Micro-organisms purify water

Before being discharged into rivers and lakes, waste waters must be purified. One of the systems used for this purpose is activated-sludge purification. This process utilises the water self-purification principle, in which the micro-organisms (bacteria, protozoa and metazoa) use the organic substances contained in the water, transform them and remove them from the water. The aerobic process is the most commonly used one, i.e. oxygen is supplied to trigger the biological processes in which the organic substances are oxidised. When this reaction is over, water is purified and flakes of biological material known as “activated sludge” are produced. This activated sludge consists of organic and inorganic components and different species of micro-organisms (especially bacteria). Ciliate protozoa play a very important role in the purification process. There is competition for food in the oxidisation tanks, i.e. the place in which purification takes place: bacteria are eaten by small predators (ciliate protozoa), which in their turn are eaten by larger organisms (carnivore protozoa or metazoa); so, bacteria need nutrients which consist of the organic matter contained in the waste water. The presence of ciliate protozoa in the activated sludge proves therefore that there are bacteria as well; if there are approximately 10 million individuals per litre, it means the purification plant works. By studying ciliate protozoa, used as bio-indicators, one can check if the plant works (low oxygen content, too many organic substances, excessive sludge extraction) and take corrective measures.

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