Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
3.3 Settled sewerage
Settled sewerage refers to sewerage for conveying wastewater that has been
settled, for example in a septic tank. The origin of settled sewerage is to
convey overflow from septic tanks where the soil cannot cope or absorb the
overflow. This usually occurs when the groundwater table is high, or where the
soil permeability is low, or where there are rock outcrops. It can also be used
when effluent from septic tanks pollutes groundwater and it is necessary to
convey the effluent off-site and treat it. Because there are no solids that can
potentially sediment in the sewerage pipes, there is no requirement for the
self-cleansing velocity. Smaller pipes and lower gradients can be used. The cost
of settled sewerage is between a third and a half of conventional sewerage.
Originally developed in South Australia to overcome problems with failing septic
tanks, it has been used quite widely worldwide to upgrade septic tank systems.
Where there is no existing septic tank, an interceptor box or tank can be
used. It functions like a septic tank and designed in the same way (Figure
2.14). To reduce cost the wastewater from a group of houses can be connected to
one interceptor tank. Just like in a septic tank, accumulation of sludge has to
be removed regularly from an interceptor tank.
Figure 2.14: Interceptor tank in settled sewerage