Secondary treatment of wastewater is the stage of wastewater treatment designed to substantially degrade the biological content of the sewage. This usually uses biological processes. Municipal and industrial plants usually use aerobic biological processes. Activated sludge type of systems are most common in medium to large-scale plants. Anaerobic treatment is recommended when there is plenty of caloric value in the wastewater combined with relatively high wastewater temperature.
The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O). In an anaerobic process the anaerobic bacteria transform organic matter in the wastewater into biogas that contains large amounts of methane gas and carbon dioxide. It is an energy-efficient process often used to treat industrial wastewater that contains high levels of organic matter in warm temperatures. It can also be used as a pre-treatment prior to aerobic treatment.
In the biological treatment, the pollution is decomposed by aerobic biological bacteria that live in floc-like colonies called activated sludge.
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