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Understanding water quality
Understanding water quality

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5 Sewage treatment

Sewage treatment has two main aims: to control the spread of disease, by isolating the sewage so that viruses and other pathogens die, and to break down the sewage into relatively harmless substances to protect the environment into which it is discharged. In the UK around 97% of the population are connected to waste water treatment works by sewers, but this is very different on a global scale. In developing countries the figure is less than 5%; most of the urban sewage is discharged into surface waters without any form of treatment. Many cities in developing countries lack sewer systems, let alone treatment plants. Sewage is often drained into rivers or lakes that may also be used as water sources— with obvious problems for health. For many developing countries, contaminated water containing bacteria, parasites and viruses derived from human waste is a major cause of death.

Communities situated on coasts commonly discharge untreated sewage into the sea. This is an acceptable method of disposal if the outfall is far enough offshore, if currents do not bring the sewage back to land, and if the discharge is not too great. In these circumstances, the organic matter can be broken down by bacteria in the sea. Sea discharges are not always satisfactory, and a few of England’s beaches, for example, still breach the EU Directive on bathing water quality. However, by 2018, 97.7% of England’s beaches met at least the minimum standard of the Bathing Water Quality Directive (DEFRA, 2018).

In inland areas of industrialised countries, sewage is usually treated to reduce the amount of oxygen-consuming organic material before it is discharged into lakes or rivers. Sewage treatment aims to reduce biodegradable material and material in suspension, remove toxic materials and eliminate pathogenic bacteria. It converts sewage into a liquid effluent, mainly water, leaving behind a sludge. The discharge of effluent to streams and rivers in the UK is controlled through a system of discharge consents by the EA, SEPA and the EHS. These limit the total volume, the BOD and the suspended solid concentration. There is no fixed standard, as the character and use of rivers varies greatly; for example, if a small quantity of effluent is discharged into a river with a high capacity for self-purification, it may be harmless provided the effluent is not toxic. Effluents from industry often have to conform to additional standards, which may include limits on water temperature or content of toxic substances.