Why is there so much sewage in the river Thames?

London's Victorian built sewage system, now much overloaded, combines rainwater drainage with sewage and so it is prone to overflowing in periods of heavy or sudden rainfall. Rather than letting it back up into streets and houses, a system of Combined Sewage & Stormwater Outfalls (CSO's) are deliberately opened to discharge into the Thames at 63 points along the tideway.

There are over 60 major discharges every year, including the much reported event in the summer of 2004 – at least 650,000 tons were discharged in one go with drastic consequences for the river's wildlife. However, this was not particularly unusual - over 60 million tons are discharged yearly. 2/3rds of which is directly from the inadequate sewage treatment facilities - possibly a greater load of sewage in the river than ever before.

Thames Water is legally permitted to discharge sewage at any time into the tideway. Discharges consist of raw sewage, activated sewage sludge, light industrial waste and rainwater with some land contamination.

Thames Water have closed many small sewage works in the London area. All the sewage is now handled by five main sewage works in Mogden (Isleworth) in the West and Riverside, Longreach, Beckton and Crossness in the East. Land from old works and pumping stations has been sold for lucrative residential developments (some despite objections from the Environment Agency). The capacity of the treatment works has been increased but not by nearly enough, and the pipes and tunnels under London are no longer adequate to handle both the sewage and drainage together (see aims).