The most basic answer is to make dirty water clean. Treatment facilities simply compress the organic decomposition processes which take place in nature. This is performed by a combination of physical, biological, and chemical treatment stages. Nature (receiving waters) can only accept small amounts of sewage before becoming polluted, that is, natural bacteria feed on the sewage organics and create an abnormal amount of dissolved oxygen uptake. Dissolved oxygen which exists in minute amounts (10 parts per million @ 20ºC), is required by all marine life for survival. One of the principle objectives of wastewater treatment is to prevent as much of this "oxygen-demanding" organic material as possible from entering the receiving water.
Other objectives of wastewater treatment include, removal of objectionable items, nutrients and heavy metals.
The degree or level of treatment required often depends upon:
- The nature and quality of the receiving water, ie., salt water, inland fresh water bodies, estuaries. Federal/Provincial regulations. Quantity of flow from the treatment plant. Uses and existing quality of receiving waters. The amount of mixing between a plant effluent and receiving waters.
- Assimilation capacity of the receiving water.
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