The best of the crop

Maize grown on contaminated land is being used to successfully power an AD plant for Severn Trent and reduce carbon output. Natasha Wiseman reports

The drive to cut carbon footprint is being taken up by all water companies and Severn Trent Water (STW), which serves more than 8M customers across 21,000km2 in central England and mid-Wales, is no exception. The utility has sought new ways to generate energy, focusing on Combined Heat & Power (CHP) from sewage gas and hydropower, and in 2011-12 produced 22% of its electricity needs from renewable energy sources.

However, the company has one of the most ambitious targets in the UK and aims to be selfgenerating 30% of its electricity requirements by 2014-15. To help achieve this goal, STW approached construction support company Interserve to design and build an Energy Crop Anaerobic Digestion (AD) Plant at Severn Trent's Stoke Bardolph site, near Nottingham.

Using a 2,000 acre site of contaminated farmland owned by Severn Trent, the UKs first large-scale crop CHP plant entered the planning stages three years ago.

Martin Dent, STW's renewable energy development manager, said: "In 2007 we became aware of a new emerging technology across mainland Europe where crops could be grown for energy production. Here at Stoke Bardolph we had an interesting dilemma as we could grow around 37,000t of crops each year, but due to the land being contaminated, any crops grown are unsuitable for human consumption."

Severn Trent engineers visited specialist plants across Europe and began talking with Interserve to see if they could embrace the new technology and bring together the right ingredients for the project.

Dent said: "We had to ensure that the design and construction complied with Severn Trent and UK specifications, but through our previous working relationship we were confident Interserve were the right partner to deliver this complex plant.

"They knew all the right practices and together we were able to bring German process specialist Schmack Biogas on board. Schmack were selected as the preferred process supplier, having already designed and constructed more than 230 plants worldwide."

Dent explained how design meetings were held over many months in the UK and in Germany: "We used a partnership approach to bring together skills and share knowledge."

Electricity production
Building started in June 2009 and by October 2010 the plant was operating at full capacity - two months ahead of schedule and thus producing surplus electricity. Each day around 100t of crop silage is fed into the plant and over a period of 90 days, bacteria breaks down the organic matter to produce a methane rich biogas. The biogas is then fed into two CHP plants, which produce electricity for export into the adjacent Stoke Bardolph Sewerage Treatment Works to help power the treatment process.

The plant has been designed to generate 15GWh of electricity each year, which is equivalent to supplying around 4,500 homes. The sewage works serves the city of Nottingham and previously imported around 1MW of electricity from the national grid, but now that the crop AD plant is up to power, the sewage works' entire electricity demand is satisfied. Any surplus electricity is exported to the grid helping the company to offset against electrical imports elsewhere. The sewage works has become a net exporter of electricity.

Contaminated land
The maize crop is being grown on farmland that has been used for the safe recycling of sewage sludge for over 120 years. It contains elevated levels of heavy metals, rendering it unsuitable for food crops, but is extremely fertile, making it ideal for growing crops for energy production.

The energy crop programme is a key part of the utility's renewable energy expansion programme, exploiting the potential of the company's contaminated landholding. Some 35,000t of maize and around 2,000t of wheat are now used to generate renewable energy.

Dent explained the logistics: "The crop is harvested, with around 1,000t collected every day, and brought to holding tanks on-site which can cater for 38,000t."

Fertiliser
The plant also produces 30,000t of digested organic plant material per year, which can be used to make fertiliser high in nitrogen, phosphorous and potassium. This fertiliser is reapplied to the farmland and saves the company around £100,000 a year that would have been required for artificial fertilisers and contributes to the annual new carbon reduction of 7,400t C02e per year.

But enabling Severn Trent to add to its renewable energy technologies - which also include electricity being produced from sewage gas its 36 main sewage sludge treatment centres - was not without its design and planning difficulties.

Chris Brooks, water business manager for Interserve, said the project created a "huge number of challenges", starting with the ground condition at the 2,000 acre Stoke Bardolph Estate.

He said: "We had to come up with a number of innovative and best practice techniques across the project to deliver a sustainable, value for money product. For example we had to create access over the existing effluent channel, which presented us with a number of technical difficulties. We designed and built a Bailey bridge to overcome this."

Sustainable construction
Brooks explained how specialist on site_ground stabilisation compaction techniques were employed to improve bearing capacities and in turn negate the need for extensive and expensive foundation piling and the need to import up to 20,000t of further material.

Furthermore suitable foundation material was sourced on site from borrow pits to make up for a shortfall in material.

Brooks said, "This approach generated one of the greatest sustainable benefits associated with the construction phase, avoiding the need to export and import a significant quantity of material, reduced vehicle movements and associated emissions."

He stressed the importance of the partnership between Interserve and contractor Schmack Biogas to bring in the right expertise. The German company was working in the UK for the first time.

Brooks said: "In conjunction with Severn Trent and key suppliers, we had to take an intelligent approach to the delivery and planning as it was essential we got everyone around the table and dealt with any issues early.

"This ultimately meant we were able to deliver electricity from the project two months ahead, enabling Severn Trent to produce additional energy."

He continued: "It is absolutely key that we focus on optimising the plant's potential and we will continue to work with Severn Trent to do that. The design, construction, commissioning and ultimately operation of this project inevitably drew on our extensive AD experience gained in the water industry.

"The successful delivery of this project has clearly been a further demonstration of our AD capability and our ability to engineer, procure and construct highly technical and complex projects." Looking to the future, Brooks said:

"It is envisaged that the success of the Stoke Bardolph energy crop AD plant will be a catalyst as well as offer a viable blueprint for the development of similar AD projects elsewhere in the UK."


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