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. Often the concentration of organic waste is so great that the biochemical oxygen demand (BOD) depletes the water's oxygen supply, killing fish and plants. In order to avoid these problems, it is now recognized that all sewage except unmixed storm sewage must be treated before it is discharged.
Sewage treatment is classified as primary, secondary, or tertiary, depending on the degree to which the effluent is purified. Primary treatment is removal of floating and suspended solids. Secondary treatment uses biological methods such as digestion. Complete, or tertiary, treatment 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.
Domestic sewage must be treated to produce discharge water that is free of odors, suspended solids, and objectionable bacteria. (Coliform bacteria, which inhabit the lower intestines of mammals, while not pathogenic of themselves, are taken as an index of contamination of watercourses.) In rural areas, sewage can be stored in a holding tank, e.g., a septic tank; naturally occurring anaerobic bacteria can decompose the solids, which then settle to the bottom. While suitable for small systems, this method has several disadvantages. First, anaerobic decomposition produces noxious gaseous effluents, and it is fairly slow. Second, harmful bacteria may still be present in the liquid effluent.
In large urban systems, a combination of processes must be used. Decomposition can be speeded by forcing air through the mass so that aerobic bacteria can be used. This oxidation process is typically combined with filtration, either in sand or in granular activated carbon, and with several hours of aeration. The liquid can then be discharged, often after being disinfected with chlorine. The liquid may be also treated by microfiltration, reverse osmosis, and hydrogen peroxide and ultraviolet light to produce very clean water that can be reused. Another method of removing solids is to allow the liquid to stand in large tanks until the solids fall out and form a sediment, but the process is slow and requires the accumulation of large volumes of liquid.
Once solids are removed, they are treated in one of several ways. Most often they are removed in a semiliquid mass referred to as sludge. Sludge may be transferred to tanks where it is digested by aerobic or anaerobic bacteria. Gaseous byproducts of this digestion are collected for use as fuels. After digestion, solids may be dried and enriched with plant nutrients for use as fertilizer. In other cases, with or without digestion, they may be dried and incinerated at 1200 to 1400 degrees F (650 to 760 degrees C). In other cases solids are buried in landfills or dumped far at sea, although environmental objections to such dumping has led to its drastic curtailment.