Sewage treatment

Sewage enters the sea via short and long sea outfalls, stormwater drains and rivers. The discharge of raw sewage into coastal waters is still widespread in the UK and in many other countries. In Europe, increasing efforts are being made to treat sewage effectively before discharge. This clearly reduces the impact of the sewage both on the marine environment and on coastal amenities. Sewage pollution of coastal and estuarine waters is usually most severe during the summer when temperatures are raised, river outflows reduced and seaside populations often increased.

Untreated domestic sewage consists mainly of waste water and solids from toilets, sinks and drains, which includes detergents (often containing phosphorus), other chemicals and plastics (e.g. from panty liners). Industrial waste is also dumped into sewers and oil and run-off from road systems may enter from storm drains. Raw sewage discharged into the sea may therefore contain large quantities of metals such as arsenic, cadmium, copper, mercury and lead as well as organic matter, petroleum products, fats, solvents and dyes. Thus there is considerable potential for human health risk and for ecological damage when untreated sewage is discharged into the sea. Currently over 80 per cent of Britain's large coastal discharges (serving more than 10 000 people) receive no treatment or are just screened. Hopefully this situation is changing as new legislation comes into force (see Section 10.1.4).

There are three main levels of sewage treatment:

1 Primary treatment. This involves screening and settling to remove larger solids and addition of chlorine or exposure to UV light to kill bacteria and most pathogens. However, there remains the problem of disposing of the sewage sludge.

2 Secondary treatment where the sewage is digested by bacteria in a treatment plant before chlorination. This can reduce the effluent's biological oxygen demand by as much as 99 per cent.

3 Tertiary treatment in which, in addition to the above, nitrate and phosphate are removed.

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