Definition - What does Secondary Treatment mean?
Secondary treatment is the portion of the sewage treatment process in which microorganisms remove dissolved pollutants from liquid sewage after solids have been removed. Biodegradable soluble compounds are removed from the water by acting as nutrients for bacteria and protozoa and the process requires aeration and temperatures conducive to bacterial oxidation.
Safeopedia explains Secondary Treatment
Solids in waste-water can be removed by allowing them to settle out, or float so that they can be removed, but the soluble pollutants in the water are still present, making it unsafe for release into the environment. The EPA requires effluent to have less than 30mg per liter biochemical oxygen demand (BOD). Water that has undergone secondary treatment is still subject to regulations for its disposal. Tertiary treatment may be required before effluent can be released.