Severn Trent Water, one of the UK’s biggest water utilities is on course to complete the first commercial scale dedicated crop digestion plant. Each year 37,000 tonnes of maize is fed into £15 million worth of machinery investment, enabling Severn Trent Water to produce 15GWh of electricity which will be utilised to help run the sewage treatment works.

Within the plant, maize silage is anaerobically digested over a period of around 90 days, during which time the majority of the organic matter is broken down to produce a methane-rich biogas. The gas is then used as a fuel in combined heat and power plants (CHP) to generate both electricity and heat. The electricity produced is used to help power the adjacent sewage works, reducing the amount of fossil fuel-derived electricity imports from the national grid to zero. – Interserve Case Study on Severn Trent

With wholesale energy continued pricing fluctuations, the project is an integral part of its renewable energy expansion programme. By 2013, Severn Trent aims to generate 30 percent of its energy demand through renewable sources and even allows them to produce surplus electricity to be sold back to the grid for profit.

A second environmental benefit of the process is that during the cultivation of the maize crop, the plants naturally convert carbon dioxide into oxygen. The heat given off from the CHP engines is re-used in the process to maintain optimum operating temperature. And finally, after the maize silage has been digested it is recycled back to the fields as a fertilizer to help grow next year’s crop. This makes the plant highly efficient and represents an annual carbon saving of around 8,500 tonnes of CO2 equivalent. – Interserve Case Study on Severn Trent