In biological treatment, also known as secondary treatment, organic matter that is soluble or in the colloidal form is removed. Secondary treatment is done with the help of aerobic (works in the presence of oxygen) and anaerobic (works in the absence of oxygen) micro­organism. Biological process utilizes biochemical reactions; typical examples are biological filtration (trickling filters), oxidation ponds and the activated sludge process.

Chemical processes utilize the chemical properties of the impurities or of the added reagents. Commonly used chemical processes are adsorption, precipitation, coagulation and disinfection (the removal of nitrate and phosphate bacteria is carried out by passing chlorine/ ozone gas.

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Advance waste treatment may involve physical, chemical or biological processes or their various combinations depending on the impurities to be removed. These processes are employed to remove residual soluble non-biodegradable organic compounds, including surfactants, inorganic nutrients and salts, trace elements and dissolved inorganic salts. The advance waste treatment process are expensive and are used only when water produced is required to be of higher quality.

After these treatment stages, the water is clean to supply in house-hold and discharge in water bodies. The sludge is the by-product of the treatment process and it can use in agriculture as manure.

Sewage treatment is done with the help of septic tank, which is an anaerobic suspended growth biological treatment system. They are made up of concrete and situated under the ground, having distributor box and porous rods. In septic tank anaerobic bacteria bio­chemically oxidize the organic matter. Septic tanks are usually adopted for small communities of population up to 300.

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