Supply Chain - Renewables & Energy Management -

The projects are being delivered by the Green Gas Solutions Team at SGN Commercial Services via a Design /Build /Finance /Operate /Maintain (DBFOM) model. In this novel model, Yorkshire Water retain control of process volumes, despite the gas to grid plants being owned and operated by SGN Commercial Services.

The biomethane produced will be shipped by Centrica Energy on behalf of Yorkshire Water. SGN Commercial Services will be paid a monthly service charge to cover capital repayment and O&M services for the duration of the project terms. SGN’s performance commitment is on the gas to grid plants availability, with gain/pain share against the contract availability target.

This model ensures that the parties are in control of the aspect that they are best at, and crucially, correctly motivated to perform optimally. Another great feature of this model provides Yorkshire Water with the option to either sell the green gas certificates associated with their biomethane or retain and retire them to offset their own greenhouse gas emissions.

Both project sites currently have anaerobic digestion (AD) plants that process sewage sludge to make it safe for onward applications. The AD process also yields biogas as the organic matter in the sewage sludge is broken down by a carefully controlled culture of bacteria.

For decades, this biogas has been combusted in reciprocating engines to drive generators to produce green electricity. However, as the UK electricity grid decarbonises, the option to clean up this biogas and inject it into the gas grid represents far more benefit from a fossil carbon abatement perspective. The new gas to grid plants at Knostrop and Blackburn Meadows will do just that; removing impurities and biogenic CO2 from the biogas to leave pure methane that can be injected into the local gas network.

The gas to grid plants at both Blackburn Meadows and Knostrop STWs have been sized to take into account biogas production volume forecasts, as the local populations of Sheffield and Leeds continue to grow. The raw biogas capacities are 1,300 Nm3/h and 2,900 m3/h respectively.

The product gas is >99% methane, and this is passed forward to the grid entry unit for metering and analysis ahead of grid injection. It is at this stage that a small quantity of propane must be added to ensure the calorific value (CV) of the biomethane being exported is equal to that of the gas network.

The waste stream of biogenic CO2 will initially be vented, although plans are already being developed to add CO2 capture facilities that will provide the possibility of a carbon negative process. On both projects, the membrane plants are from DMT Environmental Technology, and the grid entry units are of SGN Commercial Services’ own design and are being built in partnership with Bohr Engineering.

In both cases, the existing AD and hence G2G plants are located in the middle of the wider Yorkshire Water sites. In both cases therefore, SGN CS are laying new pipelines to convey the product biomethane out of the site to connect to the local gas network.

Having progressed design to a suitable point, and in their capacity as the asset owner and site operator, SGN Commercial Services was required to submit a planning permission for both projects. Planning permission was successfully obtained in the latter part of 2024 and detailed design, allowing procurement to commence. SGN worked closely with the supply chain to optimise package plant designs to ensure safe and reliable operation.

The Blackburn Meadows project is slightly ahead of Knostrop in programme terms, with the compound area now cleared and ready for civils and, at the time of writing (June 2025), the cross-site biomethane export pipeline is being laid. The pipeline between the G2G plant and the site boundary is well underway but work to date on this element has not been without its challenges. Wastewater treatment works are notorious for a congestion of buried services and redundant underground structures. These challenges have tested the skills and experience of the project team, but a route has now been successfully mapped.

The project team has visited the supplier’s manufacturing facilities to view progress and witness factory performance tests. The majority of the mechanical G2G equipment is made in mainland Europe; which helps with the projects’ embodied carbon credentials.

The containerised/modular nature of G2G equipment facilitates extensive factory testing. This, in turn, minimises costly and potentially more hazardous site works conditions. The increased plant capacity and high-pressure connection introduce additional technical challenges for the Knostrop project, but the design is substantially complete, and the majority of plant is now on order.

On both projects, the compound areas are leased from Yorkshire Water for the duration of the term. At Knostrop, the opportunity is being taken to include offices and stores to facilitate the level of planned and reactive maintenance that will be required to uphold (and hopefully exceed) the contract availability commitments.

The Blackburn Meadows G2G Plant is due to start commissioning at the end of 2025, with commercial operation expected early in 2026. Knostrop will be following on shortly after. Once complete, the two projects will allow Yorkshire Water to cumulatively generate 225 MWh of green gas per year. That’s enough to heat 20,000 UK homes, offsetting the release of 44,000 tonnes of fossil CO2.

Should SGN Commercial Services subsequently be appointed to deliver the aforementioned CO2 capture feature at both sites, Yorkshire Water could generate as much as 15,000 tonnes per year of biogenic CO2. This biogenic CO2 has value as an industrial product, offsetting the use of fossil derived industrial CO2.

More tantalisingly however, this biogenic CO2 could potentially be sequestered into permanent storage. If this were the case, Yorkshire Water would be taking CO2 out of the atmosphere and locking it away, allowing them to claim and/or sell carbon removal credits. The market value for these credits is still an emerging picture but they are likely to be extremely lucrative.

Another exciting prospect that is currently being investigated is the option to trim the amount of propane being added to the biomethane as it is injected into the grid. Using a concept solution that SGN Commercial Services developed for the SGN network, this trim is achieved by assessing the instantaneous ratio of network flow and export flow. Not only could this trim save biomethane producers a significant amount of money, but it would also further enhance the fossil carbon abatement impact of the project(s).

With the Knostrop and Blackburn Meadows Gas to Grid projects progressing well, Yorkshire Water are looking at further anaerobic digestion sites across their estate where G2G could bring environmental and commercial enhancements.

With the objectives of increasing resilience and throughput of the sludge processing facilities while enhancing sustainability, the Ringsend Sludge Line Enhancement Project saw Murphy design, build and commission a major upgrade of the advanced anaerobic digestion plant bringing the design average annual throughput of 55,000 TDS/annum.

This challenging project was delivered within a broad range of operational and construction constraints, and was completed on time and on budget in 2025.

The existing sludge plant at Ringsend Wastewater Treatment Plant comprised an advanced anaerobic digestion plant using a Cambi thermal hydrolysis process (THP) followed by four mesophilic anaerobic digesters operating in parallel with a design average annual throughput of approximately 42,000 TDS/annum. The digestate produced is then dewatered, after which it is either exported for use as an enhanced biosolid in agriculture or it further processed within the thermal dryers.

Ringsend WwTP is located on a very confined site, with insufficient space to consider solutions for expansion such as the construction of additional anaerobic digesters.

Consequently, the Ringsend WwTP Sludge Line Enhancement (SLE) Project focussed on solutions which leveraged process technologies designed to maximise utilisation and throughput of existing assets. This included refurbishment and capacity upgrade works to the advanced anaerobic digestion plant and ancillaries to bring the design average annual throughput of 55,000 TDS/annum with the ability to handle a peak throughput of 210 TDS/day.

The key elements of the project scope are summarised as follows:

The first major element at the core of the solution to deliver the expanded capacity within the advanced anaerobic digestion plant was use of Haskoning’s Ephyra® sludge digestion technology.

Ephyra® is based on plug flow digestion, where the hydraulic retention time (HRT) and solids retention time (SRT) are separated. Space constraints meant that the construction of the fifth digester at Ringsend was a not a feasible solution for the expansion of the works. The ability of the Ephyra® to optimise the performance of the existing digestion tanks made it the ideal solution for the challenge posed at Ringsend. Delivering increased volatile solids reduction, reliable operation at lower retention times and improved biogas production, the Ephyra® process allowed Murphy to meet the project objectives of increased throughput and enhanced sustainability.

Ringsend is the largest plant Ephyra® process has been delivered on to date and is also the first time it has been installed on digesters receiving hydrolysed sludge from a Cambi thermal hydrolysis process.

The second major element of the expansion was the delivery of additional throughput capacity within the thermal hydrolysis process by upgrading the existing system to a Cambi® Mk II process.

Working in partnership with Cambi, this upgrade entailed a major retrofit of the Stream 3 of the existing THP (the largest of the three pre-existing streams accounting for approximately 50% of plant throughput). The Cambi Mk II process is an advanced version of Cambi’s thermal hydrolysis process, designed to significantly improve sludge treatment performance, capacity, and energy efficiency.

The main changes that achieve Mark II status include the following:

In addition to delivering increased capacity within the THP the also entailed a refurbishment and reconfiguration of post-THP sludge cooling to enhance efficiency through energy recovery and align the system with the new plug flow configuration of the digesters. Energy recovered through the new pre-coolers installed is reused in pre-THP dilution of the sludge to reduce steam uptake and improve energy efficiency.

The final key element in the expansion of the advanced anaerobic digestion plant was the expansion of pre-THP dewatering capacity. Delivered in two phases, this required the partial relocation and recommissioning of the existing sludge dewatering, followed by the addition of a fifth centrifuge providing an additional 3,000 kgDS/hour capacity at this stage (an increase of 25%). Murphy worked supply chain partner GEA in the design and delivery of this upgrade.

Design and delivery of the project had to factor in a number of key constraints and considerations.

These constraints and considerations presented unique challenges to the Murphy team in the design, construction, commissioning, and integration of the works. At each stage of delivery these challenges were met through a combination of a collaborative approach and a willingness to leverage the latest technological innovations underpinned robust delivery processes which ensured a structured approach to each aspect of project delivery.

Murphy engaged project stakeholders in a meaningful sense from the outset of the project in collaborative design workshops. Through HAZOP, ALM and constructability workshops, Murphy worked with the client and their representatives, the plant operators and key technology providers to optimise both design and construction methodologies to ensure efficient delivery with minimum disruption.

The Murphy team leveraged BIM capabilities as a collaborative tool to improve coordination and communication between project stakeholders across design and construction phases of delivery. Constructability workshops harnessed 4D construction modelling and 3D visualisation and walk throughs as key tools in evaluating designs, planning the works, and established a clear shared understanding of key construction and interface tasks amongst all project stakeholders.

This collaborative approach allowed the collective expertise, experience and knowledge of the plant amongst stakeholders to be harnessed in driving innovation. Not only did this allow refinements and improvements to the design and planned interface works, but it also allowed Murphy to develop design changes which allowed us to reduce the impact of construction works on available capacity within the existing plant.

Most notable, was a significant reduction in the duration during which one of the four anaerobic digesters would need to be fully removed from service, from the originally planned six months to an actual duration of less than four months, and the successful delivery of the additional pre-THP dewatering capacity, six months ahead of schedule.

The Sludge Line Enhancement Project has effectively achieved its goals of significantly expanding and modernising the sludge processing facilities at the site.

Completed within the scheduled timeframe and budget, the upgrade has undergone comprehensive performance testing and will ensure that the Ringsend Wastewater Treatment Plant can reliably and sustainably accommodate the increased demand from the growing population of the Greater Dublin area .

Performance of the solution has garnered considerable interest, and also that this is the first time Cambi THP technology has been combined with the Haskoning Ephyra® sludge digestion technology.

Testing carried out on the project entailed both baseline testing of the previous process performance and testing of the new enhanced process to allow benchmark evaluation of performance improvement.

In all key performance metrics analysed the upgraded system has exceeded target performance with system availability and peak throughput successfully demonstrated at the same time as both volatile solids destruction and biogas production within the advanced anaerobic digestion plant improved by more than 20%.

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.

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.

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.

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 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.

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.

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:

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.

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.

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.

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:

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.

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.