How does a sewage treatment plant work?

Question

Can anyone tell me how does a sewage treatment plant work?

Answer 1

A sewage treatment plant consists of 3 main sections; settlement zone, treatment zone, clarifier zone.

The waste material is rendered harmless and light sludges and non-biodegradable waste is exported by a vacuum tanker generally once per year.

The Sewage Treatment Plant discharges to a soak-away, to a ditch or a water course, subject to the EA consent to discharge issued.

Answer 2

These sites walk you through the process at different levels, for both municipal sewage and industrial wastewater.  Start with the first link which has an animated walk through.  The details of the systems are too great to reproduce here.

Answer 3

Irin is right, but to expand slightly:

The key factor in sewage treatment - whether on a domestic or municipal scale - is the use of microbes. What we think of as waste, they treat as food. Because domestic sewage treatment facilities work on such a small-scale, it is possible to upset the ecology of the system and inhibit the functioning of the microbes by the use of relatively small quantities of harsh chemicals.

The microbes convert much of the waste material into gases and water, but there is always material left which they cannot consume. This is the sludge that must be periodically pumped out of the tank.