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Tertiary treatment refers to water treatment processes beyond secondary treatment.
It is advanced cleaning of wastewater that goes beyond the secondary or biological stage, removing nutrients such as phosphorus, nitrogen, and most biochemical oxygen demand (BOD) and suspended solids.
Granular media, such as sand or anthracite coal, or a membrane are used to filter out most of the solids remaining after secondary treatment. After filtration, finish water is usually disinfected with chlorine or ultraviolet light to inactivate bacteria, viruses, and other microorganisms prior to discharge or recycling.