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Sewage treatment thus involves the reduction in concentration of suspended (insoluble) and dissolved impurities to required levels. The overall process is characterized by some important features:

● the quantities involved are very large (they are often measured in ‘ population equivalents’, where waste production is of the order of 150–200 litres per day per capita for domestic wastes only, with at least as much again from other sources)
● the concentration of impurities in the influent sewage, under normal conditions, is actually quite low (0.5% of suspended solids and 1% of dissolved organics are quite high figures), and
● the concentration of impurities can vary very widely, even from hour to hour (while a large rain storm can make a tremendous difference).

Municipal treatment has been developed over a very long period, and was basically codified by the Royal Commission into Sewage Disposal (working from 1898 to 1915). From the investigations of the Commission the two-stage treatment process  became established as the satisfactory method of achieving a purification good enough for the intended disposal route.

Considerable development of the treatment process has occurred over the last few decades, mainly to improve the efficiency of the two-stage process, but also to introduce a third stage where disposal needs required it, and to reduce energy demands,
taking advantage of parallel developments in equipment and processing techniques.

The standard process begins with the primary stage, which involves the removal of suspended solids that can be separated easily, by screening or sedimentation. These solids are then treated separately from the main process line, to render them
fit for safe disposal.

The settled sewage from the primary stage passes to the secondary stage, which deals with residual suspended material and the bulk of the dissolved material by an aerated biological digestion process. This is followed by a second stage of settlement to separate the sludge resulting from this secondary treatment, which is also removed for separate treatment and disposal.

The clarified effluent following secondary treatment may be clean enough for discharge to a water course, or it may need still further treatment, in a tertiary stage, for a variety of polishing processes to suit the effluent discharge requirements. There may well then be some tertiary sludge, also needing disposal.

Each of the above processes is carried out in a range of equipment, most of which is part of the filtration process spectrum. There is very little overlap among the treatment processes, although some newer processes combine more than one stage in a
single process. Accordingly, the equipment items used in each are fairly distinct.

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