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5.2 The Environment Agency regulates the release of sewage effluent to ‘controlled waters’ by way of ‘consents to discharge’. The consent limits the quantities of the various pollutants that can be released and helps to keep the quality of ‘controlled waters’ within acceptable limits. ‘Controlled waters’ cover all watercourses from rivers, lakes, reservoirs and underground resources through to estuarine and coastal waters. European legislation, principally the Urban Waste Water Treatment Directive (UWWTD) 75 , together with UK regulations, set the general standards for sewage treatment. Policy 3 Disposal of wastewater in London The Mayor proposes the following hierarchy for the disposal of wastewater: 1 Discharge wastewater to a foul sewer 2 Discharge wastewater to the combined sewer, as a last resort. This is the ideal hierarchy but it is recognised in many areas there is limited choice. Combined sewers 5.3 In the mid 1800s, Sir Joseph Bazalgette designed and initiated the building of London’s combined sewers. The sewers, still in operation today, remove wastewater and rainwater in the same pipe from properties in central London. In order to avoid the flooding of streets and properties with raw sewage during intense rainfall events, Bazalgette designed a series of overflow outlets from the combined sewers (see Figure 5.2) into the tidal River Thames and its tidal tributaries ( together referred to as the Thames Tideway. There are now some 57 such outlets, known as Combined Sewer Overflows (CSOs), which allow diluted storm sewage (excess sewage and rainwater) to spill untreated into the Thames Tideway. 5.4 The expansion of the area served by the combined sewers, together with population growth and an increase in impermeable surfaces, has resulted in greater flows through the sewers in wet weather. During dry spells the sewers have enough capacity to cope with flows. However, during rainy 71
72 periods the sewers quickly fill up with rainwater. Such is the strain on the system nowadays that even relatively moderate rainfall can trigger an overflow. Discharges occur at some CSOs between 50 to 60 times each year. Widespread heavy rainfall can lead to over a million tonnes of untreated sewage and rainwater legally discharging directly into the rivers. Despite much improvement in the Thames this is clearly unacceptable in the 21st century. 5.5 It also fails to comply with requirements of the Urban Waste Water Treatment Directive 76 . This requires wastewater to be collected and transported for treatment (generally secondary) before discharge. However, the Directive recognises that overflows will occur because it is not possible to construct collecting systems and treatment plants that will treat all wastewater under all conditions. It therefore requires Member States to adopt measures to limit pollution from storm water overflows. 5.6 The Thames Tideway has a delicate oxygen balance, particularly in the summer months. A relatively small volume of freshwater flowing over Teddington Weir together with storm discharge from the CSOs and sewage treatment works can adversely affect the quality of the Tideway in three prime ways: • A rapid drop in the dissolved oxygen puts wildlife at risk • A rise in the levels of pathogens can lead to greater public health risks for those using the watercourses directly • Sewage-derived litter is offensive, and reinforces the perception that the river is lifeless (see paragraph 4.39). Although only ten per cent of litter in the Thames Tideway is sewage derived, it may well be concentrated locally. In the summer of 2007 skimmer boats were brought into operation which removed most of the waterborne sewage derived litter. 5.7 Concern is often expressed about the health effects of storm discharges from the CSOs. The City of London and the Health Protection Agency undertook a study of the health risks to recreational users of the Thames between January 2005 and March 2006 77 . The stretch between Putney Bridge and Kew Bridge was chosen for study because of the concentration in recreational use by 26 clubs. Less than one per cent of days when river water samples were taken were acceptable by the World Health Organisation standard. However, the number of cases of illness reported was considerably lower than expected. One factor is likely to be under-reporting. However, the report also suggests that it is quite possible that users have developed a measure of immunity or improved tolerance to the pathogens. 5.8 The Thames Tideway Tunnels (comprising the Thames Tunnel and the Lee Tunnel) have been devised to mitigate these problems. The Thames Tunnel will be 32.2 kilometres long and will intercept all unsatisfactory CSOs along the length of the tidal Thames from west London to Beckton. The first
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