For decades now engineers have chosen sewage treatment solutions from a small range of technologies that are consistent with the water-carriage of the sewage (in pipes) to a waterway where the sewage will be discharged.

Conventional treatment methods are classified into stages. The preliminary stages involve grit removal and the screening of gross solids from the sewage. Primary treatment removes some of the suspended solids from the sewage and almost always consists of sedimentation of those solids in tanks. Secondary treatment utilises aerobic micro-organisms to break down some of the organic matter in the sewage and the two main methods of doing this are biological filters or activated sludge treatment.

Tertiary treatment is an additional stage that has been added since this time. It involves additional filtering or oxidation of the effluent. A nutrient removal stage may also be added. However the basic primary and secondary treatment processes that are most often used had been invented and were in use by 1920. They have since been improved upon and refined, and the underlying principles are better understood, but there have been no breakthroughs or revolutions in conventional sewage treatment since then.

The skill of the modern sewerage engineer lies in the ability to choose, from within the paradigm, the cheapest treatment process for a given situation that will perform the minimum treatment necessary to conform with local regulations and standards without offending the sensibilities of a large portion of the public. An early American engineering text argued that 'Changing the character of the sewage merely for the sake of making it less offensive or dangerous is a waste of money unless it is necessary.'(Metcalf and Eddy, 1935)

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