Thames Water recently announced plans to upgrade the Crossness Sewage Treatment Works (STW), one of Europe’s largest treatment plants, serving two million people. The scheme, which totals £400m, will include a new wind turbine.
The STW already burns sludge to generate 20% of its energy and the wind turbine will raise this to 50%, generating half of the total energy needs for the sewage treatment works. The upgrade will also reduce Crossness’ carbon footprint.
The work is in response to new Environment Agency standards, which will demand increased capacity and effluent quality by March 2014. The investment will aim to minimise smells as well as increase capacity to cope with heavier and more intense rainfall and meet new climate change needs.
Improvement work has started and the new equipment should be fully operational by 2014.
MAJOR LONDON SEWAGE WORKS
The earliest part of Crossness was built in 1865, as part of Sir Joseph Bazalgette’s pioneering plan to manage London’s sewage. Situated on the south bank of the Thames in Abbey Wood in the London Borough of Bexley, it serves Richmond Upon Thames, Wandsworth, Merton, Lambeth, Southwark, Lewisham, Greenwich and parts of Sutton and Bromley.
On the eastern boundary of the site is a 20ha nature reserve which maintains one of the last areas of grazing marshland in London. The reed-fringed ditches, reed beds and wet grasslands support a variety of wildlife and wading birds.
At approximately 70ha, the plant treats enough sewage to fill 20 Olympic-sized swimming pools each hour. The sludge-powered generator has been running since 1998 and can produce up to 6MW of energy – about three quarters of the site’s electricity needs.
During periods of heavy rainfall, wastewater is diverted into storm tanks until the rain has subsided. During storms, so much wastewater enters the STW that partially treated wastewater has to be discharged into the Thames. Thames Water admits only partial success in controlling odour when this happens, with a successful prosecution by the London Borough of Bexley for odour nuisance in 2004.
The sewage is processed in five stages:
• The inlet works screen items like wood, bricks, rags, paper, plastic and grit.
•Primary settlement uses gravity to settle the finer solids as raw sludge.
•The remaining ‘settled sewage’ flows to the biological treatment plant for secondary treatment, where polluting organic matter and ammonia is consumed by bacteria.
•The resulting ‘mixed liquor’ (activated sludge and effluent) passes for final settlement. The treated effluent flows over the weirs as clean water, where it is strictly monitored before being discharged to the Thames.
•Sludge treatment thickens and mixes raw and activated sludge before they are pumped to the sludge-powered generator.
The total flow arriving at Crossness will remain approximately the same but the proportion of flow fully treated will rise significantly. That means less risk of storm overflow into the river. Less wastewater entering the Thames will help boost oxygen levels, which will in turn improve the habitat for fish and other aquatic life.
Crossness also aims to reduce the number of days that litter and untreated sewage flows into the river by using two new boats to remove rubbish (thousands of sewers become blocked each year because of household items being flushed away rather than properly disposed of).
STW proposes to keep a similar design for the new works and machinery as the existing equipment. The works will therefore be relatively low lying and will not be obvious outside the site. The new development will stay well within the boundary of the STW. However, the wind turbine will be visible from some distance.
Thames Water is submitting a planning application to the London Borough of Bexley requesting permission to undertake the improvements. The council is consulting a number of organisations including local residents’ groups, the Greater London Authority, the Environment Agency and Natural England. Ofwat has approved funding.