Supply Chain - Renewables & Energy Management - DSEAR Assessment

Deephams Sewage Treatment Works (STW) is located in Edmonton, North London. In November 2018, Thames Water Utilities Limited (TWUL) issued take-over to the AECOM, Murphy, and Kier (AMK) joint venture for the replacement of the entire sewage treatment stream. The project increased the treatment capacity to 989,000 population equivalent and met the new final effluent flow to full treatment capacity of 5,750 l/s and the discharge consent quality concentration of 10/5/1/1mg/l (solids/BOD/NH₃-N/P). As described in the UK Water Projects 2018 article Deephams STW - A630 Major Project, the project utilised integrated fixed film activated sludge (IFAS) media in the temporary and permanent phases of the works to increase the biological mass in the aerobic phase of the activated sludge process.

Biogas yield

Since the commissioning of the new sewage stream, TWUL operational excellence has resulted in the yield of biogas exceeding the best estimates of its generation from the anaerobic digestion plant. The unintended consequence being that both the CHP and boiler demands are met but regular flaring of excess gas occurs. To avoid these environmental emissions, the biomethane project at Deephams STW was born. During 2020, TWUL created a design, build, maintenance and operation framework agreement for the provision of biomethane plants and in March 2021 a contract was awarded to SGN Commercial Services Limited for the project at Deephams STW. The plant is designed for a maximum 1,150Nm³/hr flowrate of biogas, with a maximum biomethane injection rate to the Cadent medium pressure grid of 825Sm³/hr after enrichment with the addition of propane. [caption id="" align="alignnone" width="1200"] Deephams STW Biomethane Plant flow diagram[/caption]

Engineering technical solution development

The biomethane plant gas-to-grid treatment stream comprise the following stages:

• Biogas transferred from the anaerobic digestion plant.
• Pre-treatment and dewatering.
• Compression and methane separation.
• Enrichment, gas quality monitoring, odorisation, and injection into the grid.

The design concept was to allow flexibility for the biogas to be diverted to either the biomethane plant or to the retained CHP capacity. Thereby, permitting biogas to feed the biomethane plant during normal operational periods but to be diverted to the CHP during high electricity demand periods from the STW plus fulfil any obligations to export electricity off-site to the electricity grid during high consumer demand periods. [caption id="" align="alignnone" width="1200"] Membrane unit - Courtesy of SGN Commercial Ltd[/caption] Biogas transferred from the anaerobic digestion plant: Upstream of the gas holder, a connection was provided to supply the biomethane plant. The biogas header operates at between 5–25mbar. Biogas is saturated with water vapour, and its composition is 50-60% methane (CH₄), 40-50% carbon dioxide (CO₂), and small amounts of hydrogen sulphide (H₂S), siloxanes, nitrogen (N₂), Oxygen (O₂), and other trace components such as volatile organic compounds (VOCs). Compression and methane separation: The pre-treated biogas is then analysed and if it is within specification transferred to the compression stage (two 600Nm³/hr compressors capable of 50% turndown ratio) which creates a driving force for membrane separation of the CO₂ and CH₄. The compressed biogas is further cleaned to reduce oil, water and any remaining contaminants to negligible levels to protect the membrane system’s operation and performance. Highly selective Evonik Sepuran membranes supplied by DMT Environmental Technology are used to permeate the CO₂ from CH₄. The polymer of which the membrane is formed allows the larger CH₄ molecule to be retained within the gas stream whilst the membrane polymer which is subjected to heat and pressure allows the smaller CO₂ molecule to permeate through its structure and separate from the CH₄. The CO₂ passes through a vent and is then discharged to atmosphere. [caption id="" align="alignnone" width="1200"] Compressor room - Courtesy of SGN Commercial Ltd[/caption] Enrichment, gas quality monitoring, odorisation, and injection into the grid: The resultant biomethane gas (retained methane) is checked for compliance with the Gas Safety Management Regulations (GSMR); once satisfactory, it is conveyed to the grid-entry-unit for final conditioning. The biomethane has a lower calorific value, approximately 5%, than the indigenous network natural gas so requires enrichment with propane (a heavier hydrocarbon with a higher calorific value). Following the addition of propane, the gas passes through the Honeywell UK limited gas-to-grid entry unit where it is tested. If it is of acceptable quality for onward conveyance into the Cadent gas network, it is metered and undergoes its final conditioning phase with the addition of a sulphur compound or ‘stenching’ agent to introduce the distinctive ‘natural gas’ odour. In the event of there being a problem in the Cadent gas network there is a remote operated valve (owned by Cadent) that can shut off the supply of biomethane gas from Deephams STW. If at any phase of the process stream ‘gas’ is out of standard or specification the biomethane gas is rejected by the grid-entry-unit. These rejected gases are then either diverted to the waste gas burner (WGB) or returned to the biogas storage.

Deephams Biomethane Project: Supply chain - key participants

• Biomethane plant design/delivery: SGN Commercial Services
• DSEAR EPD: SLR Consulting
• Civil/electrical engineering: Bridges Limited
• Pipelines: South East Eco Utilities Limited
• Biogas upgrading plant: DMT Environmental Technology
• Gas-to-grid entry unit: Honeywell UK Limited
• Siloxane & H₂S removal filters: CPL Activated Carbons
• Propane tanks: Flogas Britain Ltd

[caption id="" align="alignnone" width="1200"] Honeywell UK Limited grid entry unit - courtesy of SGN Commercial Ltd[/caption]

Non-engineering technical challenges

As with most projects, the complexity of its implementation was not solely restricted to the engineering solution. Multiple other challenges were confronted and delivered, including:

• Third party agreements.
• Permitted development rights.
• Waste operation environmental permit.
• Application of Thames Water standards and gas industry standards.
• Renewable energy grants.
• Future opportunity.

Third party agreements: Engagement with Cadent was a key delivery component to ensure timely compliance with their guidelines; a pre-requisite for the biomethane to be injected into the national gas transmission grid. The first key component being to obtain a Grid Connection Agreement which permitted the physical connection to the medium pressure main; the second being granted the Network Entry Agreement which required the installation of Cadent approved plant to demonstrate the provision of ‘in-specification biomethane’ being ready for injection into the grid. This was achieved by utilising the Honeywell UK limited gas-to-grid entry unit; a tried-and-tested solution that Cadent had previously approved. [caption id="" align="alignnone" width="1200"] (left) Cadent-owned remote operated valve and (right) the electrical HV room - Courtesy of SGN Commercial Ltd[/caption] Permitted development rights: A key driver in achieving timely commissioning of the biomethane plant was for the project to be implemented under TWUL’s permitted development rights as a statutory sewerage undertaker. TWUL requested of London Borough of Enfield (LBE) an EIA Screening Opinion under Regulation 6 of the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 for the proposed biomethane plant to agree that an EIA was not required (as if full EIA is required, permitted development rights are removed). LBE agreed and issued an EIA Screening Opinion confirming that full EIA was not required. TWUL hence notified LBE that the scheme would be implemented under their permitted development rights as Statutory Sewerage Undertaker. Subsequently, LBE agreed that the development constituted Permitted Development under Part 13 B (f) of the regulations, as none of the exclusions to development permitted under Part 13, Class B applied. Waste operation environmental permit: The waste operation environmental permit for the CHP plant was varied to include an ‘R3: recycling/reclamation of organic substances (which are not used as solvents)’ activity for upgrading the biogas to biomethane, in a biomethane plant, for injection into the national gas transmission grid. In essence, the change had the effect of extending the CHP’s green permit boundary line. As a result of biogas being converted to biomethane, compliance with the requirements of the end-of-waste quality protocol for biomethane was a prerequisite. It defined the point at which the biogas (which is termed as a waste) ceases to be waste; and the by-product biomethane can be used as a product without the requirement for waste management controls as the point-of-injection into the Cadent gas network. [caption id="" align="alignnone" width="1200"] Propane tanks - Courtesy of SGN Commercial Ltd[/caption] Application of TWUL asset standards and gas industry standards: As the design developed, it was necessary to agree the order of precedence of TWUL corporate asset standards and gas industry standards. It was concluded that the conditioning plant up-front of the biomethane plant is typical of the gas conditioning plant for CHP unit, hence, TWUL adopted corporate asset standards to that area. Once the biogas meets the 15bar high pressure forwarding gas compressors (the ‘compression’ phase described above), then international and gas industry (IGEM) standards were adopted. Renewable energy grants: It was identified that the project qualified for the Renewable Heat Incentive (RHI) accreditation scheme. A 20-year long-term financial support programme for renewable heat that closed on 31 March 2022. Notwithstanding the benefit of the project contributing towards TWUL goal of net zero carbon emissions in 2030, the financial benefits of meeting the RHI timeline were a driver. If the deadline was not met, it was planned on selling the biomethane into the Road Transport Fuel Obligation (RTFO) scheme. A non-guaranteed revenue stream dictated by market forces of supply and demand for the biomethane commodity. Following the closure of the RHI, TWUL is currently seeking an amendment to the Green Gas Support Scheme (GGSS) to allow retro-fits. This would support biomethane gas-to-grid projects like the Deephams STW project being built at other TWUL sites. Future opportunity: To further enhance the benefit of such biomethane plants; investigations are on-going into the practicality of adding a carbon dioxide capture plant into the process. A key driver for its implementation will be whether validation is granted that the carbon dioxide is of food industry quality; plus assessing whether the technology currently available is commercially viable. [caption id="" align="alignnone" width="1200"] Deephams STW Biomethane Plant - Courtesy of SGN Commercial Ltd[/caption]

Conclusion

The Deephams STW biomethane plant gas-to-grid solution is helping Thames Water Utilities Limited achieve its goal of net zero carbon emissions across all operations by 2030. Within the first year of operation, it has produced an average of 550m³/hr of biomethane being injected into the gas grid with 97% availability. This is enough gas derived from the biomethane plant to heat 4,250 homes in the London Borough of Enfield. The potential for roll out of similar projects across TWUL’s estate is large; estimates indicate that there are twenty-four potential STW sites that could install biomethane plants to generate enough gas to heat over 50,000 homes. The Deephams STW biomethane plant project remains a suitable engineering example where technology provides a viable opportunity to produce renewable energy whilst helping to reduce carbon emissions.

Mogden STW is one of Thames Water’s Tideway treatment works and in AMP6, a combined heat and power (CHP) installation was constructed at the site. Once the ongoing (AMP7) sludge and AD facility upgrade is completed, biogas yield is set to increase again and, in line with its net zero strategy, at the start of 2022, Thames Water opted to add a biomethane gas-to-grid plant at Mogden during AMP7.

Project brief and site-specific challenges

The gas-to-grid plant at Mogden was conceived to complement the AMP6 CHP facility. As such, the plant was sized to process a third of the site’s anaerobic digestion (AD) biogas output.  In order to successfully complement the CHPs, the control of the gas-to-grid plant would need to be appropriately integrated into the existing process control system.

With neighbours on all boundaries and an ever-increasing local population, the Mogden STW site is especially constrained for space. The design development and delivery of the gas-to-grid project was further complicated by the aforementioned AMP7 sludge and AD facility upgrade. The layout, plant configuration and plant control would all need to consider construction, interim and legacy phase site arrangements.

Innovative deployment of a fully proven solution

On placement of the design and build order, SGN Green Gas Solutions set about tailoring a well proven membrane upgrader-based solution for the specific requirements of the Mogden project. Fundamentally, the solution is fairly typical of a current generation biomethane gas-to-grid plant.

The biogas supply for the gas-to-grid plant was taken off the existing supply to the CHP engines. To help overcome the challenges routing this supply to the main gas-to-grid plant compound, SGN Green Gas Solutions located the biogas dehydration equipment close to the point of connection. A combination of above-ground stainless steel and buried PE pipework could then be used to convey the dry biogas.

SGN Green Gas Solutions developed an elongated design for the main plant to work given space constraints. The plant layout had to accommodate HV/LV transformers to supply the membrane upgrader and associated ancillaries and an appropriate route was found for the biomethane product pipework.

Construction

A remotely operated valve delineates the biomethane plant from the gas network and due to its location, the SGN team had to design and install a 315mm diameter, PE export pipeline over a complex 1.2km route crossing a road with high traffic flows into a medium pressure (MP) system. Although convoluted, this connection option guaranteed year-round export capacity.

With established supply chain partnerships and relationships, SGN Green Gas Solutions was able to sequence design, procurement, site works and plant installation to optimise the programme and limit the amount of work required on site. The grid connection pipeline commenced in the summer months of 2022.

Mobilisation on the Mogden site was in January 2023 and civils work peaked in June. Package plant items arrived in quick succession throughout August, and by the end of November 2023, site installation and site commissioning were substantially complete.

[caption id="attachment_13204" align="alignnone" width="1200"] Activated carbon pre-treatment units - Courtesy of SGN Green Gas Solutions[/caption]

Operational flexibility and integrated control

Given the aspiration to operate the gas-to-grid plant in conjunction with the existing CHPs, a significant amount of thought went into the control philosophy for the plant and the control interface between the plant and the wider site. Being a large sewage treatment works, Mogden has a sophisticated process control system, power management system and SCADA.

The existing CHPs normall operation includes automatic start, ramp and stop based on the common level of the site’s floating-roof AD tanks. It was agreed that the biomethane plant start, ramp and stop commands would also need to be driven from the existing process control system based on configurable set-points. The values used for these set-points would then allow the site’s process control engineers to optimise the conversion of biogas in real time based on prevailing conditions.

When there is surplus in electricity generation capacity, more of the site’s biogas can be directed to the gas-to-grid plant to be used (and to an extent ‘stored’) in the gas network.

Renewable generation capacity with intelligent features like this help the UK better match energy supply and demand as the country transitions away from fossil fuels.

Commissioning & commercial operation

Commissioning of the gas-to-grid plant commenced in November 2023, with the first green gas being injected in December. From January the plant effectively commenced commercial operation, with optimisation work, performance tests and reliability trials continuing through the spring.

The output from the plant will provide sufficient green gas to supply over 4,000 local homes, offsetting 8,000 tonnes of fossil carbon emissions each year. The project has further shown how gas-to-grid technology can be rapidly deployed and fully integrated in a high-profile municipal wastewater treatment context.

Ballygowan is a small village situated in County Down, within the Ards & North Down Borough Council area. The towns of Comber and Saintfield are both approximately 4 miles to the north-east, and to the south respectively, with the city of Belfast 7 miles north-west. The 2021 Census indicates that the population of Ballygowan is approximately 3,138. The existing WwTW was constructed in the village in the 1950s and comprised an inlet works, a CAS basin, a storm tank and a small sludge holding tank, as well as an MCC building to control the works and was designed to treat a population equivalent (PE) of 2,700. The existing works was therefore beyond capacity to treat the present day population, and the structures had exceeded their design life. An extensive upgrade was required to provide effective and robust wastewater treatment services for the future catchment area, allowing economic growth, while improving the local environment and adhering to a much tighter discharge consent of 7mg/l BOD, 10mg/l total suspended solids, and 1.5mg/l ammonia. The new fully automated works will serve a PE of 4,513.

Primary drivers for the upgrade

• The existing wastewater treatment works (WwTW) was undersized for the current flows coming into the works.
• The existing works was not complaint with the Northern Ireland Environment Agency’s discharge consent standard for Ballygowan WwTW.
• Future planning applications for residential properties in the Ballygowan area had been put on hold due to the existing WwTW operating beyond its intended design capacity.
• The existing works was not capable of fulfilling the requirements of NI Water’s long-term strategy to enhance wastewater services and ensure environmental compliance and was expensive to operate and maintain.
• An even tighter NIEA standard was imminent.

[caption id="" align="alignnone" width="1200"] (left) Existing Ballygowan WwTW - Courtesy of NI Water and (right) demolition in progress: (top) the existing storm tank and (bottom) the existing CAS basin tank - Courtesy of  GEDA Construction[/caption]

Project description & scope

Designed for a 2045 population equivalent of 4,500, the new Combined Activated Sludge Plant (CASP) at Ballygowan WwTW, was to be constructed east of Dickson Park in Ballygowan, in a green field site that bordered the southern boundary of the existing site:

• The preliminary treatment process comprising of 6mm inlet screening and forward flow-control equivalent to Formula ‘A’.
• A new storm storage management facility including a CSO storage tank, and two storm tanks providing a total capacity of 224m³. The CSO storage tank and two storm tanks are equipped with a tipping bucket system for effective cleaning of the storm tank floor.
• Flows equivalent to Full Flow to Treatment (FFT) are pumped forward via a new interstage pumping station.
• FFT is then split with equal flows going to two CAS basin tanks. The selector chamber was carefully designed to assist with controlling filamentous bulking in activated sludge systems which can be a common operational challenge issue in activated sludge plants.
• Downsteam of the flow split chamber, there are two CAS tanks complete with final settlement tanks (FSTs) accommodated centrally, each aeration stream comprising of an anoxic zone to aid denitrification, and three individual aeration zones equipped with tapered aeration diffuser grids. The design and construction of the biological treatment process on this site was critical and to ensure compliance with an extremely stringent discharge consent of 1 mg/l ammonia alkaline was dosed at this stage in the process, detailed below.
• Due to the restricted footprint available for the site, the CAS tanks were selected to accommodate the final settlement stage within the biological treatment footprint. Two FSTs were constructed, with the CAS (anoxic and aeration zones) wrapped around the FSTs.

[caption id="" align="alignnone" width="1200"] CAS basins  - Courtesy of NI Water[/caption]

• Following final settlement, the treated effluent gravitates to a tertiary treatment plant for further solids removal prior to discharge at the final effluent outfall location.
• A new combined RAS/SAS pumping station was constructed adjacent to the selector/flow split chamber to aid best use of available footprint. This pumping station houses two RAS pumps to replenish the bacteria within the biological treatment system and maintain the required MLSS levels for effective treatment. A further two SAS pumps are also provided to pump away sludge to the new sludge storage facilities should the MLSS levels within the treatment process exceed the desired levels.
• The new Ballygowan WwTW includes two sludge storage tanks, one mechanical sludge thickener and a polymer chemical dosing plant for processing sludge prior to being tankered offsite.
• To ensure a robust solution was provided for the client to meet the 1 mg/l ammonia consent, a supplementary alkalinity chemical dosing plant was provided for dosing into the aeration lanes. The provision of this dosing plant which utilises sodium carbonate ensures there is sufficient alkalinity available within the biological treatment process for effective and complete nitrification under all flow and load conditions.
• The works also comprises a new MCC room which controls all mechanical and electrical plant equipment and instrumentation, a welfare unit, a standby backup generator, and associated access and drainage.
• As the treatment works was built in close proximity to a neighbouring residential area, the odour model highlighted areas of the treatment process that required covers with passive odour abatement being provided.
• The site was also landscaped to include a site bund to reduce the visual impact on the neighbouring residential area of Dickson Park.

[caption id="" align="alignnone" width="1200"] New storm tank being utilised during construction - Courtesy of GEDA Construction[/caption]

Main challenges for design/construction

Discharge consent The key challenge around the design and construction of the new Ballygowan WwTW, was ensuring that the new WwTW would comply with a very tight water order consent (WOC) set by NIEA. The WOC is one of the tightest consents in Northern Ireland. The discharge consent was originally advised as 7:10:1.5 mg/L (BOD/TSS/ammonia), however the design and construction was further challenged when NIEA confirmed that the ammonia consent would be restricted to 1mg/l ammonia. GEDA Construction, in conjunction with their MEICA and process subcontractor Water Solutions Ireland Ltd, reviewed the design and construction proposal to provide a solution which incorporated minimal changes, minimising costs to NI Water, but still providing effluent treatment compliance. Site location The WwTW site is bordered by agricultural land to the north and east, and has a housing development to the west. To construct the new WwTW and keep the existing plant operational, NI Water purchased an area of land (formerly a football pitch) to the south of the site belonging to the local council. The existing WwTW access road is through Dickson Park housing estate and runs directly alongside a children’s playpark. In advance of construction the project team took the proactive decision to look at creating a temporary construction access to the south of the site through agricultural land so that all construction traffic would not have to travel near Dickson Park or the children’s playpark. Extensive consultation took place with local landowners, the residents’ association and roads authority and the temporary road was successfully constructed ahead of construction start. Due to the close proximity of the new WwTW to the housing development, consideration was also given to any potential visual and odour implications to the neighbouring housing development. It was decided at an early stage to design the structures in such a way that they would have minimal visual impact. This meant designing and constructing structures as low in the ground as possible to keep them out of view of the neighbouring properties. An earth bund was also constructed to the west boundary of the site and planted with trees/bushes and shrubs in order to further hide the treatment works as well as helping to combat any odours coming from the site. [caption id="" align="alignnone" width="1200"] Ballygowan WwTW under construction (December 2021) - Courtesy of NI Water[/caption] Interface between new and old works during construction The majority of the new WwTW works was to be built on a greenfield site which allowed GEDA Construction to build most of the new structures without having to work around existing infrastructure. However, in order to construct pipelines which would eventually take the flow into the new works, and to construct the new WwTW access road, it was necessary to demolish the existing storm tank in the old works prior to the new works becoming live. GEDA Construction carried these works out by utilising the newly-constructed storm tank and incorporating a carefully developed phasing plan. A temporary 300mm gravity fed sewer was laid from the existing inlet works to the new storm tank, which in turn rendered the existing storm tank surplus to requirements, as all storm flows had been diverted away from it. GEDA Construction also had to lay a temporary 200mm pumped sewer from the new storm tank complete with temporary pumps to send the storm flows back to the existing pumping station in order for the influent to then go through the treatment process. This in turn allowed us to demolish the existing storm tank and construct the new permanent inlet sewers to the new works ahead of turning flows. This process had to be carefully managed both hydraulically and biologically to ensure the treatment process was optimised and effluent compliance was maintained at all times. [caption id="" align="alignnone" width="1200"] New permanent high-level sewer under construction - Courtesy of GEDA Construction[/caption] Turn of flows Influent flows were successfully diverted into the new works in early May 2022. In order to bring the new biological treatment process online, several tanker loads of seeded sludge were brought to site from another treatment facility and were introduced into the new CAS basin tanks to promote bacteria growth. With flows diverted, and effective treatment established through a rigorous sampling regime, approval was received from NI Water to proceed with the demolition of the remainder of the existing treatment works. Once the existing works were decommissioned and demolished, the entire new WwTW site was able to be finished and access roads completed. Demolition Post turn of flows in early May 2022, the existing treatment works was decommissioned and then demolished. Reusable mechanical and electrical assets from the old treatment process were removed for future use by NI Water, while the effluent left in the old CAS basin and pumping station was removed from the site to allow demolition to commence. Effluent was filtered through the new treatment process to minimise unnecessary removal and disposal from site.

Project team

As principal contractor, GEDA Construction had overall responsibility to deliver the project on time and within budget, which they succeeded in doing by working collaboratively with NI Water, project managers Tetra Tech, key suppliers and subcontractors to identify and plan for key project milestones. McAdam provided civil, structural and geotechnical design services, including the initial enabling and investigatory works carried out at the Early Contractor Involvement (ECI) stage, as well as the detailed design for construction. Water Solutions Ireland (WSI) were responsible for designing and delivering a full turnkey MEICA, process design and build solution for the new works.

Ballygowan WwTW: Supply chain - key participants

• Civil contractor: GEDA Construction
• Project managers: Tetra Tech
• MEICA contractor: Water Solutions Ireland
• Civil, structural & geotechnical design: McAdam
• Electrical design: SBE Design
• Mechanical design: McWall Drawing & Design
• PLC software & commissioning: Ashdale Engineering
• Odour control: Anua Clean Air UK
• Storm screens: Eliquo Hydrok Ltd
• Grit removal plant: Jacopa
• 6mm inlet screening: Hydro International (M and N Electrical and Mechanical Services)
• Aeration blowers: Aerzen Machines
• MCC: Greenville IDC
• Pumps, mixers, fine bubble diffusers: Xylem Water Solutions
• Tertiary treatment plant: WPL
• Chemical dosing: Richard Alan Engineering
• Instrumentation: Park Electrical Systems
• Flow measurement: Siris Environmental Ltd
• Flow measurement: AMPM Flow & Energy Services Ltd
• Penstocks, valves & actuators: Flow Technology Services
• Storm tank cleaning tipping bucket: CSO Group Ltd
• GRP kiosks, FST scrapers & site-wide metalwork: Victoria Engineering
• 6mm bar screen, skip rails & trolleys: Graham Steelwork Engineering
• Potable & FE service water boosters: Dutypoint Ltd
• Lifting equipment: Columbus McKinnon Co Ltd
• Solar panels & EV charger: Solmatix Renewables Ltd

[caption id="" align="alignnone" width="1200"] CAS basins in operation - Courtesy of NI Water[/caption]

Solar

It was recognised that a large area of land within the footprint of the existing WwTW site would be available for use following demolition of old structures. As a result, NI Water asked GEDA Construction and Water Solutions Ireland to investigate the feasibility of installing solar energy within the space available. Following a successful outcome of the feasibility study, it recommended that a 62Kw solar PV array would be installed in the free space which would provide renewable energy to run some of the WwTW operations and boost the sustainability of the site. A 22KW dual EV charger unit was also installed which is used by NI Water’s growing fleet of electric vans. Solmatix Renewables Ltd installed a ground-mounted solar PV panel system consisting of 164 solar panels within the footprint of the old works. New electrical invertors and controls housed within the new MCC kiosk were installed and cabled directly onto the new MCC. The solar system has been set up as zero export, meaning everything that is being generated on site, is used on site through the plant and other electrical equipment. The estimated payback period on NI Water’s £70k investment is 6.5 years (savings of £11,000/year on electricity costs at Ballygowan WwTW). [caption id="" align="alignnone" width="1200"] Solmatix Renewables Ltd solar farm - Courtesy of NI Water[/caption]

Alkalinity

A challenging issue encountered during the design and construction of the new works at Ballygowan WwTW was the high acidic nature of the incoming influent. During review of the design and aided by a sampling regime of the influent under varying flow conditions, it was determined the site required alkalinity dosing to allow the wastewater treatment process to provide adequate nitrification (in other words ammonia reduction) within the biological treatment to meet the 1mg/l ammonia discharge consent. This problem required NI Water and GEDA Construction to design, supply, install and commission a supplementary packaged chemical dosing system to increase the alkalinity levels and achieve a stable pH within the wastewater to safeguard optimum treatment. NI Water shared some past negative experiences encountered with similar alkalinity dosing plants which was invaluable to ensure that a well thought out, comprehensive solution was selected, which would deliver the requirements of supplementary dosing but reduce the ongoing operation and maintenance requirements encountered on other schemes for NI Water. The agreed solution was to provide a new proprietary sodium carbonate (powder) make up and dosing system, all housed within its own GRP kiosk and standalone control panel.

Odour control

Due to the proximity of nearby residential housing, the design and execution of odour containment and treatment was a major consideration for this project. Early odour models and specialist supplier engagement was imperative to ascertain the potential effects of odours. GEDA Construction provided a cost-effective, proven solution through a carbon filter plant dedicated for the inlet screening and grit removal works, and a second Mónashell Biofilter plant from Anua Clean Air UK was provided solely for the sludge treatment and storage tanks. Both systems, along with covering large areas of the works with solid top flooring, safeguard against the negative effect odours can cause. [caption id="" align="alignnone" width="1200"] Ballygowan WwTW nearing completion - Courtesy of NI Water[/caption]

Collaboration and PR

Collaboration was instilled throughout every aspect of the planning, design and construction phases for the new Ballygowan WwTW. NI Water and its project management team developed a proactive communication and PR strategy for project which included erection of storyboards; circulation of project brochure; issuing of regular updates to residents; briefings for elected representatives; circulation of press releases and a number of events and initiatives with the local residents’ association. A collaboration initiated with the local council also saw the creation of a small orchard by local children on an area of disused council land adjacent to the WwTW, which GEDA prepared for planting out.

Outcome

By working in partnership with each other, as well as myriad stakeholders, NI Water, Tetra Tech, GEDA Construction and the wider supply chain were able to successfully overcome difficult challenges to deliver a sustainable, state-of-the-art wastewater treatment solution, capable of meeting its stringent consent standard and serving the community for many years to come.

In line with the Scottish Government’s move towards a Low Carbon Economy, Scottish Water has committed to achieving its strategic ambition of ‘Beyond Net Zero’ by 2040. Driven by this ambition, the public utility tasked its commercial subsidiary, Scottish Water Horizons, to review its asset base to identify and accelerate opportunities for renewable energy generation. Previously, most hydropower projects developed within the water industry have focused on raw and clean water sources. By adopting a different perspective, Scottish Water Horizons identified an opportunity to generate green energy via hydropower at a wastewater treatment works (WwTW).

Background to the project

This pioneering project will see Scottish Water harness hydropower from wastewater and the scheme is:

• The first mid-process wastewater hydropower install in the UK that optimises energy potential.
• The first containerised hydropower install in Scotland which not only reduces construction carbon emissions; it also saves time and costs. This solution can also be replicated on other wastewater assets as well as other applications including raw and treated water.

Design

Hydropower opportunities across Scottish Water’s estate are identified and developed through a combination of data analytics techniques (flow and pressure data in particular), GIS analysis, whilst also tapping into the first-hand knowledge of local system operators.

One of the sites with hydropower potential that emerged from this exercise was Hamilton WwTW in South Lanarkshire, near the city of Glasgow. Hamilton WwTW is a fairly typical site with the flow passing through initial 6mm inlet screening, grit removal, primary settlement, aeration tanks and final settlement unit operations before returning to the local water course which is the River Clyde.

The aeration tanks which are in operation are relatively new to the site, having been installed approximately 17 years ago with the original aeration lanes now treated as a redundant asset.

The new aeration lanes were built above-ground and therefore sit proud of the rest of the works.

[caption id="attachment_10115" align="alignnone" width="1200"] Above-ground aeration lanes - Courtesy of Scottish Water Horizons[/caption]

Hamilton WwTW serves a population of around 57,000 people and therefore has a high electrical consumption. Thus, any electricity generated from the hydro turbine can be utilised on site, helping to green Scottish Water operations and reduce reliance on grid electricity.

After an initial desk-based investigation into the feasibility of the site, on-site operators at Hamilton WwTW were engaged, with loggers deployed to further understand the flow and head parameters. It was determined that capturing the flow between the new aeration tanks and the final settlement would provide 10m of head which is further extended to 11.5m by use of a draft tube, and an average annual flow of 432 l/s.

Through a process of data verification, site surveys, and extensive liaison with both internal and external stakeholders, a clear opportunity was identified to develop a 48 kW hydropower scheme.

Turbine location within the WwTW

By installing the turbine mid-process this allowed Scottish Water Horizons to capture not only the flows through the WwTW but also harness the recirculating flows. This makes the turbine an integral part of the works and therefore every attempt was made to incorporate safeguards to ensure the turbine will not conflict with the overall site operations. This has been addressed by adopting several measures:

• A level sensor has been installed at the aeration tanks to ensure that the turbine does not cause the tanks to either empty or overflow.
• A bypass valve has been designed in which, should the flow be too high for the turbine to function efficiently and safely, the flow will be redirected and continue onto the final settlement tanks uninterrupted. This bypass valve also allows for safe maintenance of the system.
• If a mechanical problem was to occur on the turbine system, the bypass valve will open automatically, therefore imposing no conflict to site operations.

A secondary benefit was realised;  when the effluent passes through the turbine it becomes aerated, thus aiding the treatment process. As a result, the quantity of air required to be used for aeration can be reduced on the existing asset which further reduces the energy consumption on site.

[caption id="attachment_10495" align="alignnone" width="1200"] (left) The original flume and (right) container is on site with thrust restraint in progress on the left - Courtesy of Scottish Water Horizons[/caption]

Varying flow rates

As Hamilton WwTW is an operational site the flow rates are largely influenced by the level of rain in the catchment. Loggers captured the average annual flow rate to be 432 l/s.

The section of the works where the turbine is located is subject to large flow fluctuations varying from 300 l/s to 1200 l/s. This prompted the incorporation of a bleed valve at the inlet to the pipework arrangement which allows excess flows to be diverted away from the turbine, allowing the turbine to continue generating at exceptionally high flow rates.

Effluent considerations

As this is Scottish Water’s first hydropower scheme to be installed on a wastewater asset, there was the aspect of using effluent to be considered. By using the flow from the gravity return pipe at the aeration tank, the effluent had been through several treatment processes such as screening, grit removal and primary settlement to remove the bulk of solids as well as through the aeration process itself. At this point in the treatment process the reactivated sludge and water combination will be denser than fully treated effluent therefore may boost the turbines power output.

Scottish Water Horizons had to consider that finer particles may find their way into the system, and it was important to understand the implications this could have on the turbine and use this knowledge to select a suitable turbine design choice.

Turbine design choice

A Crossflow turbine was selected as the most suitable machine for Hamilton WwTW.

[caption id="attachment_10110" align="alignnone" width="1200"] Crossflow turbine from CINK Hydro-Energy k.s. - Courtesy of Scottish Water Horizons[/caption]

This cost-efficient turbine solution allows for dependable operation over a wide range of flows whilst maintaining efficiency^[1]. The turbine is also capable of ‘self-cleaning’ to an extent. If debris becomes stuck on the blades, it is typically dislodged by either the outflowing water or the centrifugal force generated by the turbine rotating. This addressed concerns about using effluent to power the turbine as the maintenance risk of the turbine becoming clogged with debris such as rags has been partially mitigated against.

A Crossflow design turbine also requires lower maintenance due to the simplicity of the system with only three moving parts. It is typically only the rotor that has a bespoke design to match the hydraulic profile of the application.

Hamilton WwTW Hydropower: Supply chain - key participants

• Design & project delivery: FES Support Services Ltd
• Specialist hydro consultant: Glen Hydro
• Civil works: Calforth Construction Ltd
• Mechanical installation: Dustacco Engineering Ltd
• Turbine supplier: CINK Hydro-Energy k.s.
• Valves: Glenfield Invicta

Construction work

Construction started on site in April 2023 with the removal of the existing flume taking place in May. A temporary flume was then installed to allow the effluent to flow past the working area. This mitigated the risk of contractors encountering effluent when working.

[caption id="attachment_10112" align="alignnone" width="1200"] (left) Old flume removal and (right) temporary flume in situ - Courtesy of Scottish Water Horizons[/caption]

Works then started on the concrete sub-structure and tail-race upon which the turbine house is installed.

The existing pipework was retrofitted with a bespoke ‘Y’ section of pipe along with relevant bypass and bleed valves. A level sensor was installed at the outlet to the aeration tanks which communicates with the turbine control system adjusting the valves and turbine guide vanes to match the flow conditions.

A pre-fabricated containerised turbine house

Once the concrete foundations were completed the containerised turbine house was then delivered to site; the pre-fabricated unit having been assembled by CINK at their factory.

The container was crane lifted into place. Adopting off-site construction methods allowed Scottish Water to reduce its carbon footprint associated with construction and condensed time on site.

Once the container was in situ and all pipework installed, testing and commissioning began. This included pipeline stress tests and multiple mechanical and electrical tests on the equipment to prove the system functioned as the manufacturer intended.

This construction approach has an eye on standardisation which could lead to wide ranging applications within Scottish Water’s assets based on the lean principle of ‘minimum viable product’. This includes minimising capital and carbon expenditure whilst maximising energy recovery.

[caption id="attachment_10111" align="alignnone" width="1200"] Bespoke ‘Y’ piece in situ showing bypass and manual isolation valves - Courtesy of Scottish Water Horizons[/caption]

Conclusion

The system was connected to the grid (G99 accreditation) at the end of November 2023 and is now generating on site, providing green electricity to the works. This helps to not only reduce the reliance on importing electricity from the grid but also reduces on-site energy costs.

The scheme is estimated to generate 420,000 kWh of green electricity per year which will offset around 14% of the site’s annual energy demand. It is also projected to save around 64 tonnes of carbon emissions each year.

Re-using existing land and assets also meant a reduction in carbon emissions during construction, supporting Scottish Water’s strategic ambition of going ‘Beyond Net Zero’.

From conception to starting on site, the Hamilton Hydropower project took around 21 months, with a construction period lasting a further nine months. This shows the efficiency of the containerised methodology which will enable future similar schemes to be delivered at pace.

This project has demonstrated that hydropower generation can be achieved on a wastewater asset and can offset the heavy cost of electricity on wastewater sites. There are many similar opportunities available across other wastewater sites and other industries such as petrochemical and outfalls from power plants. This would drive down the carbon associated with capital investment and operational activities.

References

^[1] CINK Hydro Energy, “Crossflow Turbines,” [Online]. Available: https://www.cink-hydro-energy.com/crossflow-turbines.

Cardiff and Afan WwTWs are in South Wales and are owned and operated by Dŵr Cymru Welsh Water. The Cardiff WwTW serves a population equivalent of 974,869 with a flow to full treatment of 6,060 l/s, and Afan WwTW located on the TATA Steelworks site at Port Talbot, has a flow to full treatment of 2,423 l/s - a combined flow capable of filling an Olympic size swimming pool in just five minutes. At both sites, surplus activated sludge is pumped to a sludge treatment building where it is mixed with polymer, thickened, and then pumped (along with imported sludge) to a Cambi advanced aerobic digestion (AAD) process for further treatment. Methane produced in the Cambi AAD is used in the CHP engines to produce electricity.

Background

The CHP plants at both Cardiff and Afan treatment works were constructed in 2010, however the CHPs and associated ancillary assets at both sites face continued degradation, exposing Operations to volatile energy prices and increased maintenance costs. These assets are not currently performing to their designed capability, meaning the potential energy recovery savings are not being fully realised.

Samples of the biogas entering the CHPs have highlighted that the siloxane composition is at unacceptable levels, causing a risk to the CHP condition. The scope will include for replacing the existing siloxane plant with a new carbon filter which will increase delivered pressure of biogas to the CHPs and boilers to help increase performance.

[caption id="attachment_13304" align="alignnone" width="1200"] (left) Cardiff WwTW and (right) Afan WwTW - Courtesy of Google Earth[/caption]

Project drivers

Morgan Sindall Infrastructure was appointed as principal contractor and principal designer to successfully deliver a scope of work at both sites to meet the following main drivers for the project:

• Increased CHP plant availability.
• Ability to run all CHP engines and boilers while using the gas clean-up equipment.
• Operational health & safety: the ability to work safely on boilers while the associated CHP boiler is running.
• Reduce energy and maintenance costs.
• Operational health & safety at Afan WwTW: relocate the CHP exhaust bursting discs to outside of the operational building.

Being one of the designers working for Morgan Sindall Infrastructure on Welsh Water’s AMP7 framework, and because of their previous experience on similar projects, Arcadis were selected by Morgan Sindall as the design consultants for the Cardiff and Afan WwTWs CHP Optimisation Project.

Due to the commonality between the two sites, both projects were developed using the same principal contractor, design consultants, sub-contractors, and supply chain. This helped realise significant design and programme savings across the two sites.

CHP process description

For both sites, at each CHP engine the hot exhaust gases are directed to a dedicated composite steam boiler to recover heat. Each CHP package includes a biogas booster pump which is used to increase the pressure of the biogas supplied to the CHP engine.

[caption id="attachment_13326" align="alignnone" width="1200"] ADD process diagram - Courtesy of Dŵr Cymru Welsh Water[/caption] The pipework from the biogas booster pump to the container includes connections for a gas analyser and an in-line gas meter, as well as a safety slam-shut valve. The gas analyser monitors the levels of methane, carbon dioxide, oxygen, and hydrogen sulphide in the biogas. The readings are used by the CHP control system to adjust the engine performance, or to trip on high levels. The in-line gas meter monitors the flow rate of biogas to each CHP engine and the gas slam-shut valve is used to isolate the container in the event of a gas leak or fire alarm.

Design solution for both sites

• Removal of siloxane plant and installation of carbon gas cleaning.
• Chiller installation to include:
• Shell and tube heat exchanger.
• Recuperator.
• Lagging to exchanger and recuperator.
• Inlet and outlet temperature probes.
• Condensate pot (drained locally).
• Parker WFB2380CSB3180A-316 biogas cooler/separator.
• Parker ICE183B4003501P15FF0LA000 biogas process chiller.
• Parker HDF220BE biogas float drains.
• Gas booster to ensure boiler and combined heat and power operation, to include:
• Control panel booster and chiller with SCADA access and HMI.
• Gas booster motor 1 and 2 starters.
• VSD control.
• Pressure and temperature protection.
• Pipework and manifolds.
• Interconnecting pipework between the booster set and chiller including valves and purge points.
• Safety measures to enable a CHP to be operated and allow boiler safe isolation.
• Gas flow meter installation (on each digester).
• Afan Works only: Relocation of the CHP exhaust bursting discs to outside the existing boiler house.

[caption id="attachment_13308" align="alignnone" width="1200"] New biogas booster set install at Cardiff WwTW – courtesy of Morgan Sindall[/caption]

Design figures

• Maximum biogas throughput:
  • Cardiff WwTW: 3,000 Nm³/hour.
  • Afan WwTW: 2,500 Nm³/hour.
• Assumed biogas composition:
  • CH₄/CO₂/N₂/O₂): 65/35/0/0 %.
• Assumed biogas methane content range: 60-70%.
• Dehumidified gas temperature from chiller: 5⁰C.
• Combined siloxane level required following carbon filter treatment: 10 ppm.

Supplier selection

Early in the project it was recognised that the management of multiple key suppliers and lead-in times of up to six months were two key risks to a successful delivery. Selecting the same suppliers, contractors and designers across both projects, together with a clear collaborative approach from the start helped mitigate these risks.

1. Endress+Hauser was selected as suppliers of the biogas flow meters due to their proven accuracy at a similar project completed at Five Fords WwTW in North Wales.
2. CC Jensen Ltd was selected as a specialist carbon filter supplier they based on their work on previous CHP installations.
3. Dunphy Combustion Ltd was selected based on their technical capabilities and previous performance at Cog Moors WwTW.
4. Uniflare Group was selected based on programme delivery and the previous work undertaken at Cog Moors WwTW.

[caption id="attachment_13305" align="alignnone" width="1200"] Installation of foam probe and suppression nozzles at Cardiff WwTW - Courtesy of Morgan Sindall[/caption]

Cardiff & Afan CHP Optimisation: Supply chain - key participants

• Principal contractor & designer: Morgan Sindall Infrastructure
• Consulting designer: Arcadis
• Mechanical installation & biogas scrubber supplier: CC Jensen Ltd
• Electrical installer: Protocol Control Systems Ltd
• Biogas flow meter installation: Marches Biogas Ltd
• MCC modifications: GPS Group
• Systems integration: GPS Links
• Biogas boosters & chillers supplier: Uniflare Group
• Steam boiler dampers & ducting supplier: Dunphy Combustion Ltd
• Biogas flow meter & control panel: Endress+Hauser

Project benefits

As part of its response to climate change, and to reduce greenhouse gas emissions, Welsh Water have invested in a renewable energy generation and energy efficiency portfolio, split as follows:

• 45% hydro electric
• 45% Combined heat & power
• 5% wind energy
• 5% solar energy

Source: latest figures taken from dwrcymru.com - June 2024 [caption id="attachment_13309" align="alignnone" width="1200"] Installation of new Endress+Hauser prosonic gas flow meters at Cardiff - Courtesy of Morgan Sindall[/caption]

At Cardiff WwTW, the current average power generation is 20.25 GWh, and at Afan WwTW is 10 GWh. The target to achieve a 3 GWh improvement at each site will generate a combined revenue of £930,000 per annum and contribute significantly to Welsh Water’s renewable energy totals while reducing greenhouse gas emissions. The carbon benefit is assessed to be approximately 1,000 tonnes/year for both sites combined.

Project status

At the time of writing (June 2024), both projects are progressing well, and at Cardiff WwTW the civils works are now complete. The mechanical installation is expected to be complete by the end of June, with handover back to Welsh Water targeted for August 2024.

The Afan project trails Cardiff, providing the opportunity to benefit from any lessons learnt during the Cardiff installation. The work at Afan WwTW is scheduled to be commissioned in August and handed back to the Welsh Water to operate in October 2024.

These projects will meet all the client drivers to improve operational health & safety, reduce energy and maintenance costs, and increase CHP availability and output. The carbon benefits will also contribute to Welsh Water’s overall carbon reduction strategy and journey to net zero.

Between 2015 and 2020, Affinity Water reduced abstraction from chalk groundwater sources by 42 million litres per day (MLD) and aims to cut an additional 36 MLD by 2025. These efforts are crucial for sustainably providing high-quality drinking water to their customers. While Affinity Water does not manage sewage, it plays a key role in maintaining the ecological health of rivers by reducing water abstraction and making other environmental improvements. To protect chalk streams and their wildlife, Affinity Water is importing more water from Anglian Water’s Grafham Water Reservoir. Affinity Water has acquired Sundon Reservoir, an underground clean water facility reservoir near Luton, originally built by Anglian Water. As part of the plans, a treatment facility was constructed to adjust the Larson-Skold Index and free chlorine levels, as well as refurbishing the reservoir with a dividing wall for improved resilience.

Purpose of the new water treatment facility

The treated water from Grafham Water Treatment Works (WTW) has a different water chemistry to that of the vast majority of treated water in Affinity’s network.

The Larson-Skold Index (LSI) is an empirical scale that measures the corrosivity of water towards mild steel. During an early feasibility study, it was noted that the LSI of the water from Grafham will be much higher than that of water treated by Affinity. Therefore, if Affinity Water is to import such an increased amount from Grafham WTW to Sundon, then further water treatment would  be required.

Reservoir additions and modifications

The first phase of this project saw some work being undertaken on the existing concrete reservoir structure. The tank was bypassed and drained, with work being carried out in off-peak months as to make the most of the lower demand period. A reinforced concrete in situ dividing wall was constructed down the centre of the tank to split the single cell reservoir into two. This provides storage resilience during decennial inspections.

[caption id="attachment_14285" align="alignnone" width="1200"] Sundon Reservoir and Water Treatment Facility - Courtesy of Ward & Burke[/caption]

The incoming twin mains were brought together and a new DN800 ABB flow meter was installed. Tapering up to DN1200, the main was brought above ground into the new building before diving back down and on towards the reservoir. Existing inlet, outlet, overflow and washout pipes were reused for one cell, while new ones were installed for the second cell.

Other modifications included new access hatches, ladders, pipe hop-overs and overbanding on existing concrete joints. A baffle curtain was installed in each cell to channel the water down to one end of the reservoir and back before leaving, creating the contact time required for the chlorine to reach ‘breakpoint’ (see the following section).

As such, the reservoir used for storage as a primary purpose, was also being used as a contact tank. Prior to returning the cells to service, the structure was inspected, hydrostatically tested, thoroughly cleaned and subsequently sampled.

Chemical dosing

Sodium hypochlorite dosing: There are two components to the dosing of sodium hypochlorite:

1. To destroy the chloramines (breakpoint).
2. To leave a free chlorine residual in the outgoing water.

The dosing is calculated using the results from the import water quality analysers; Hach’s ammonia analyser and Swan Analytical’s free and total chlorine analysers. To determine the free chlorine levels after breakpoint chlorination had been reached, a vessel was used to simulate 30-minute old water before being analysed. This 30-minute value was arrived at experimentally through a bench test in outline design. Finally, the calculated dose per litre of water is then multiplied up to be flow proportional using the new flow meter.

[caption id="attachment_14281" align="alignnone" width="1200"] Water quality analysers - Courtesy of Ward & Burke[/caption]

Bulk quantities of sodium hypochlorite at 14-15% w/w concentration are stored in vertical cylindrical tanks, manufactured by Forbes Technologies Ltd. The tanks are manufactured from GRP with a uPVC lining inside, with typical connections for inlet, outlet, drain, vent and overflow. Pressure instruments installed at low level are used for chemical level readings as to remove any working at height. The duty tank is selected using an actuated valve on the tank’s outlet, taking the chemical into the dosing skid.

Grundfos manufactured the dosing skid for this project. Two duty/standby configured Grundfos DDA120 smart pumps are used in a standard dosing skid configuration with components including, but not limited to, pulsation dampers, loading valves and PRVs.

Duty/standby outlets are provided for two dosing hoses that connect the dosing skid in one building, to the point of application (POA) cabinet in another building. The dosing hoses used, stretching a distance of around 50m, is PF Detect dual contained, leak detection hosing from FT Water Treatment. This hose-inside-a-hose has two copper wires in a helix sandwiched between the two hoses. In the case of the internal carrier hose bursting, chemical leaks into the gap between hoses and by capillary action, fills the gap until a short is created across the two copper wires, indicating a fault at the control panel. This automatically switches to the standby dosing line immediately, preventing any further leakage. Note that the leakage is contained thanks to the outer hose’s extremely high burst pressure. This has allowed for transfer of chemicals through ducts over a great length without using catchpots and rigid outer piping.

The POA cabinet is situated adjacent to the static mixer on the water main. It is of typical construction with a loading valve and flow switch. Finally, withdrawable injection lances are used in the DN1200 static mixer to dose the sodium hypochlorite into the water main.

[caption id="attachment_14287" align="alignnone" width="1200"] Chemical injection at static mixer - Courtesy of Ward & Burke[/caption]

Sodium hydroxide dosing: The Larson-Skold Index (LSI) is an empirical scale to measure the corrosivity of water towards mild steel. This is an issue for Affinity’s network that has not seen such a high LSI as it can lead to pipe deterioration and water discolouration. It is a ratio of sulphate and chloride content, to alkalinity content (in the form of bicarbonate and carbonate).

The sulphate and chloride content of the import water is measured using some of Hach’s EZ4000 series analysers, and does not change very much day on day. As such, in order to reduce the LSI, alkalinity is increased by the addition of sodium hydroxide. The alkalinity is measured at the import, as well as after dosing, using a Hach EZ4000 series also. A trim function is coded in to bring the dosing closer to the set-point if it is slightly out. As a bi-product of this dosing however, the pH of the water would increase, and this is where carbon dioxide dosing comes in (see following section).

The sodium hydroxide is stored in bulk quantities at a concentration of 50% w/w. Similar tanks are used, however are of HDPE PE100 construction, also manufactured by Forbes.

The connections and instrumentation are identical to that of the sodium hypochlorite tanks, with the addition of a temperature transmitter and heater. Sodium hydroxide at this concentration has a freezing temperature of +7°C and as a result, the tanks’ contents are maintained at 25°C. The temperature transmitter informs of any drop in temperature before it is too late. It should be noted that the tanks’ stainless-steel fill and outlet lines were all heat traced and lagged also.

Grundfos also manufactured the dosing skid for this chemical. Three DME940 pumps were used in a duty/assist/standby configuration, along with the usual components as above. The same PF Detect hosing was used to get the chemical to the POA cabinet and these lines were heat traced and lagged. Furthermore, the dosing skid and POA cabinet each contained heaters.

Components in the POA cabinet were identical to that of the sodium hypochlorite, with withdrawable stainless steel injection lances in the static mixer to complete the dosing.

[caption id="attachment_14286" align="alignnone" width="1200"] (top left) Sodium hypochlorite storage and dosing, (top right) sodium hydroxide storage, (bottom left) sodium hydroxide dosing, and (bottom right) point of application cabinets - Courtesy of Ward & Burke[/caption]

Carbon dioxide dosing: Working in tandem with the sodium hydroxide dosing, the gaseous carbon dioxide is dosed into the water to bring the pH back down to the set-point. This means that both dosing systems must be working at the same time. It should be noted that the carbon dioxide does not affect the LSI at all.

As with most industrial gas use, the storage vessels are rented from the gas supplier. Nippon Gases are the supplier of carbon dioxide for this project. Delivery and storage is in liquid form, with the storage vessels being insulated with a vacuum layer, akin to a Thermos flask. Vaporisers are also rented from Nippon Gases UK and convert the liquid carbon dioxide into gaseous form for dosing. The gas is heated using trim heaters before the pressure is reduced, prior to going through flow control valves and a flow meter to control quantities for dosing.

Sundon Water Treatment Facility: Supply chain - key participants

• Client: Affinity Water
• Principal designer & contractor: Ward & Burke
• CFD analysis: Wilde Analysis
• Precast concrete: Shay Murtagh Precast
• Electrical installation: Enisca Ltd
• System integration: Riverside Automation
• Concrete repair: Concrete Repairs Ltd
• Stainless steel pipework: ABC Stainless Ltd
• Dosing skids & pumps: Grundfos
• CO₂ storage & vaporisation: Nippon Gases
• Water quality monitors: Swan Analytical UK Ltd
• Water quality monitors: Hach
• Water quality monitors: ABB Ltd
• MCC & panels: Motrol (now Profitec)
• Static mixer: Statiflo International
• Permanent generator: Bells Power
• Solar panels: GeoGreen Power
• Chemical filling kiosk: Northern Pump Suppliers
• Chemical storage tanks: Forbes Technologies Ltd
• PF Detect dual contained hoses: FT Water Treatment
• DI pipework: Saint Gobain PAM UK
• Heat tracing & lagging: Jade Insulation
• Heating & ventilation: Air Technology Systems Ltd
• Lightning protection: Bailey International

New buildings & compound

Two new buildings were constructed:

1. The chemical building to store chemicals and dosing equipment.
2. The control building to house the static mixer, water quality monitors and main MCC.

[caption id="attachment_14282" align="alignnone" width="1200"] Chemical delivery area & carbon dioxide vessels - Courtesy of Ward & Burke[/caption]

Despite their differences in size, both buildings were constructed with an in situ RC base, with precast concrete columns, beams and wall panels. Purlins in the chemical building were precast too. The precast concrete avenue was first explored due to a requirement to construct buildings with a 60 year design life. Due to the nature of the off-gassing of sodium hypochlorite degrading metals over time, a steel portal frame construction was not an option. Since concrete walls were required for satisfying security ratings anyway, the precast concrete route was a perfect option. An additional benefit of this design for manufacturing and assembly (DfMA) approach was the considerable reduction of on-site construction risks related to working at height and handling of tall and heavy rebar cages. Other notable benefits included fewer concrete wagons on local roads, less material waste and reduced construction time.

Both buildings were clad with insulated Kingspan cladding to the planning permission colour and security rated doors supplied by Bradbury were installed. Heating and ventilation systems in all rooms were designed and installed by Air Technology Systems, with duty/standby fans in rooms with critical ventilation requirements. Finally, lightning protection systems were installed by Bailey International on all buildings, as well as other plant at risk on site.

A fenced slab was constructed for all the carbon dioxide equipment in proximity to the road for deliveries. Outside the chemical building is the chemical delivery area for the sodium hypochlorite and hydroxide. It features a security rated fill kiosk with connections for each of the tanks, a tepid water emergency shower and blind tank with an interlock valve. This valve’s purpose is to isolate the rainwater drainage system during a tanker discharge to contain any spill. The valve must be shut to release a key to open to the fill kiosk door to prevent human error.

Electrical

A new MCC with ICA section was supplied by Motrol (now Profitec) in collaboration with Enisca Ltd completing the electrical installation. Said MCC was housed in a dedicated room in the control building with a trench beneath it, cast in situ during base construction allowing for easy and tidy bottom cable entry. A remote IO (RIO) panel was installed in the chemical building with an HMI to monitor and control the process.

[caption id="attachment_14280" align="alignnone" width="1200"] New MCC - Courtesy of Ward & Burke[/caption]

With Anglian Water owning the pumping station feeding Sundon, there was a focus on ensuring raw signals of reservoir level were provided to both water authorities. In contrast, any other interfaces were minimised as far as reasonably practicable to expedite any future maintenance or works that would have previously required both company’s inputs.

Solar panels were installed to provide power to the MCC at all times, with any surplus required made up by mains power. In the event of a mains power outage, an auto-changeover in the MCC will call on the permanent generator to kick in to keep the site running until mains power is restored.

Commissioning

As of August 2024, the plant is in its commissioning phase, with chemical dosing being brought online after extensive offline testing. In the coming months, Affinity Water will focus on fine-tuning and optimising the plant’s chemical usage to meet network requirements.

Finham STW, situated near Coventry in the West Midlands, is Severn Trent’s fifth largest wastewater treatment works and treats over 115 megalitres of effluent a day from half a million people. The works processes indigenous sludge on site as well as accepting exports from local, satellite sites, producing final effluent, ‘cake’ and biogas as outputs. The two separate elements at Finham comprised a Thermal Hydrolysis Plant (THP) and a Gas to Grid (G2G) project, which were delivered as a single advanced digestion scheme. Sludge is anaerobically digested to produce biogas which, prior to the advanced digestion scheme, powered two, 60-cylinder, combined heat and power (CHP) engines providing recirculated heat for digestion and electricity for the works. Following a competitive tender, the design and build contract for the purpose-built, brand-new, advanced digestion scheme was awarded to Costain by Severn Trent in 2019, with construction starting on site in early 2020.

Overview - Gas to Grid Project

The gas to grid (G2G) phase of works refines biogas from the digesters through a biogas upgrade unit (BUU) and blends it with propane to ensure the correct calorific value for export to the national gas grid. This phase was complete first with a sectional completion date of March 2021, determined by the Ofgem industrial Renewable Heat Incentive (RHI) scheme deadline. The first export of biogas to the Cadent Gas Network was successfully delivered by the key milestone and ensured that Severn Trent secured RHI-funded unit rate for its gas export. [caption id="" align="alignnone" width="1200"] Gas to grid process diagram - Courtesy of Costain[/caption]

Overview - Thermal Hydrolysis Plant

The thermal hydrolysis plant (THP) phase of the project was around 75% of the total value of the scheme and constructed alongside the G2G initially, before continuing to completion once the G2G section was handed over. The process utilises a Cambi Ltd designed THP which uses steam from two-gas boilers to hydrolyse sludge produced by the primary settlement tanks (PSTs), activated sludge plant (ASP) and imported sludge from regional sewage works around the midlands. This process increases the gas yield of the sludge when digested to increase G2G throughput. Process commissioning started in January 2022 and contractual completion was in July 2022. [caption id="" align="alignnone" width="1200"] Thermal hydrolysis plant process diagram - Courtesy of Costain[/caption]

Gas to Grid Project

Digesters, boosters and gas holder Biogas is produced by sludge digesting in 8 (No.) digesters on site. The pressure is regulated in each digester with a ‘floating roof’ which rises and falls with the volume of gas present. The project saw 6 (No.) boosters installed, 3 (No.) 1000m³/hr boosters to transfer the biogas from the digesters into the gas holder between 15-25mBar and 3 (No.) to provide a biogas feed to the boilers. A 2700m³ inflatable gas holder was constructed on a 400t concrete suspended slab to store biogas and provide the BUU with a consistent flow and pressure of gas. BUU, GEU and ROV The biogas upgrade unit (BUU) uses membrane technology to refine the biogas and export it to the grid. The BUU was procured from Italy and required careful management to navigate the international challenges posed by COVID-19 and Brexit. The BUU has a throughput flow rate of 720m³/h to 1800m³/hr. The upgraded gas is then sent through the grid entry unit (GEU) which monitors the characteristics of the gas and rejects non-compliant gas. The compliant gas is then exported to the gas grid through the remote operated valve (ROV) kiosk which is operated by the gas utility, Cadent, down 800m of newly laid medium pressure gas main in the highway which was installed using horizontal directional drilling (HDD) to minimise impact to the public. [caption id="" align="alignnone" width="1200"] (left) Inflated gas holder and (right) biogas upgrade unit and carbon storage silos - Courtesy of Costain[/caption]

Thermal Hydrolysis Plant

Sludge The THP process uses a blended mixture of sludge from surplus activated sludge (SAS) from the aeration settlement plant (ASP), primary sludge from the primary settlement tanks (PSTs) and liquid imports from satellite sewage works. Sludge ‘cake’ is also imported into the new THP cake bunker where it feeds directly into the THP silos. The blended sludge is transferred by progressive cavity pumps via 2 (No.) duty/duty 220m long buried sludge mains. Screening The transferred sludge is screened through 3 (No.) Huber Technology Strainpresses with 6mm screens that capture and remove solids in the sludge to prevent wear and tear damage to the THP process. Screenings are dropped from the elevated platform into 2 (No.) roll-on-roll-off skips ready for incineration off site. The elevated strain presses were craned onto the steel frame which was assembled in 3 days. The screened sludge in then moved into two, 1200m³ buffer tanks that keep the sludge moving to prevent settlement before it is dewatered. [caption id="" align="alignnone" width="1200"] (left) Sludge transfer pumps and (right) elevated strain press in front of the buffer tanks - Courtesy of Costain[/caption] Dewatering The screened sludge is dewatered from approximately 3% to 20% dry solids for transfer to the THP feed silos. 3 (No.) centrifuges work in a duty/duty/standby arrangement and backwash with final effluent (FE) as required. THP & boilers The cake is diluted with FE to 16% dry solids and fed into one of the 2 (No.) THP streams which process up to a maximum of 60 tonnes of dry solids per day (tds/day) per stream. The sludge is heated to around 95°C in pulper to become ‘homogenised’. This sludge is then pumped sequentially through each of the streams 4 (No.) reactors for batch treatment. Steam from the boilers is used to heat the sludge between 50-180°C where it is held at 6-bar pressure to undergo pasteurising hydrolysis for around 30-45 minutes. After hydrolysis, sludge is passed into the flash tank, where the pressure and temperature of the hydrolysed sludge are decreased by flashing steam back to the pulper. The flashtank provides the necessary thermal buffer capacity to release the excess energy contained in the sludge before entering the downstream pumps. The process kills all pathogens and, in turn, increases the potential gas yield of the sludge. 2 (No.) boilers, procured from Italy, produce 192°C steam from potable water, operate at 12bar and can run on either natural gas imported from the grid or biogas produced from the gas to grid plant. 2 (No.) CHP engines are situated locally to the boilers and the exhaust gas heat is recovered by boilers to optimise performance, these CHP engines can also run on either natural or biogas. The boilers are operated by BOAS accredited operators who were transitioned across from Costain to Severn Trent upon handover. The warm hydrolysed sludge is then blended with UV FE to reduce the temperature to around 42°C where it is fed back into the digesters via a bank of 8 (No.) sludge return pipes traversing 500m across the site footprint. [caption id="" align="alignnone" width="1200"] (top left) Centrifuges, (top right) THP silos, (bottom left) the boiler and (bottom right) the boiler house - Courtesy of Costain[/caption] UV treatment and sludge cake A UV plant was constructed to ensure no pathogens were introduced to the sludge post-THP which would normally be present in non-UV treated FE. This ensures that the biosolids product produced by the works is ‘enhanced’ quality and can be sold to farmers to fertilise fields growing a wider range of crops than those which can have ‘standard’ biosolids. A pair of Bollfilters with 25µm strainers from Bollfilter UK Ltd screen the FE prior to UV treatment to ensure that stringent disinfection standards are achieved and to prevent particulates damage to the plant. Other THP highlights The THP installation also included a polymer dosing plant, FE and potable break tanks, odour control unit, deep drainage and centrate pump stations with 700m of buried return pipework to head of works, a new FE pump station and a 1200m³ cake bunker. [caption id="" align="alignnone" width="1200"] THP reactors - Courtesy of Costain[/caption]

Finham STW Advanced Digestion Plant: Supply chain - key participants

• Design & build: Costain
• Geotechnical investigation: Geotechnics Ltd
• Digester to G2G pipework: Alpha Plus Ltd
• Gas import/export pipe: Morland Utilities Ltd
• Mechanical installation: HL Engineering Contractors Ltd
• FE pump station: Arthur Civil Engineering Ltd
• G2G mechanical installation: Field System Design Ltd
• Pre-THP mechanical installation: Balmoral Ltd
• LV electrical installation: AVRS Systems
• Roads: Hercules Site Services
• Groundworks & FRC: Costain
• Trace heating & lagging: Thermal Design Ltd
• Thermal hydrolysis plant (THP): Cambi Ltd
• Biogas upgrade unit: Air Liquide UK Ltd
• Odour control unit: Air-Water Treatments Ltd
• Bollfilters: Bollfilter UK Ltd
• Gas holder: Power Plastics Ltd
• Centrifuges: Alfa Laval Ltd
• Buffer tanks: Balmoral Tanks Ltd
• MCCs, controls, switchboards etc: CEMA Ltd
• CTM imported cake reception silos & hoppers: CTM Systems Ltd
• GRP kiosks: Industrial GRP Ltd
• UV pathogen control: Lintott Control Systems Ltd
• Lightning protection & earthing: Omega Red Group
• Grid entry unit (GEU): Orbital Gas Systems
• Polymer storage & anti-foam dosing: Richard Alan Engineering Ltd
• Indigenous sludge screens: Huber Technology
• Indigenous buffer tanks air mixing: Stem Drive Ltd
• Lifting equipment: T Allen Engineering Services Ltd
• Steam boilers: Thorne Industrial Boiler Services
• Flare & G2G boosters: Uniflare Ltd
• Boiler & THP operational support: PBJ Engineering Services Ltd

Construction

The vast proportion of the civil installation was self-delivered, this allowed for flexibility in the installation programme to align with constantly changing delivery and subcontract dates affected by COVID-19 and material shortages. The delivery team consisted of over 100 operatives at its peak, covering a range of trades. The construction phase included the installation of over 3.4km of buried drainage, 3 (No.) cofferdams and over 200 crane lifts. The mechanical plant was largely designed for manufacture and assembly (DfMA) off site and was able to be delivered, lifted in and installed in a relativity short period of time. One of the highlights was the installation of the 175 tonne THP which was installed in 10 working days. [caption id="" align="alignnone" width="1200"] (left) Cake bunker cofferdam and (right) cake bunker - Courtesy of Costain[/caption]

Health & safety

The safe delivery of the project was extremely successful and managing the interface between multiple contractors working in a small footprint was priority throughout. The delivery and commissioning phase saw over 490,000 hours worked with a AFR (Accident Frequency Rate per 100,000 hours worked) of 0.00. A total of 2,315 different people contributed to the scheme and over 3,200 hazard and safety observations were raised. The design team were proactive in holding hazard and operability (HAZOP) reviews with the client from the outset of the project, using the integrated 3D design model to identify potential challenges in a virtual environment prior to construction. Due to the volatile nature of substances on site, stringent safety measures were adopted in line with the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). Procedures put in place during G2G commissioning and handover benefitted from cross-sector learning and Costain specialists from the Energy sector helped shape the measures put implemented. [caption id="" align="alignnone" width="1200"] CHP engines- Courtesy of Costain[/caption]

Commissioning and handover

Commissioning of the G2G took place in 2021 with the first export to the grid in March 2021 before the Government’s commercial RHI deadline. The gas quality standard is set by the end user, Cadent, who dictate a specific calorific value that the gas must be for export. This is monitored by the grid entry unit (GEU) which recirculates non-compliant gas back to the gas holder to be refined further. This refining process was complex and required close collaboration between the client and contractor. The gas to grid achieved sectional handover to the client first and was taken over whilst construction continued the THP. The THP was commissioned in 2022 and required complex commissioning to convert the digesters from conventional sludge to hydrolysed sludge. This was done in a phased approached, whilst closely monitoring the biology of the digesters, over the course of 3½ months. The health of the digesters directly impacts the sludge feed rate through continued monitoring of several factors including % dry solids, pH, ammonia content etc.

Biodiversity

Planning conditions set out a requirement to create a mitigation area to enable net gain in biodiversity on the Finham site. The client’s solution was to establish a wildflower meadow at least equal in area to the new hardstanding constructed to promote a thriving insect and wildlife environment. Whilst Severn Trent were considering the logistics and cost of how to fulfil the planning requirements, Costain’s self-delivery team were busy excavating over 15,000 tonnes of muck for the THP build. [caption id="" align="alignnone" width="1200"] Muck shift in progress - Courtesy of Costain[/caption] Costain worked collaboratively with Severn Trent to draft plans for the reuse of the muck in the proposed BIA and began the process of sampling and testing the muck. A comprehensive Materials Management Plan approved under the CL:AIRE regulations was produced detailing the proposal. Upon approval from all parties, the muck shift operation began. Costain operated three 30 tonne articulated dumpers (ADTs) to transport the muck across the site. Each dumper averaged 25 trips per day and the full operation was completed in just a little under two weeks. The overall benefits were:

• 15,000t of waste product eliminated.
• No requirement for Severn Trent to import material.
• Net saving after labour and plant costs.
• Reduced carbon emissions by 76.2 tonnes.
• Reduced road traffic by up to 1000 lorry journeys.
• Site participation in World Environment Day.
• Contractor/client collaboration and mutual benefit.
• Long lasting environmental legacy.

[caption id="" align="alignnone" width="1200"] Finham THP & G2G - Courtesy of Costain[/caption]

Project awards

The advanced digestion project has been nominated for several construction awards including the Constructing Excellence Sustainability Award, the Pipeline Industries Guild Award, and was successful in winning the NEC Sustainability & Climate Resilience of the Year award 2021. Department for Business, Energy & Industrial Strategy (BEIS) also visited the project during its construction to showcase the Government’s investment in domestic, renewable energy generation.

In recent years, the village of Boherbue in north-west Cork has undergone considerable expansion, resulting in increased demand for housing. This growth has presented challenges, particularly for the existing wastewater treatment facility, which has struggled to cope with the surge in population. Additionally, the plant has fallen short of the Environmental Protection Agency’s treatment standards, underscoring the pressing need for an upgrade. Of particular concern is the discharge of treated wastewater into the River Brogeen, a protected conservation area that serves as the habitat for the freshwater pearl mussel. In collaboration with Uisce Éireann (Irish Water) and Cork County Council, a comprehensive project has been undertaken to modernise and enhance the capacity of the treatment plant. This initiative not only addresses the immediate needs of Boherbue’s growing community but also ensures compliance with environmental regulations.

Early Contractor Involvement

The project, delivered under the innovative Early Contractor Involvement (ECI) framework, aimed to address the overloaded and outdated infrastructure while ensuring compliance with regulatory standards and meeting the evolving needs of the community. The innovation in this project lies in its multifaceted approach to efficiently treat wastewater, reduce carbon emissions, and protect natural ecosystems.

Leveraging the ECI framework allowed for the integration of innovative solutions such as solar energy, natural sludge drying reed beds, and the preservation of existing constructed wetlands for stormwater overflow. This innovative approach not only sets a new standard for sustainable wastewater treatment but also ensures long-term environmental benefits for Boherbue and its natural surroundings.

[caption id="attachment_16527" align="alignnone" width="1200"] Google Earth image of Boherbue[/caption]

The scope of works for the upgraded facility included:

1. A new inlet works.
2. A stormwater storage tank.
3. Biological treatment processes.
4. Tertiary solids removal cloth filtration system.
5. Sludge drying reed beds.

A description of these key features follows.

New inlet works

An inlet works with fine screening to 6mm in 2D and a manual bypass screen to 19mm with a stone trap upstream of the screens were installed. The screens were designed to cater for the Phase 2 Formula A flows of 23.2 l/s screens are to be designed to cope with a minimum of 150% of the design capacity for works under 2,500 PE, i.e. a design capacity of 35 l/s based on the Design Formula A. The hydraulic design of the inlet works was also based on the fine screen(s) being blinded by a minimum factor of 70%. The screen selected by the ECI team was the Definitive Ecology SF/T3 in-tank screw screen, with SDACA (specially designed anti-clog auger) system to ensure compliance with the screening handling requirements.

[caption id="attachment_16526" align="alignnone" width="1200"] Inlet works preliminary treatment - Courtesy of Glanua[/caption]

Stormwater tank

Flows exceeding the plant’s peak design flow, overflow by gravity (weir discharging via overflow screen) to the stormwater holding tank, which has been designed to provide a retention time of two hours for the difference between the Design Formula A and FFT flows plus 40 l/PE for SWO compliance as only 1:7.1 dilutions will be available for the WwTP effluent in the Brogeen River (based on DWF and 95th percentile river flow).

The stormwater holding tank is provided with a Eliquo Hydrok CWF flushing bell in order to automatically clean the tank once the storm flows have abated and the tank has been emptied; which is an innovative approach to this, requiring no energy input.

The CWF Storm Flush system is a highly effective non-powered cleaning system for all stormwater retention tanks, utilising the sewage stormwater for the flushing process thus requiring no additional fresh water. Storm return pumps located in a sump within the storm tank, which will reduce retention of larger volumes of waste and reduce the risk of odour.

Biological treatment processes

The secondary treatment stage for Boherbue includes the following:

• Two anaerobic selector tanks with three separate selector cell volumes.
• Two 624m³ aeration tanks with 109m³ of anoxic volume each for denitrification.
• Settlement provided by two 5m diameter/3.5m side wall depth secondary settlement tanks.

[caption id="attachment_16524" align="alignnone" width="1200"] Secondary aeration treatment - Courtesy of Glanua[/caption]

Each aeration tank is 15.6m length x 5m width x 4m working depth. Aeration is provided to the tanks using a fine bubble diffused aeration system designed to achieve an overall peak standard oxygen transfer rate of 66.15 kgO₂/h (i.e. 33kg O₂/h per cell).

The aeration system comprised three screw blowers from Kaeser Kompressoren operating on a duty/duty/standby basis, with automatic changeover of the standby blower on a time-basis; ensuring equal use of the equipment. Air from the blowers is forwarded to each aeration cell by an air header, which feeds individual branches into the aerobic cells. The diffusers are disc membrane units from Xylem Water Solutions.

Tertiary solids removal systems

Following settlement, secondary treated effluent is passed onto a tertiary solids removal treatment stage for final effluent polishing prior to discharge to the Brogeen River. The TSR process was required in order to ensure compliance with the WwDL ELVs, and in particular with the phosphorus ELV of 0.25mg/l, through coagulation/flocculation and filtration treatment.

[caption id="attachment_16528" align="alignnone" width="1200"] Cloth filter tertiary treatment - Courtesy of Glanua[/caption]

During the design stage and evaluation of the ECI team, the supplier Eliquo Hydrok was found to have the most reliable, energy and cost-efficient equipment that includes both flocculation and filtering stage. The package unit includes a flocculation stage comprised of one stream with two chambers that split into the cloth filters.

Sludge drying reed beds

The implementation of sludge drying reed beds (SDRB), which significantly increases sludge concentration to up to 40% dry solids (DS) after 10 years of operation is a key innovation. This revolutionary approach reduces sludge export volumes by more than tenfold compared to conventional methods, thereby minimising transportation and dewatering costs while substantially reducing the carbon footprint of the Boherbue WwTP. SDRB sludge can also be directly land spread without the need for additional chemical conditioning, further enhancing its environmental benefits.

There are numerous benefits in the inclusion of the sludge drying reed beds vs traditional sludge management practices. These include but are not limited to the following:

• Reduction of carbon footprint of the plant by approximately 45 T over a 10-year period.
• Carbon capture via natural plant growth.
• Up to 25% organic matter removal due to mineralisation.
• Dewatering process completed with zero energy consumption.
• At full design loads, the SDRBs will remove over 49 (No.) 20,000 litre tankers from the road per annum.

[caption id="attachment_16533" align="alignnone" width="1200"] Sludge drying reed beds to process wastewater sludge naturally - Courtesy of Glanua[/caption]

The construction of the sludge drying reed beds (SDRBs) commenced in early February 2023, incorporating earth embankments and precast concrete wall sections for formation. Each cell was meticulously lined to ensure water-tightness, with feed and drainage pipework installed for optimal functionality. Reeds were planted in late April to establish and grow during the summer months, aided by the importation of sludge to promote growth. This comprehensive approach ensures the effectiveness and sustainability of the SDRBs in treating wastewater and reducing the carbon footprint of the Boherbue WwTP.

Solar technology

The use of solar technology underscores the project’s commitment to renewable energy sources, contributing to further reductions in carbon emissions. By leveraging these innovative technologies and practices, the initiative sets a new standard for sustainable wastewater treatment, offering a replicable model for other treatment plants to follow. This holistic approach not only improves operational efficiency but also promotes environmental stewardship, paving the way for a cleaner and more sustainable future in the wastewater sector.

50Kw solar panels from The Low Carbon Energy Company Ltd have been installed which will produce 42,500Kwh of electricity per year. This alone will introduce a carbon saving of 13.7T/CO₂ per year which shows significant savings versus a traditionally built wastewater treatment plant of this kind.

[caption id="attachment_16532" align="alignnone" width="1200"] Solar panels for clean, renewable energy - Courtesy of Glanua[/caption]

Reuse of constructed wetlands

The integration of the existing constructed wetlands into the wastewater treatment facility which was core to the design development brought a multitude of benefits, ranging from low operational costs, support for diverse ecosystems as well as sustainable water management. The wetlands improve water quality of streams and rivers in surrounding area creating conditions for aquatic life to thrive, and also create a scenic area in the village, promoting biodiversity and providing a habitat for small animals.

Boherbue WwTP: Supply chain - key participants

• Precast concrete: Glanua Group
• Designers: Jennings O’Donovan & Partners
• Designers: Construct Engineering
• Precast concrete: Shay Murtagh Precast
• Precast concrete: Carlow Tanks
• Precast concrete: FLI Precast Solutions
• Precast concrete: Tracey Concrete
• PLCs & automation: UMAC Systems
• Inlet screens: Definitive Ecology
• Tertiary solids removal cloth filters: Eliquo Hydrok Ltd
• Aeration blowers: Kaeser Kompressoren
• Aeration diffusers: Xylem Water Solutions
• Solar panels: The Low Carbon Energy Company Ltd
• CWF Storm Flush system: Eliquo Hydrok Ltd
• Steel modular buildings: Shanette

[caption id="attachment_16530" align="alignnone" width="1200"] (left) Precast concrete aeration tanks reduced construction time and safety risks and (right) view of final clarifiers and cloth filters - Courtesy of Glanua[/caption]

Innovative construction methods

Throughout the entire design process, Glanua Group and Uisce Éireann’s PMO team evaluated several project aspects such as energy efficiency (BEP), TOTEX costs, and environmental impact.

• Energy efficiency of all motors and equipment (BEP).
• Process and equipment selected on TOTEX costs (including CAPEX, OPEX and capital replacement at end of life).
• The impact to the local environment was carefully assessed for all design options considered during the planning phase.

Modular and off-site construction

Modular/off-site construction was used to reduce the carbon footprint and on-site safety risks, with precast panels and BCAR-compliant steel modular buildings used to promote efficient delivery and enhance safety.

Precast panels were instrumental in constructing the main aeration tank, reducing construction time and mitigating safety risks compared to traditional in situ methods. Precast wall sections were used to separate the sludge drying reed beds sections to reduce the footprint size and again to reduce construction time and safety risks. Throughout the project, precast process tanks were used extensively contributing to streamlined construction processes and improved safety measures.

A BCAR-compliant steel modular building from Shanette was used for the control room, laboratory, and caretaker’s office facilities. This has a significantly lower carbon footprint than that of a conventional block-built structure and in turn reduced health and safety risks significantly during construction due to fabrication being completed before assembly on site.

[caption id="attachment_16529" align="alignnone" width="1200"] Off-site fabrication of steel building, fully assembled as works have been completed (April 2024) - Courtesy of Glanua[/caption]

Sustainability

The construction of the Boherbue Wastewater Treatment Plant is an example of Uisce Éireann and Glanua Group’s commitment to sustainability across various dimensions, including environmental, social, and economic considerations.

The inclusion of sludge drying reed beds demonstrates dedication to environmental sustainability. These beds offer an innovative solution for sludge management, reducing the environmental impact of waste disposal while also providing opportunities for ecological conservation.

By using nature-based solutions to treat and manage sludge, the carbon footprint associated with traditional disposal methods is minimised; thereby mitigating environmental harm. The SDRBs also significantly reduces operational costs, providing a long-term cost-effective solution.

The integration of solar technology underscores the commitment to renewable energy and carbon reduction. By harnessing solar power to supplement the energy needs of the wastewater treatment plant, reliance on fossil fuels is reduced, contributing to the transition towards a more sustainable energy future. This not only reduces operational costs but also mitigates greenhouse gas emissions, thereby supporting climate action efforts and environmental preservation.

Socially, the decision to reuse the existing wetlands demonstrates dedication to maintaining and sustaining biodiversity in the Boherbue community. By incorporating natural elements into the design, not only is disruption to local ecosystems minimised, but also to the overall health and vitality of the local environment.

Darran O’Leary, Programme Manager at Uisce Éireann said:

“We are delighted to have delivered this important project for the community of Boherbue. The modernisation and improvement of the wastewater infrastructure will accommodate further growth in the area and ensure that cleaner and safer treated wastewater is being discharged into the Brogeen River.

“It also adds biodiversity value to the area and remains sensitive to the surrounding landscape. This, coupled with the use of solar energy, demonstrates our commitment to putting sustainability at the heart of everything we do”.

Boherbue WwTP upgrade works is a flagship project for sustainable and carbon saving treatment processes and will serve as a reference project for future similar projects within the industry.

By demonstrating the feasibility and benefits of integrating renewable energy, advanced treatment technologies, and ecological conservation practices, this project sets a precedent for industry-wide innovation and best practices.

It complies with stringent Environmental Protection Agency (EPA) discharge consent standards, while minimising impact on the existing landscape, demonstrates a commitment to environmental sustainability, operational efficiency, and community well-being, making it a catalyst for positive change within the wastewater treatment sector and beyond.