Sewage works. Aerial view of aeration tanks at a sewage works. This stage is used to treat sewage after most of the solid waste (sludge) has been removed in settlement tanks. In an aeration tank, air is blown into the sewage to promote the growth of micro-organisms that feed on harmful bacteria. This is followed by further settlement to remove any remaining solid waste. A worker (centre) is monitoring the process. The cleaned water will be pumped into a river. Sludge can be treated and used as fertiliser, but here the sludge is used to produce methane to burn and generate electricity. This is the Severn Trent sewage works, Burton Joyce, Nottinghamshire, UK. Photographed in 2004.