Sewage Treatment Sewage systems collect wastewater and treat it before discharging it back into the environment. These systems consist of intricate networks of underground conduits, or sewers, that convey the sewage through the treatment process to the point of disposal. Sewage systems also handle the flow of rainwater, either separately or as part of a single system. Separate systems are generally preferable because, in single systems, heavy rainfall can overload treatment plants, with the result that untreated overflow can become a source of pollution. In separate systems, rainwater is often allowed to flow into streams untreated because it is assumed to be relatively clean.
Sewage is processed in three major steps, called primary, secondary, and tertiary treatments.
Most areas do not use all three, and different areas use the treatments in different ways. The initial, and sometimes the only, method of cleaning wastewater is primary treatment, which consists of removing floating chunks and fine particles of solid waste. The simplest form of primary treatment is a cesspool, now found primarily in rural areas. A cesspool is a big tank with a porous bottom and sides that lets the liquid wastewater filter into the ground while holding the solid waste. Septic systems are somewhat similar, though the tank is connected to a drainage field so that more waste can be dispersed over a wider area.