2.3 In 2000, Thames Water set up the independently-chaired Thames Tideway Strategic Study to identify the environmental harm that storm sewage overflows cause, and to identify options to address them. In the case of the Tideway, the Study concluded that existing measures failed to adequately limit the impact of overflows, and that existing guidance was not well placed to deal with discharges on the scale of a catchment the size of London's. The study reported three areas of harm:
• Aesthetic: sewage-derived solids creating offensive conditions both on the river and the foreshore.
• Ecological: reduced oxygen levels in water following discharges, resulting in fish mortality or undesired behaviour (eg avoiding certain stretches).
• Health: increased levels of pathogenic microbes in water following discharges, posing a higher health risk to river users.
2.4 The Study proposed criteria to measure satisfactory performance against their three areas of harm and to identify appropriately protective solutions:
• Dissolved oxygen standards. Sewage discharges reduce dissolved oxygen levels, causing harm to fish. Four duration-based dissolved oxygen thresholds were set. These allow dissolved oxygen below a certain threshold for a given 3 kilometre stretch of river, for a defined number of tides; with progressively fewer tides allowed for lower thresholds.
• Flow-based criteria to identify unsatisfactory CSOs. In 2005, the Environment Agency set thresholds for each of the three areas of harm which defined CSOs as unsatisfactory based on their average discharge volume, loading, and the sensitivity of the surrounding area (Figure 4). Failure of any one threshold resulted in unsatisfactory status. Solutions were required to limit pollution from all unsatisfactory CSOs, but criteria defining acceptable limits were not devised.
• The 'four spills' criterion - A third criterion was also established to use in place of the first two, in order to reduce modelling effort. Thames Water and the Environment Agency agreed that a maximum of four spill events in a modelled 'typical year'6 would deliver satisfactory environmental performance.
Figure 4 Environment Agency methodology for categorising unsatisfactory Combined Sewer Overflows (CSOs) | ||
Objective | Criterion for 'unsatisfactory' status | Inputs |
Aesthetic | Average discharge above a threshold giving rise to aesthetic harm, with lower thresholds set for areas of greater public access and activity. Historical complaints received about a CSO were also sufficient to warrant unsatisfactory status. | Sewer model flow predictions from 21 rainfall events. Complaints records. Environment Agency assessment of high visibility, or heavily-frequented areas of the Tideway. |
Ecological | CSOs whose average polluting load during storms is above a threshold likely to cause dissolved oxygen deficits which harm the Tideway ecology. | Sewer model flow predictions from 21 rainfall events. Dissolved oxygen standards. Environment Agency assessment of sensitivity of three sections of the Tideway. |
Health | Average discharge above a threshold giving rise to unacceptably high health risks to recreational users with lower thresholds applying to areas of heavier recreational use. | Sewer model flow predictions from 21 rainfall events. Environment Agency assessment of heavily-used areas of the Tideway. |
Source: Environment Agency, An Assessment of the Frequency of Operation and Environmental Impact of the Tideway CSOs, (2005) | ||
The Department accepted these three measures as a valid basis for appraising potential solutions.
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6 The rainfall data used to represent the 'typical year' is from 1979-80, a year chosen by modelling consultants WRc as being the most representative of a 34-year historical rainfall series.