Biological Treatment:

Biological treatment utilizes biological activity to stabilize or remove impurities and they are particularly useful for the removal of organic impurities. Biological processes may be aerobic, anaerobic or facultative and are mostly done to purify waste water but not commonly used for treatment of water.

Figure: Typical Water Treatment Processes

In practice, there are several processes which combine various modes of operation although one may predominate. For example, in the filtration of water through a bed of sand, the processes involved include physical straining, sedimentation, chemical and molecular attraction reactions, and some biological activity.

In many cases of water treatment, it is necessary to use several processes in sequence to achieve the desired natural quality. Physical processes like screening and straining often form first stage of the treatment chain and these are frequently followed by further physical or chemical processes for water treatment. Figure shows some typical process chains for a number of situations.