Parts of a Wastewater Treatment Plant

All 15 parts on the diagram

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Start here: 10 main steps 10

Influent The untreated wastewater flowing into the plant.
Bar screen A rack of bars that catches rags, sticks and other large objects.
Grit chamber A tank where sand, gravel and other heavy bits settle out.
Primary clarifier (sedimentation tank) A still tank where solids settle to the bottom and scum floats.
Aeration tank Air is mixed in so microorganisms can break down the waste.
Secondary clarifier A second settling tank, where the microorganisms sink out.
Disinfection tank (chlorine contact tank) Chlorine or ultraviolet light kills the germs left in the water.
Effluent The treated water that leaves the plant.
Digester A closed tank where bacteria break down the sludge.
Biosolids Treated sludge, which may be spread on land as fertilizer.

The water's path 2

Scum Grease, oils and other light material that floats and is skimmed off.
Sludge The solids that settle to the bottom of a clarifier.

The solids' path 3

Thickener A tank where sludge settles again and loses some of its water.
Biogas Methane-rich gas from the digester; it can be burned as fuel.
Drying bed A shallow bed of sand where digested sludge is dewatered.

How this list was compiled: Read 2026-09-30; a name is kept only where two of these three use it. (1) US Environmental Protection Agency, Primer for Municipal Wastewater Treatment Systems (EPA 832-R-04-001, 2004), https://www.epa.gov/sites/default/files/2015-09/documents/primer.pdf : influent passes through preliminary treatment; bar screens; a grit chamber where sand, grit and small stones settle; primary sedimentation in a sedimentation tank, where the settled solids are primary sludge; the aeration tank, where wastewater is mixed with air and microorganisms; a sedimentation tank (secondary clarifier) for removing the excess biomass; disinfection by chlorine, ozone or ultraviolet radiation; effluent, the treated wastewater; digestion, which produces methane gas; dewatering, including drying beds; biosolids, used as a soil conditioner or fertilizer. (2) US Geological Survey, Water Science School, A Visit to a Wastewater Treatment Plant, https://www.usgs.gov/special-topics/water-science-school/science/a-visit-a-wastewater-treatment-plant : screening; grit settles out; sedimentation tanks, where sludge settles and scum (grease, oils, plastics and soap) is skimmed off; aeration tanks; thickening; large tanks called digesters; a chlorine contact tank to kill bacteria; the treated water, called effluent, is discharged to a river or the ocean. (3) Wikipedia, Sewage treatment, Bar screen, Clarifier, Activated sludge, Secondary treatment and Sewage sludge treatment: influent, bar screen, grit chamber, primary clarifiers, scum, aeration tank, secondary clarifier, disinfection, effluent, thickening, digesters, biogas, drying beds, biosolids. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. A schematic, not to scale. Licence: own work

How it works

A wastewater treatment plant does two jobs at once. It cleans the water until it can go back to a river, and it deals with the solids taken out along the way. Each job is a row of tanks.

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The water's path

Influent arrives from the sewers. The bar screen catches rags and sticks, and the grit chamber lets sand sink. In the primary clarifier, sludge settles and scum floats. The aeration tank bubbles air through the water so microorganisms can break down what is dissolved. They settle out in the secondary clarifier. After the disinfection tank, the effluent leaves.

The solids' path

Sludge from both clarifiers goes to the thickener, which removes some water, then to the digester, where bacteria break it down and give off biogas. A drying bed takes out more water. What remains is called biosolids.

Follow the arrows: each row reads left to right.

Questions and answers

What are the parts of a wastewater treatment plant?

For the water: a bar screen, a grit chamber, a primary clarifier, an aeration tank, a secondary clarifier and a disinfection tank. For the solids: a thickener, a digester and a drying bed or other dewatering equipment. Plants differ, and many add further steps.

What are the stages of wastewater treatment?

Preliminary treatment screens out large objects and grit. Primary treatment lets solids settle. Secondary treatment uses microorganisms and air to remove dissolved waste. Disinfection comes last. Some plants add advanced, or tertiary, treatment before the water is released.

What is the difference between a primary and a secondary clarifier?

Both are still tanks where solids settle. The primary clarifier comes first and removes solids that arrived with the sewage. The secondary clarifier comes after the aeration tank and removes the microorganisms that grew there.

What happens to the sludge?

It is thickened, then broken down by bacteria in a digester, which also produces methane-rich gas that can be used as fuel. After dewatering, the treated solids, called biosolids, are spread on land as fertilizer, sent to a landfill or burned.

Where does the water go after treatment?

The treated water, called effluent, is released into a river, a lake or the ocean. That is why treatment plants are usually built on low ground near water.