Water and Effluent Treatment Magazine
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18 WET NEWS JANUARY 2015 The time taken over a wind farm application meant improvements were made to Bristol STW's sludge digestion process instead. The challenge: Powering self- sufficiency The Bio-Bus is the first bus of its kind to run on gas generated from food waste and sewage treatment, says Wessex Water G ENeco, Wessex Water Enterprises was estab- lished as a business within the Wessex Water Group of companies in March 2009, to provide a dedicated team of individuals focused on seeking opportunities through the use of renewable energy. At the time GENeco was established, Wessex Water's Bristol sewage treatment works site at Avonmouth was not self-sufficient in its use of electricity, despite the fact that it used anaerobic diges- tion to destroy organic matter and combusted the resulting methane in a CHP plant to generate electricity. Initially, this was to be rec- tified through the creation of a wind farm and enable the site to become self-sufficient in electricity usage, and so dis- place fossil fuel-derived elec- tricity that would otherwise be imported to the site. During the time taken for the development of the wind farm planning application and the discharge of the plan- ning conditions, improve- ments had been made to Bris- tol's STW's sludge digestion "The new ability to upgrade biogas into biomethane means GENeco can recover much more residual energy from waste streams" THE CONCEPT • The Gas to Grid plant allows biomethane, produced during food waste and sewage treatment • The site combines gas production from three separate sources into the AD process • The organic matter in waste breaks down to produce methane, which can be harvested and optimised via AD to produce biogas • The gas produced is 98% methane, an equivalent composition to natural gas • Leftover liquid from our digesters is dewatered to produce a solid cake which is used as a nutrient-rich organic fertiliser, thus closing the process loop NEED TO KNOW 1 Every day, 56,000m 3 of biogas is produced at Bristol sewage treatment works from the AD of domestic and commercial wastes 2 The solution led to a 97% reduction in dangerous particulate emissions, which can pass easily from the lungs into the bloodstream 3 In resulted in a 80-90% reduction in nitrogen oxides, which contribute to the formation of acid rain and city smog, and have a negative effect on vegetation growth 4 Public Health England research shows that 1 in 20 of the UK population in urban centres will suffer an early death directly attributable to particulate emission THE VERDICT • The facility is the first in the UK to extract biogas from food waste (unfit for human consumption), in addition to sewage sludge • The digested material from food waste recycling is currently fuelling a third of the gas output from the plant • INSIGHT RENEWABLE ENERGY process. This resulted in greater destruction of the sludge with associated increased production of biogas and enhanced produc- tion of electricity. By the time all of the plan- ning conditions had been dis- charged, the site was self-suf- ficient in electricity from anaerobic digestion / CHPs. Bristol STW is one of Wessex Water's largest sludge treat- ment centres, and manages around 50% of the company's total sludge. The facility serves around one million people in the Bristol area.