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Biological Treatment of Waste Water

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Overview | Waste Water Composition | Waste Water Treatment
Biological Treatment of Waste Water | A Waste Water Treatment Plant

Waste water in general, and sewage in particular, usually contains amounts of hydrocarbon that need to be reduced to a certain low level or even to zero. Biological treatment can eliminate these hydrocarbons. Certain bacteria are able to 'eat' them and, in doing so, do the same thing that any living species is doing - they 'burn' their 'meal' to produce energy that keeps them warm, and lets them move and reproduce. In other words, it lets them live and gives us clean water.

Bacteria for Waste Water Treatment

Since sewage consists of a great variety of organic and inorganic components, no special bacterium can eat them all. A population of various micro-organisms is the best ingredient for a good degradation. There are, however, certain groups of bacteria that are specialised to deal with certain hydrocarbons, surrounding conditions and nutrition. They are closely related to the way biological waste water treatment is performed.

'Aerobic' bacteria require oxygen to burn hydrocarbons. Aerobic treatment technology can usually be recognised by air blown into waste water basins, making the whole thing blubber around. Providing air this way requires compressors that consume power, causing treatment costs to increase. Therefore, a technology is desired that does not need to have compressors blow air into the waste water. Two different kinds of bacteria satisfy this need.

Waste water treatment in an 'anoxic'1 environment is performed with a different type of aerobic bacterium. No air or oxygen needs to be blown into basins. Instead they obtain their oxygen from groups such as nitrate (NO3-) or nitrite (NO2-). These compounds are chemically reduced to molecular nitrogen (N2), providing the bacteria with their oxygen.

The third group of bacteria and the related technology are called 'anaerobic' - they do not need oxygen. Instead they often produce it as their waste. In the early days of life on our planet, these bacteria dominated, as the atmosphere contained very little oxygen. Oxygen was, in fact, a poison. Producing this waste gas led to the first major extinction of species and is also linked to the greenhouse effect.

Biological Reactions

Aerobic bacteria need nitrogen and phosphate, as well as carbon and oxygen, to live. Both elements are widespread in nature and known to be important ingredients in any kind of manure. The optimum ratio of the elements carbon, nitrogen and phosphorus in the nutrition of bacteria has been determined as 100:5:1.


Biological activity similar to that in waste water treatment can be observed in the production of compost. Organic waste is eaten by micro-organisms, producing heat that stimulates them to eat more, reproduce and die. Death of the biomass is the final stage in the process and is what makes compost a good fertiliser.

1Devoid of oxygen.

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