main sewage sludge treatment centres. The company recognised that many synergies exist between with the anaerobic digestion of sewage sludge and energy crops and the biomethane from both processes can be used to power combined heat and power plants (CHP).
The energy crop AD plant has been designed to generate approximately 15GWh of electricity each year. This is equivalent to supplying around 4500 residential properties. Each day around 100 tonnes 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 feedstock is being grown on Severn Trent’s own farmland which is classed as dedicated land as it has been used for the safe recycling of sewage sludge for over 120 years. The land potentially contains elevated levels of heavy metals rendering it unsuitable for food crops. However the land is extremely fertile making it ideal for growing crops for energy production.
The estate produces around 35,000 tonnes of maize and around 2000 tonnes of wheat which are the primary feedstock for the energy plant. An independent study was commissioned to report on the environmental impact of mono-cropping maize (growing maize year after year after year). The report concluded that maize is one of only a few crops which is suitable for mono-cropping with positive environmental benefits to be had for allowing the land to be left as stubble over the winter months providing habitats for hares and lapwings amongst other species.
The biogas produced is fed into 2 combined heat and power plants which produce electricity for the nearby sewage works and heat which is required to maintain the AD process. The plant also produces 30,000 tonnes/ annum of digested organic plant material which composite makes for an excellent organic fertilizer high in nitrogen, phosphorous and potassium. This fertilizer is reapplied onto our farmland saving around £30k a year on artificial fertilisers that would have been required and contributing to the annual net Carbon reduction of 7,400 tCO2e / annum.
This plant is producing 1.6% of the total amount of electricity Severn Trent needs when supplying its water and wastewater services to over 8 million people living in the midlands and mid-wales. This project has successfully combined the company’s expertise in farming, operating AD plants and producing significant amounts of renewable energy. It is hoped that this energy crop AD plant will act a catalyst for the development of similar projects elsewhere in the UK to hopefully realise DEFRA’s vision for 1000 farm-based AD plants in the UK by 2020.
Now that this plant has successfully been completed, Severn Trent is also assessing its other land holdings to see where this can be repeated.
Benefits: This energy crop project provides: • 1.6% of Severn Trent’s total electricity needs • an annual carbon saving of 8,500 tonnes of CO2 equivalent.
• 15,000 GWh of electricity (enough to power more than 4,500 homes)
• a sustainable use of contaminated land • 5000 tonnes of solid fertilizer • 25,000 tonnes of liquid fertilizer.
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