The challenges for improving water quality are outlined below as part of the London State of the Environment Report 2010.
Although the evidence shows that water quality in London has improved over the past few decades, it still faces a number of challenges that must be dealt with if water quality is to be maintained and further improved.
Growth and sewerage capacity
Water quality in London is under pressure from major urban development and the growing population of the capital. The London Plan has identified a requirement for a further 33,400 homes per year. This will put more strain on London’s sewer network and sewage treatment facilities, which are already at or close to capacity in places. It is likely to increase the amount of impermeable surfaces, and increase urban run-off in the capital.
Much of London has a combined drainage system, which carries both surface water (rain) and sewage. Moderate and intense rainfall events currently cause London’s sewerage system to exceed capacity and overflow, discharging more than 50 million cubic metres, per year, of untreated sewage from outfalls into rivers. This is called a combined sewer overflow or CSO.
London’s main sewage treatment works also discharge treated effluent into the Thames Tideway. However, if during wet weather, they are unable to adequately treat the high flows they receive, storm sewage may be released into the river to prevent the works being overwhelmed.
Urban diffuse pollution
When it rains, pollutants (chemicals, fuel and debris from the urban environment) are washed over impermeable surfaces into rivers, polluting the water and threatening life in the river. This is known as diffuse pollution and it is a significant problem in London. Diffuse pollution poses a threat to achieving the objectives of the Water Framework Directive in some areas of London.
Misconnections of foul sewage into surface water drains are a significant source of urban diffuse pollution in those areas of London where a separate drainage system is used. Misconnections happen when domestic plumbing has been connected into surface water drains instead of the foul sewer. This means untreated dirty water goes directly into London’s rivers without receiving treatment.
Sediment deposition is a major threat to all urban rivers and canals and is of major concern in some areas of London, particularly the Lower Lee catchment. Sediment reduces oxygen in the water which consequently reduces the river’s ability to sustain aquatic life. Sediment also poses a threat to water quality in the Tideway as it enters via rivers and the sea. This is particularly a problem in the middle Tideway during spring tides, as sediments are mobile. Re-suspension of organic material by the tide lowers the dissolved oxygen and quality of the water.
Water quality in London is likely to be adversely affected by climate change in the future. Climate change could lead to:
- An increase in the frequency and intensity of rainfall events that would put additional pressure on the combined drainage system.
- Longer periods of severe low flows and increasing temperatures, which would lead to a decline in oxygen levels in the river, threatening fish and other life. This would be a particular problem in the Tideway and other rivers already experiencing low dissolved oxygen levels.
- Low flows and less dilution which would exacerbate the impacts of sewage discharges and diffuse pollution on the river.
- An increased risk of eutrophication, which occurs when nutrients in rivers cause excess growth of algae, using up oxygen and blocking out sunlight. The risk is higher in rivers such as the River Lee, Pymmes Brook, River Wandle, Hogsmill River and Beverley Brook that receive sewage discharges.