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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

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