Secondary treatment En route from primary to secondary treatment, water is aerated. Secondary treatment processes vary widely between facilities. In some cases, additional settling and mechanical filtration are the only remaining step before discharge to waterways. Sometimes the water is given adequate conditions to allow microorganisms to treat dissolved and suspended biological matter.
Tertiary treatment There are a variety of possible tertiary treatments, including biological processes, chemical treatment, and microfiltration. After tertiary treatment, water is released to the environment, or may be directed toward other uses such as irrigation or industry.
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Sludge treatment In every cubic meter of wastewater, there are between 80-220 grams (about 3 to 8 ounces) of solids.18 This aspect of treatment facilities is often overlooked when volumes of treated water are discussed. Sludge is subjected to different treatment processes, from landfill disposal to digestion, drying, and finishing. Sludge is often highly contaminated and only partially treated before being used as fertilizer, resulting in contaminated surface water and soil.
Any discussion of municipal wastewater treatment must include a reminder that many municipalities provide little or no treatment of wastewater before it is ejected into waterways. According to Environment Canada in 2009, 18% of the population has only primary treatment or less.19 The Clean Water Act (1972) and supporting grants in the United States eliminated raw sewage discharge by the 1980s, but, according to the EPA, 10 million people are still served by systems that provide only primary treatment.
In both countries, the number of people served by systems that include some form of tertiary treatment prior to discharge of wastewater is remarkably small. If you have high environmental goals for your project, research the treatment process of your municipal wastewater treatment system.
Sewage discharge from the home is carried to a septic tank equipped with one or two chambers. The inlet to the tank is equipped with a baffle that forces incoming solids to the bottom of the tank. Further baffles are provided to keep floating scum from clogging the exit pipe. After initially filling, every incoming quantity of sewage forces an equal amount of effluent out the exit pipe and into a series of perforated pipes known as the weeping bed. Here the effluent is discharged into the ground where it percolates in the soil and is remediated to whatever degree the biological conditions in the soil can provide.