Supply Chain - Renewables & Energy Management - Combined Heat & Power

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.

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.

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. CC Jensen Ltd was selected as a specialist carbon filter supplier they based on their work on previous CHP installations. Dunphy Combustion Ltd was selected based on their technical capabilities and previous performance at Cog Moors WwTW. 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 Transformers: Wilson Power Solutions 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.