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Biological Treatment

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As in all biological processes that use liquid, the oxygen required by micro-organisms or bacteria must be available in a dissolved state. In "natural" aerobic purification (ie: the way in which water cleans itself in a natural environment, in ponds for example) the oxygen necessary for the biological reactions is made available in a dissolved state through either oxygen in the atmosphere being dissolved into the water or due to the function of oxygen-producing chlorophyll present in green algae. In "active mud" plants the increased concentration of bacteria there is not enough oxygen available from the atmosphere. It is therefore necessary to carry out artificial aeration.

How does it work?

In an oxygen rich environment, like aeration tanks, a complete biological process can take place. Firstly, a chemical oxidation of the reduced compounds (such as hydrogen sulphate, sulphides and sulphite) occurs. Then, through a physical and biological process, the large concentration of bacteria which has been growing in the aeration tank intervenes and forms a "colony" consisting of "biological sludge". This sludge binds together any sedimentable solids that have escaped previous treatment. During the long period of time in which the waste is inside the aeration tank, the bacteria themselves use some of the soluble organic substance present in the liquid for their own growth and development. In this way the bateria increase in size making them sedimentable.