The domestic sewage in large cities is processed by anaerobic bacteria in the primary treatment and by aerobic bacteria in the secondary treatment in sewage treatment plant (STPs). It has a high BOD as it contains both aerobic and anaerobic bacteria. It has very high amounts of suspended solids and dissolved salts. When treated in STPs, it does not really require the aeration step as the sewage contains adequate oxygen. It is processed by aerobic and then anaerobic bacteria in the secondary treatment in sewage treatment plant (STPs).

Sewage is wastewater having food residue, animal and human excreta and waste material from industrial establishments. The domestic sewage in large cities is processed by aerobic and then anaerobic bacteria in the secondary treatment in sewage treatment plant (STP).Treatment of water:-1. Primary treatment - The treatment utilizes settling tanks of upward, horizontal tanks. The water flow is induced to allow 15-45 min retention time during which 70% of the suspended solids are removed.2. Secondary treatment - The secondary treatment involves the use of aerobic bacteria for the oxidation of organic matter which produces carbon dioxide and water. After that, anaerobic bacteria are used for the anaerobic digestion process to complete the decomposition of organic matter.3. Tertiary treatment - After the secondary treatment the water is disinfected which is done with the help of chlorine, ozone or UV radiation.