At the wastewater treatment plants (WWTPs), treatment usually consists of two major stages - primary and secondary, which clean up water so that it may be safely released to a lake or river. During primary treatment, sand, grit and larger solids in the wastewater are separated from the liquid, usually with screens, settling tanks and skimming devices, removing 45% to 50% of the pollutants. However, wastewater still contains solid materials either floating, dissolved or both. Under natural conditions, these substances would provide food for organisms such as algae or bacteria that live in a stream or lake.
Secondary treatment removes these solids mainly through a biological process. Air is supplied to stimulate the growth of bacteria and other organisms to consume most of the waste material still present in the water. The wastewater is then separated from the organisms and solids, disinfected to kill any remaining harmful bacteria and released to a nearby stream, river, or lake.
Some wastewater treatment plants, such as NEORSD's Southerly WWTP, provide advanced treatment (also called tertiary treatment), which consists of oxidation of ammonia, de-nitrification, filtration and activated carbon absorption.
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