,  the pumping station will operate as a duty/standby/standby station, with the individual pumps sized to transfer 24 l/s of final effluent to maintain a minimum velocity in the rising main.

Pity Me Transfer Station: The main works at Pity Me STW comprises the following:

A 6m diameter, 9m deep wet well shaft (sunk as a caisson) with built-in emergency storage volume sized for ultimate raw flows. MCC and associated kiosk. Generator and associated kiosk. Valve chamber. Flow meter chamber and all associated interconnecting pipework and ducting.

Similar to Plawsworth, the pumping station was designed as a three pump station to transfer 34 l/s in AMP7, with a duty/standby/standby arrangement, and up to 61 l/s in AMP8, after conversion to a duty/assist/standby arrangement. In both the final effluent and raw flow scenarios, pumped flows will exit the works via a new 280mm rising main.

[caption id="attachment_18550" align="alignnone" width="1200"] Active Tunnelling constructing the new wet well shaft at Pity Me STWCourtesy of Esh-Stantec[/caption] Transfer rising mains

Approximately 2.1km of 250mm PE rising main and associated air valve/washout chambers has been installed from Plawsworth STW. The rising main pipework was installed predominantly via a trenchless technique, namely horizontal directional drilling (HDD), primarily due to lack of available access from the adjacent A167 highway to facilitate traditional open cut construction, as well as unfavourable ground conditions.

A new 280mm  rising main was installed over an 850m length from Pity Me STW. Due to a tight working corridor situated between a garden centre and the Blackdene Burn, as well as the requirement to cross beneath the A167, a highly trafficked highway, the rising mains was also installed via HDD. This use of a trenchless installation technique minimised disruption to local business and removed the requirement for traffic management on a busy commuter route.

eDNA samples taken from ponds adjacent to Pity Me in the feasibility stage confirmed the presents of great crested newts. In lieu of protracted traditional population surveys and subsequent licenced activities to mitigation including fencing and capture to clear the working area, a District Level License (DLL) was secured from Natural England and works were completed under an approved method statement with supervision from a consultant ecologist.

The 280mm and 250mm rising mains from Pity Me and Plawsworth connect to a common chamber located at the high point of the new network, where they will discharge flows to a new DN400 gravity sewer.

[caption id="attachment_18547" align="alignnone" width="1200"] Construction of new valve chamber and associated pipework at Pity Me STW - Courtesy of Esh-Stantec[/caption] New gravity sewerage

2.4km of new DN400 gravity sewerage, with associated manhole chambers, was constructed to receive the pumped flows from both Pity Me and Plawsworth STWs. Careful consideration was given to the design of the new sewerage to ensure capacity (including future growth) and self-cleansing requirements are met in all flow scenarios, including when Pity Me and Plawsworth transfer pumps operate either in isolation or in parallel.

The construction of the new gravity sewer poses some complex crossings and challenging ground conditions. The crossing of Chester Low Road, another busy road for both commuters and customers of a popular local retail park, The Arnison Centre, was originally planned for open cut excavation. However, subsequent on-site observations with regard to topography, a number of high-risk services including high voltage electric cables and a uPVC water main, and running sands within nearby open cut trenches led to the conclusion that it would not be possible to safely support the excavation during construction. Therefore, the road crossing was completed via guided auger bore. Not only did this mitigate the disruption and health and safety risks associated with damaging services, but it also minimised disruption to local businesses and residents as the road remained operational throughout the process.

East Coast Main Line crossing

Further downstream, the new gravity line is also set to cross beneath Network Rail’s East Coast Main Line. The proposal is to install a 610mm sacrificial steel sleeve via a guided auger bore beneath the embankment, once complete the 400mm NB gravity pipe will be sleeved and the anulus grouted. Extensive liaison is ongoing with Network Rail in the lead up to the under track crossing (UTX) to gain the necessary approvals and fully understand the constraints which must be met during construction. These constraints include but are not limited to 24/7 working for the duration of the tunnelling works, installation of track monitoring and an approved contractor on standby in case there is a requirement for emergency track repair.

[caption id="attachment_18549" align="alignnone" width="1200"] Aerial view of working area at Brasside STW - Courtesy of Esh-Stantec[/caption] Brasside pipe bridge

Approximately 800m upstream of the Brasside STW outfall connection, the sewer crosses a watercourse located within a steep ravine and dense woodland at the existing Brasside Sewage Pumping Station (SPS).

At the optioneering stage of the project, it was proposed that this crossing would take place via an inverted siphon arrangement. However, subsequent hydraulic modelling deemed that, due to topography constraints, it would not be feasible to achieve sufficient capacity while maintaining self-cleansing velocities in both the AMP7 and AMP8 flow scenarios without connecting to the existing outfall further downstream. This would have required construction works within ancient woodland, imposing the requirement for Environmental Impact Assessment (EIA) and potentially preventing the obtention of a Lawful Development Certificate.

Therefore, to mitigate the risk of repeat maintenance issues and potential asset failure, a pipe bridge was progressed as the preferred option having discounted a further interstage pumping station at the low point due to a lack of space within the existing Brasside SPS site and the associated operational carbon and expenditure.

The route of the pipe bridge was designed to mitigate environmental impact, by including bends within the bridge structure and above ground pipework to take advantage of existing operational land and avoid clearance of significant trees within a conservation area. The interface between the bridge structure and the above ground ductile iron pipework has been carefully modelled and iteratively designed to assess the impact of bridge movement and deflection on the pipe joints and select joints which provide appropriate tolerances for these movements.

[caption id="attachment_18552" align="alignnone" width="1200"] Construction of pipe bridge structure by Francis & Lewis International Ltd - Courtesy of Esh-Stantec[/caption] Conclusion

The design of the WINEP Durham Transfers Scheme serves to meet regulatory deadlines by achieving a sustainable multi-stage solution which meets the needs of the current and future catchment population.

At the time of writing (July 2024), construction is progressing well, and all rising main pipework and transfer station substructures have been installed. The gravity sewer is predominantly complete with the two key crossings remaining, pending final Network Rail approval to complete the rail crossing and final structural design of the pipe bridge to install the above-ground pipework adjacent to Brasside SPS.

The current construction programme shows that flows will be turned in December 2024, transferring final effluent to the River Wear in the interim and, in AMP8, transferring raw flows to Brasside STW for treatment. This will achieve the removal of treated effluent from the Blackdene Burn, in turn significantly improving water quality while meeting WINEP obligations. Furthermore, the completion of the scheme in AMP8 will serve to rationalise the NWG sewage network in the area and improve the resilience of the network by serving catchment growth and improving treatment capacity.