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From Appropedia

Secondary treatment of sewage and other 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.

Suspended processes, particularly activated sludge, are most common; fixed film methods such as roughing filters need less maintenance and control, and are more resilient, and are appropriate where cost and maintenance are major issues.

Anaerobic treatment is sometimes used, in the form of septic tanks and in biogas digesters. in the case of septic tanks the primary and secondary phases are combined in one unit. If biogas digesters are used for secondary treatment, the primary treatment phase is reduced or emitted (aiming to remove matter such as gravel and litter rather than sewage solids).^{[verification needed]}

Information on Sewage treatment#Secondary treatment can be found at Wikipedia.