Making Energy from Waste – Severn Trent

In the run up to this years Green Apple Awards at the Houses of Parliament on the 12th November, we will be reviewing some of the amazing award winning projects from last year.

Severn Trent is the UK’s second largest water utility serving eight million customers. Our business faces an increasing need for energy to ensure supply resilience as a consequence of climate change and ever tightening quality standards.

Maximising renewable energy generation from sewage, water and our landholdings is an essential part of our strategy to minimise GHG emissions. At Stoke Bardolph Nottingham, Severn Trent retains 750 hectares of farmland which has been used for over 120 years for sludge recycling practices.

The land contains elevated levels of heavy metals and, as such, is unsuitable for food production. The aim of this project was to find a way of using this land to its full potential, generating renewable energy in a totally sustainable way to help cut the company’s green house gas emissions.

PROJECT ACHIEVEMENTS

After three years of planning and construction, the £15million plant achieved full 2MWe production in October 2010, two months ahead of schedule. Since October 2010 the plant has continued to operate reliably and by March 2011, electricity production was ahead of target.

This energy crop project will provide:

● 1.6% of Severn Trent’s total electricity needs

● An annual carbon saving of 7,400 tonnes of CO2 equivalent.

● 15,000 GWh of electricity, which is enough to power more than 4,000 homes

● A sustainable use of contaminated land

● 5,000 tonnes of solid fertiliser and 25,000 tonnes of liquid fertiliser.

WHAT DID THE PROJECT INVOLVE DOING AND WHY DID YOU DO IT?

Severn Trent is the UK’s second largest water utility and serves eight million customers. Our business faces an increasing need for energy – to ensure supply resilience as a consequence of climate change and ever tightening quality standards. Maximising renewable energy generation from sewage, water and our landholdings is an essential part of our strategy to minimise GHG emissions.

In 2009/10 Severn Trent generated 183 GWh of renewable energy from sewage gas CHP and hydro, which satisfied over 20% of our electricity demand and saved 99,000 tonnes of CO2 emissions. We have an ambitious target to generate 30% of our consumption by 2014/15 by investing in other renewable energy technologies.

In Nottingham, we recently completed the construction of UK’s first commercial scale dedicated crop digestion plant to help power the city’s main sewage treatment works. The plant will produce over 15GWh of electricity each year using 37,000 tonnes of crop silage grown on our own farmland.

The farmland has been used for the safe recycling of sewage sludge for over 120 years and contains elevated levels of heavy metals, rendering it unsuitable for food crops. The land is extremely fertile making it ideal for growing crops for energy production. An independent study reported that maize is one of only a few crops which is suitable for mono-cropping and brings with some positive environmental benefits:

● The land issued for growing only one crop per year

● It is worked less intensively and requires less pesticides

● It leaves the fields as stubble over winter, providing a good habitat for lapwings and hares.

Each day 100 tonnes of maize is fed in to the plant and over 90 days the organic matter is broken down, giving off methane. The biogas is fed into a combined heat and power plant to produce electricity. The plant will also produce 30,000 tonnes per annum of digestate, which is a good natural fertiliser high in nitrogen, phosphorous and potassium. This fertiliser will be reapplied onto farmland having recently been approved for use on organic crops. The net carbon benefit is 7,400 t/CO2 per annum.

WHO AND WHAT HAS BENEFITED?

Work on this project started in 2007 when Severn Trent became aware of the development of Energy Crop AD plants across mainland Europe.

The company commissioned its own feasibility studies to assess the potential of this technology for deployment at its own farms. Severn Trent is the largest producer of electricity from sewage gas in the UK, which is generated at its 36 main sewage sludge treatment centres.

The company recognised that many synergies exist between the anaerobic digestion of sewage sludge and energy crops and the bio-methane from both processes can be used to power combined heat and power plants (CHP).

In 2008 the outline design of the plant was completed with assistance being provided by German process specialists, Schmack Biogas GmbH. Schmack were selected as the preferred process supplier, having already designed and constructed over 230 biogas plants worldwide.

In June 2009 a design and build contract was let to Interserve Project Services Ltd. Interserve, who were already one of Severn Trent’s main AMP4 contractors, were specifically selected for their excellent approach in managing health and safety and their ability to integrate with new specialist process suppliers.

By May 2010, construction of the 2MWe Energy Crop AD was completed on programme with the plant ready for the commencement of the commissioning phase.

The multiple digestion tanks were filled with around 6,000 tonnes of imported farm yard manure and the temperatures with in the tanks were gradually raised to the 40 degree centigrade digestion temperature. The biology of the plant was closely monitored during this process to ensure that a healthy biomass was produced.

After three years of planning and construction, the

£15million plant achieved full 2MWe production in October 2010, two months ahead programme. Since October 2010 the plant has continued to operate reliably and in 2011 electricity production remains ahead of target.