In most municipal systems hydrants are connected to the drinking water system except during periods of extreme water shortage. The biochemical processes that take place in water bodies have also been relied on to neutralize sewage. Aerobic, or oxygen-requiring, bacteria feed on the organic material in sewage, decomposing it. However, this process uses the oxygen that is dissolved in water. In order to avoid these problems, it is now recognized that all sewage except unmixed storm sewage must be treated before it is discharged.
Primary treatment is removal of floating and suspended solids.
Secondary water treatment uses biological methods such as digestion. Complete, or tertiary, watertreatment removes all but a negligible portion of bacterial and organic matter. Industrial wastes are treated by a number of methods, depending on the specific nature of the waste. Increasingly, governments are forcing industries to process effluents either chemically or mechanically, or both ways, so that harmful substances are removed.