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surface. The general process of removing sewage is designated as sewerage and
the entire systems of sewers including a sewage treatment plant is known as a
sewerage system.
The method of sewage treatment to be adopted in a particular case will
depend almost entirely on local conditions. It may consist only of the discharge
of the raw sewage into a stream or a large body of water. The usual methods of
sewage treatment consist either of preliminary treatment alone or of primary
treatment followed by secondary treatment.
During primary treatment the larger and heavier solid particles settle out
from the liquid. These solid particles that settle out form a slimy paste which is
known as sludge.
The partly clarified sewage that has been given primary treatment
generally contains much decomposable materials. Therefore, further treatment
which is known as secondary treatment is usually required.
An auxiliary treatment which may be used with either primary or
secondary treatment is disinfection or the killing of the most of the bacteria in
the sewage by means of chemicals.
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