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Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes. [13] Different types of sewage treatment may utilize some or all of the process steps listed below.
Secondary treatment (mostly biological wastewater treatment) is the removal of biodegradable organic matter (in solution or suspension) from sewage or similar kinds of wastewater. [ 1 ] : 11 The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option.
Sewage treatment plant (a type of wastewater treatment plant) in La Crosse, Wisconsin. Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on the environment.
The basic national standard for U.S. municipal treatment plants is the Secondary Treatment Regulation. [2] Most plants in the U.S. must meet this secondary treatment standard. The permit authority (state agency or EPA) can compel a POTW to meet a higher standard, if there are applicable water quality standards for the receiving water body.
Sedimentation has been used to treat wastewater for millennia. [14] Primary treatment of sewage is removal of floating and settleable solids through sedimentation. [15] Primary clarifiers reduce the content of suspended solids as well as the pollutant embedded in the suspended solids.
Extended aeration agitates all incoming waste in the sludge from a single clarifier. The combined sludge starts with a higher concentration of inert solids than typical secondary sludge and the longer mixing time required for digestion of primary solids in addition to dissolved organics produces aged sludge requiring greater mixing energy input per unit of waste oxidized.
The fact that constructed wetlands do not produce secondary sludge (sewage sludge) is another advantage as there is no need for sewage sludge treatment. [4] However, primary sludge from primary settling tanks does get produced and needs to be removed and treated.
Vermifilters can be used for primary, secondary and tertiary treatment of sewage, including blackwater and greywater in on-site systems and municipal wastewater in large centralised systems. Vermifilters are used where wastewater requires treatment before being safely discharged into the environment.