Sewage treatment (waste water treatment)
Two major methods are in use, biological filter beds and the 'activated sludge' process, of which the latter is more common. There are variants on this process, but one layout is shown in the diagram overleaf.
Sewage comprises domestic and industrial waste, as well as drainage water from rainfall: about 99.9% is water. Many types of microorganism are present, most of which are harmless, but some are pathogens from infected people and animals. The pathogens include bacteria which cause dysentery, viruses responsible for polio and hepatitis, and many others. The main aim of sewage treatment is to reduce the organic content and suspended material to a level which does not harm the ecology of the area (usually a river) receiving the effluent.
Although the number of pathogenic microorganisms is greatly reduced during treatment, many thousands of potentially harmful organisms are still present per cm3 of effluent. This knowledge should alert students to the dangers of swimming in the sea or rivers which receive effluent.
Effluent from sewage works contributes to the supply of water available for abstraction and use in the mains water supply, after purification.
This removes larger solids by mechanical screening and then by sedimentation in the first settling tank. From here, the liquid and sludge are treated separately.
The liquid is passed to the oxidation pond, where it is vigorously agitated and aerated. Various types of aerobic bacteria digest, break down and oxidise the
organic components, producing nitrates, sulphates, carbon dioxide and water. In doing so, they gain energy and raw materials for their own growth and
reproduction. Protozoans are also present and these consume many bacteria. Further settling out of solids takes place in the second settling tank. About 10% of the sludge from this tank is returned to the oxidation pond to provide more bacteria and protozoa: the remainder passes to the digester (see below).
It may be necessary to treat the effluent further to remove excess nitrates and phosphates, which could bring about eutrophication. Also excess nitrates may persist in drinking water, where they may be a hazard to the health of young babies.
The sludge from the different tanks passes into the digester, a large vessel containing very little air. The sludge is stirred gently and warmed. Further breakdown of organic compounds takes place by anaerobic microorganisms and, as a result, a mixture of carbon dioxide and methane is produced. The methane is burned to provide heat for the digester and electricity for the pumps in the sewage works. The digested sludge may be dumped into the sea or into landfill, spread on farmland or dried and sold to the public as a fertiliser. However, there is concern about the prolonged use of the material as fertiliser, since traces of toxic heavy metals may be present. Some sludge is now incinerated because EU legislation limits disposal at sea.
