More than 7.3 million cubic metres of sewage overflowed into the river Thames in May due to high levels of rainfall. There were eight separate incidents, with one on 26 May discharging 5.5 million cubic metres. The overflows increase pathogen levels, litter the shoreline with sewage debris, and cause oxygen levels to fall to dangerously low levels and suffocate fish.
Thames Water deploys two different types of boat to improve water quality. Skimmers collect litter from the water surface six days a week and bubblers re-oxygenate the water after sewage discharges.
Some 50 million cubic metres of sewage overflows into the Thames each year because London’s Victorian sewerage system is unable to contain large volumes of stormwater during periods of heavy rainfall. More frequent episodes of violent weather due to climate change may exacerbate the problem.
The scheme involves building a 32-kilometre tunnel under the Thames from Hammersmith in the west to Beckton in the east. Seven metres wide and sloping to a depth of 80 metres at its eastern end, the tunnel will collect sewage overflows from at least 36 discharge points and transport them to the Beckton sewage works. Another tunnel in east London will link the largest overflow, at Abbey Mills near Stratford, to Beckton.
On 30 May, Thames Water submitted a planning application for the £700 million east London tunnel. It expects to receive approval by October.
This 7km section of tunnel will handle 50% of the sewage overflows, reducing the number of incidents from 90 to 6-8 per year. The tunnelling will start at Beckton sewage treatment works using a machine that will bore 17 metres a day towards Abbey Mills.
Around 670,000m3 of excavated material, mainly chalk, will be removed during this process. A jetty bridge will be constructed so material can be transported by barge down stream. Thames Water is in discussions with possible buyers for the material.
The east London section is due for completion in 2014 - not in time for the 2012 Olympics. There are fears that sewage overflows from outfalls near the Olympic site could cause a stink during the games (ENDS Report 370, p 4 ).
Planning permission for the main section of the tunnel, which is still at the design stage, is due to be submitted in spring 2011 with building work to commence in 2012. It will create 2.5 million m3 of excavated material, mainly clay.
Four to five access shafts will be constructed on land alongside the tunnel. Environment consultants Scott Wilson have been contracted to assess where the shafts should be sited to minimise disturbance. Thames Water will launch a consultation on the proposals later this year.
Programme mangers for the entire project are CH2M Hill, who will support it through to completion in 2019.
Thames Water is also investing £400 million to expand the capacity of London’s three largest sewage treatment works: Beckton, Crossness and Mogden.