Severn Trent builds on its renewable credentials

Despite some common misconceptions, anaerobic digestion is not new. And Severn Trent for one has been using the technology for decades. Here we look at some of the water company's latest projects that are producing biogas for CHP.

As new energy and environmental directives come into play, the sewage industry is increasingly in the spotlight. And examples of energy efficiency measures by the big water companies and their framework partners are set to draw much interest from legislators, competitors and the general public.

Combined heat and power (CHP) is already at the forefront in this industry, producing heat and electricity from a single input source that would otherwise be burnt off as waste.

Severn Trent Water and May Gurney are approaching the end of an extensive project to install new anaerobic digestion technology on three of Severn Trent’s sites. The water company has been working in partnership with contractor May Gurney, a specialist in biogas optimisation, which has been carrying out the works since 2007. And it is nearing completion on the last site at Coalport, Shropshire.

The other two sites, at Newtown (Powys) and Monkmoor (Shropshire), were handed back this summer and are already producing biogas from their improved digester systems and using it to fuel CHP units.

The work at Newtown, Monkmoor and Coalport has allowed the sites to reduce their carbon footprints and produce green energy. And it will also prove cost-effective as Severn Trent will become more self-sufficient and save heavily on external energy suppliers’ ever growing costs.

At Coalport alone, the average daily sludge feed to the digesters is 129m3, which produces an average of 2,640m3 of biogas per day – enough to power the on-site CHP units that generate electricity to run the site as well as the heat to operate the boilers and perpetuate the digestion process.

Nick Chesters is May Gurney’s Cheadle, Staffordshire-based regional M&E director, and is the general framework manager for Severn Trent Water. He has overseen these three projects as part of a £4.7M long-term partnership, and is already working on plans for the next phase following their success.

He explains the significance of the work: “There is a general trend in the water industry towards sewerage treatment works capturing biogas, for a number of reasons.

“The main driver is ever increasingly stringent government legislation, with its requirements for both the reduction of waste to landfill and the production of renewable energy. The government is already aware of anaerobic digestion as a cost-effective means of harnessing the energy from the wastewater treatment process, and using it to generate not only heat and power for the plants themselves, but also the potential for extra electricity which can be pushed back into the national grid, generating further income for the water company.”

Anaerobic digestion at these three Severn Trent sites is by no means a newly introduced process – in fact, there has been a digester on site at Newtown for 28 years.

Martin Newey, May Gurney project manager, says: “There is a common misconception outside this industry, born of lack of knowledge, that anaerobic digestion is a new idea.

Many end users, such as council procurement departments, may not have realised that companies like us and Severn Trent have been working with this technology for decades. And, while some new players from overseas are seizing upon the opportunity to repackage digestion and sell it as something new, we are confident that we are still ahead of the game with our advanced expertise and genuine innovation.”

May Gurney’s work at Newtown, for example, comprised a total refurbishment of the existing digester, and also the installation of new systems to maximise its efficiency.

Nick Chesters describes the process: “The total scale of the work is around one year, from the allocation of the project to scope visits, early design work, works order, detailed design and finally to site works, which comprise approximately the last six months of the contract.

“Alongside the refurbishment of the Newtown digester, we installed a new design of membrane gas holder, a de-grit system with actuated valve, new flare stack and vent stack, new gas interface motor control centre with HMI and PLC, cabling and instrumentation, gas distribution pipework, and all civil-engineering works including walkways and fencing.”

Commissioning plan

Chesters continues: “A comprehensive commissioning plan was then implemented, involving filling the digester with sludge, switching it on and then running, testing and monitoring the whole system with the pipes and the boilers working at full capacity, in accordance with Severn Trent procedures.

“The contract at Monkmoor, meanwhile, involved works of a similar nature to Newtown – two redundant gas holders were decommissioned and replaced with a more efficient membrane-type gas bag. This new design has given substantial cost savings compared to the previous type, which was the more conventional steel gas bell in a concrete tank. A major part of this project involved keeping the plant running whilst constructing the new works, thus minimising any operational disruptions.”

At the time of writing, May Gurney is nearing the end of the final four-week test-and-run process at the Coalport site. This is the largest of the three schemes, where the improved gas delivery system is already powering a new 190kW CHP engine. The scheme is being managed by Phil Lomas from May Gurney in Cheadle. He says: “One of the main benefits from CHP is heat. The heat is transferred from the engine’s hot-water loop and interfaced with the existing boilers. This then heats the sludge in the digester to the optimum temperature for the production of gas.

“We have many years’ experience with CHP installations, however the Coalport site also required special consideration for planning and design because it is located in an area of natural beauty, adjacent to the River Severn and next to a flood plain. We used a whole-team approach, in agreement with Severn Trent’s philosophy of one supply chain.”

At Coalport, too, the installation of a new, larger gas bag will add to efficiency. These new spherical gas holders are the most immediate visual sign of the cutting-edge solutions that have been implemented at the Severn Trent sites.

The golf ball white spheres are constructed of two polyester and PVC-based skins, the outer of which is inflated by air pressure and holds up the structure.

And the inner of which is suspended within and contains the biogas.

The materials are resistant to the corrosive nature of the gas, which with its hydrogen sulphide content produces weak sulphuric acid on contact with the water in the saturated gas, and can therefore damage a steel container.

There are obvious savings from a gas membrane bag compared with a gas holder. A gas holder requires a substantial civil structure which is filled with water and houses a steel bell, whereas a gas bag simply sits on a concrete base. The bag also requires less maintenance and is not prone to freezing in winter. This produces a far smaller carbon footprint and a reduction in capital and operational spend.

The new gas holders are also in fact important pieces of process equipment, rather than the mere storage vessels their name suggests. They maintain a constant system pressure required for the correct operation of engines, boilers and waste gas flare stacks. They also have level instruments that measure the height and volume of the inner gas bag to provide signals for process control, so they need to be simple, reliable and robust.

Further efficiencies and economies have been driven into the Severn Trent project thanks to the fact that May Gurney handled the entire design process in-house, from taking the outline design produced by Severn Trent’s framework designer, through to completion.

While many main contractors might outsource the design element of such projects to an external specialist, May Gurney has its own expert team, thus both reducing cost – creating economies of scale and avoiding passing on fee-on-fee margins to the client – and also minimising risk thanks to better control of safety in design risk assessments.

Derek Shepherd is May Gurney’s design manager, who is responsible for design coordination. He explains the advantages of the full service approach: “By taking more control for Construction Design Management upon ourselves, and not passing it to a third party, we are especially confident in all aspects of the projects. There is less risk, and by not having to verify somebody else’s design we also save time without doubling up effort.”

Better still, the in-house design team have contributed to better environmental performance. Derek Shepherd believes that, by having an independent and impartial designer, they were able to identify all products, materials and suppliers based on performance and an overall design approach to the system. This was rather than it being based on any existing commercial relationships.

The final phases of all works on the Severn Trent sites have been to leave each plant and its surroundings in a cleaner and more presentable state than before. In line with both companies’ commitments to social responsibility and continuous improvement, an inevitable part of the project was to take into consideration the surrounding environment and local communities.

Alistair Jones, regional director at Severn Trent Water, is based at Newtown. He says: “There is a public nature trail that runs right past this site, and we have had much interest from walkers who are wondering what the big white gas holder is. We are aware of our responsibility to local residents, and have taken steps to preserve the area with investment in the adjoining stretch of nature trail, and measures to protect and conserve local colonies of geese and endangered Great Crested Newts.

“In fact, in accordance with the law, for the duration of the works on site, we had installed a small extra perimeter fence solely to protect such protected species. And in future we are confident we will be providing a cleaner environment not just for them, but for everyone.”

Martin Newey sees a big future for biogas optimisation. “For all the same environmental, legislative and economic reasons that have motivated Severn Trent Water, others will surely follow suit,” he says. “We are excited about the opportunities for growth and development in this area. A whole industry is expanding around the re-emergence of anaerobic digestion, which enables waste material, such as food waste, to be used as a resource to produce renewable energy.

“The huge increase in available volumes of biogas, rising oil prices, rising demand for new renewable fuels and bio energy will stimulate investment in biogas utilisation technologies that will see biogas refining for use as vehicle fuels or injection into the national grid.

“This latter opportunity will certainly not have escaped the notice of water companies. While sites with anaerobic digestion processes already benefit from self-sufficiency by producing their own energy, we need only look to countries like Sweden to see examples of how extra income can be generated by selling electricity and biomethane and at the same time contribute to government climate change, waste management and wider environmental objectives.”

Renewable resources

Severn Trent is certain to be at the forefront as the industry moves in this direction. As the leading producer of renewable energy within the water sector, Severn Trent is progressing with its investment programme to further develop greener energy.

Having set itself the target of almost doubling self-generation from renewable resources to 30% of its total energy use by 2013, Severn Trent has clear plans to develop usage of existing technologies as well as introduce new and upcoming technologies.

Severn Trent operates 30 CHP plants across its region using methane gas produced from the sewage treatment process. In 2005, this accounted for 51% of all renewable energy derived from sewage gas in the UK and about 1.3% of all renewable energy generated in the UK. Current investment plans include schemes to extend the usage of CHP plants across the region, install more water turbines in its dams, generate power from energy crops, and generate power from wind turbines at appropriate locations.

As a result of the success of the Monkmoor, Newtown and Coalport projects, the company is already in advanced talks with May Gurney about a further nine CHP projects.

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