How does a sewage treatment plant work?

Written by don shepard
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Operation of a sewage treatment plant generally works the same no matter your location in the U.S. Removing total suspended solids (TSS) and biochemical oxygen demanding waste (BOD) are the main goals of a sewage treatment plant. Extracting these pollutants reduces the chance of fish kill due to eutrophication and disease carrying microorganisms forming on the suspended solids. Eutrophication is basically the decreasing of oxygen to levels unfit for life. U.S. federal law requires wastewater treatment to be completed.

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Preliminary and Primary Sewage Treatment

Once wastewater arrives at the sewage treatment plant, preliminary treatment begins with wastewater flowing through large bar screens that removes big objects and particles such as sand and gravel. Wastewater then runs through the comminutor that shreds organic matter such as paper. This readies the wastewater for primary treatment. After flowing to settling tanks, oily materials float to the surface where they are removed by skimmers and solids fall to the bottom where sludge is formed and collected. According to the Muncie Wastewater Treatment Facility Guide aneorobic digesters process the sludge for disposal.

Secondary Sewage Treatment

Removing BOD and TSS continues in secondary treatment of wastewater. Effluent from primary wastewater treatment is mixed with activated sludge in aeration tanks. Activated sludge has microorganisms to decompose organics. Microorganisms require oxygen, so diffusers are equipped at the bottom of the tank to allow passage of compressed air. Moving upward, this air helps to mix and speed up the digestion process of the microorganisms. Flowing out of the aerator the waste water again goes to sewage treatment plant clarifiers where microorganisms cling together, forming flocs and settling at the bottom. These settled flocs are activated sludge that can be reused within the sewage treatment plant.

Tertiary Sewage Treatment

Flowing through the secondary clarifiers the wastewater is now ready for filtration. According to Environment and Society: Manual of Laboratory and Field Experiences, "the filter medium consists of anthracite coal, sand, garnet and gravel." This can vary depending on the location of the sewage treatment plant. This filtration further removes suspended and dissolved solids.

Chlorination and Dechlorination

Many sewage treatment plants include a final step of chlorination in order to completely remove microorganisms before releasing the former wastewater. Reacting with sulphur dioxide, the chlorine is then removed because it can harm life within the waters to which it is released. From here the wastewater flows out of the sewage treatment plant, generally into a river or stream.

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