Sewage is domestic and industrial waste, and water drained from rainfall. Although sewage is mostly harmless (it is 90% water) it contains pathogens that can cause abdominal cramps, diarrhoea, and even paralysis.
This strangely named method is the most common type of sewage treatment.
First the sewage is screened where larger solid particles, such as stones or bones are removed.
The non-solid remains go to the first settling tank where denser material (the sludge) settles to the bottom and goes to the digester, the fluid will be sent to the oxidation (or aeration) pond.
In the oxidation pond, compressed air is pumped in to vigourously agitate (stir) and aerate (give air) the liquid. Various aerobic bacteria digest and break down organic material, producing nitrates, sulphates, carbon dioxide and water. Any remaining solids are settled out more in the 2nd settling tank. About 10% of this sludge is returned to the oxidation pond.
The digester is a large vessel with little air. The sludge is stirred and heated with anaerobic bacteria, these produce methane which is burnt to heat the digester and generate electricity for the sewage works.
Treated sludge sludge is dried and used as fertiliser on farms. The treated water will be pumped into rivers or oceans, in some cases it may be reused.
Water treatment is very important because it removes pathogens from water supplies that could cause lethal diseases. In developing countries where there is a poor sewerage infrastructure, millions of children die of diahorrea.
Good water treatment is particularly important in areas that do not receive much water. In California for example, sewage is cleaned and then 'recycled' back into the water supply rather than being dumped because of shortages in water supply in such a highly populated yet dry area.