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Wastewater runs down our drains in homes, schools, businesses, and factories– but where does it go? Well, most likely, it’ll go down into a sewage system. In places like New York City, it’ll come together and flow into a wastewater treatment facility. Here are the ways that wastewater is treated. 

Preliminary Treatment

First, influent, or incoming wastewater, flows into the treatment facility through the sewer system. As it enters, it passes through bar screens and removes leaves, twigs, and litter (plastic bags, food wrappers, bottles, sanitary wipes, etc.) from the water itself. The trash is subsequently collected and transported to landfills. Then sewage pumps push the wastewater from the screens to the beginning of the facility. 

Primary Treatment

Then, the wastewater moves through the primary settling tanks, slowing the water flow down. This is where the lighter solids, like grease and small plastic material, are skimmed from the surface. Not only that, but the primary sludge (feces, food, and paper fibers) sinks down to the bottom of the settling tank. It’s then removed for thickening and digestion. 

Secondary Treatment 

Throughout the secondary treatment, air is added to aeration tanks in order to help provide a good and healthy environment for the oxygen-loving microorganisms that are present in the sewage. These microorganisms will eat the wastewater’s organic material, yielding heavy particles that can be removed easier. 


The aerated wastewater flows to the final settling tanks, where secondary sludge settles to the bottom. It’s then removed and combined with the primary sludge for thickening and digestion. However, some of the secondary sludge is moved back into the aeration tank to help aid the microorganisms and process the incoming wastewater. 


Sodium hypochlorite, the same chemical that’s found in household bleach, is used to disinfect the wastewater and remove the remaining disease-causing bacteria and microorganisms. It’s then released into local waterways. 

Sludge Treatment 

The sludge that is collected in the primary and secondary treatments is thickened and used to separate the water from the solid material. The sludge is put into a tank called a digester, a low-oxygen environment heated to 98ºF. These microorganisms that thrive in a hot environment will digest the sludge and are converted into methane gas or biogas. After 15 days, the sludge goes through a dewatering process, further removing water from solids using large centrifuges. The remaining solids– biosolids– will be composted, added to agricultural soils, or processed even further for beneficial uses.