Wastewater Treatment Plants

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Do you ever wonder where the water from your home goes after it disappears down the sink or after you flush your toilet? If you live in a town or city, all of this water flows through the sewer system, ending up at your local Wastewater Treatment Plant (WWTP), to be cleaned up and discharged into a river or stream.

WWTPs are normally located close to a river or stream for two reasons: having the plant in a low-lying area allows the sewer system to use gravity to help deliver the wastewater to the treatment plant, and it positions the treatment plant to discharge the treated wastewater (referred to as effluent) into the river or stream.

wastewaterOnce the wastewater enters the plant, it goes through various stages of treatment, depending on the city and the composition of the wastewater. Some areas demand advanced treatment to remove waste that is resistant to conventional treatments.

The first stage, or primary treatment, is designed to remove large debris and solids from the water. The water is pumped into sedimentation tanks, where solids and suspended sediment is allowed to settle out of the bottom, and scum rises from the top. This material is removed and incinerated or sent to a landfill. Primary treatment removes about 60 percent of solids from wastewater.

If you live in a rural area, your house probably has a septic tank to treat the waste from your own house. Septic tanks act similarly to sedimentation tanks in WWTPs, by allowing large solids to settle to the bottom and scum to rise to the top. As new water flows in, the settled water is pushed out and goes to a drain field where it is absorbed by the soil. Some homes may construct a pond to absorb the water from the septic tank.

Secondary treatment involves aerating the wastewater. The water is aerated (or shaken up) to release dissolved gases such as hydrogen sulfide. Aeration also replenishes oxygen, which is consumed by decaying organic matter found in wastewater. Bacteria are also used at this stage to consume nutrients and organic materials. The wastewater then goes to settling tanks, allowing the bacteria to settle out. Secondary treatment can remove up to 90 percent of all solids and organic material. Most cities and townships require a minimum of secondary treatment for wastewater.

Tertiary treatment can vary depending on the treatment plant and the composition of the wastewater. Tertiary treatment may involve the use of chemicals to remove remaining nutrients (nitrogen and phosphorous). It may also include the use of filter beds or other types of treatment.  Tertiary treatments are applied where constituents of concern remain after primary and secondary treatment.

Oakland2The final stage of treatment is disinfection, which removes bacteria, viruses and other microorganisms before the water is discharged back into the environment. In America, this is most often accomplished through chlorination. Adding chlorine is a cheap and effective method of disinfection, but it also requires an additional step to remove the chlorine before the water is discharged to a river or stream. Residual chlorine in effluent can be harmful or fatal to aquatic species, and ideally, effluent should contain little to no chlorine. Other WWTPs may use ultraviolet light, or ozone to remove microorganisms.

A summary of the wastewater treatment process can be found here.

WWTPs are required to monitor their effluent (treated wastewater discharged to the environment). The WWTP must acquire a permit allowing them to discharge the treated wastewater, which establishes limits for certain constituents. Other constituents may be required to be monitored regularly, but may not have a limit placed on them.


These are the most common parameters monitored in effluent:

  • Biochemical Oxygen Demand (BOD) – BOD indicates the amount of organic material remaining in the effluent, by measuring the amount of oxygen required for microorganism to completely digest the remaining organic material. A higher BOD indicates a larger amount of organic material in the water.  Ideally, effluent will have a BOD of zero.
  • Suspended solids – The amount of solid material remaining in the effluent. Ideally, effluent will have zero suspended solids.
  • pH – A measure of the acidity or basicity of the effluent. Ideally, the pH of the effluent will match the pH of the river or stream it is discharged into.
  • Dissolved oxygen – Measures the amount of oxygen available to aquatic life once the water is discharged. The effluent should be as close to 100 percent saturation as possible.  (Dissolved oxygen concentration is dependent on the ambient temperature and atmospheric pressure).
  • Total nitrogen/total phosphorous (nutrients) – It is important to monitor the amount of nutrients remaining in the effluent. Discharging too many nutrients into a river or stream can cause eutrophication (excessive nutrient enrichment), which can threaten aquatic life. For more information, see the Nutrient section.
  • Chlorine – If chlorine is used to remove bacteria during treatment, it should be chemically removed before the water is discharged to the environment. Large concentrations of chlorine are toxic to many aquatic species and can kill beneficial bacteria in the environment. Ideally, effluent should have no detectable concentration of chlorine.
  • Fecal coliform bacteria – Fecal coliform is a group of bacteria that includes both harmful and benign strains of bacteria, including E. coli, and is measured as an indicator of fecal contamination. It is important for wastewater effluent to be virtually free of bacteria to prevent the degradation of water quality in the receiving body of water.

Unfortunately, only a handful of the most modern and expensive WWTPs in America discharge close-to-pristine effluent. It is important to keep WWTPs up to date and operating properly to reduce the degradation of our rivers and streams.

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