Skip to Main Content

Wastewater Treatment

Wastewater Treatment

  Treating wastewater is a complex process.  The Tri-City Water Pollution Control Plant uses physical, biological and chemical treatment to clean over seven million gallons of wastewater per day, created by the cities of Gladstone, Oregon City and West Linn.  Following the treatment process, clean water is released into the Willamette River.  Below is a brief overview of the Tri-City treatment process.


Treating the WATER in Wastewater

Preliminary Treatment (Screening)
The first step in the wastewater treatment process is “screening.”  Before wastewater is piped into the plant, large contaminants such as rocks, sand, plastics and toys, known as “grit,” must be removed; if not, plant equipment may be damaged, causing the treatment process to be interrupted. Mechanical bar screens are used to remove this material.

Primary Treatment
The second step of wastewater treatment is “primary treatment,” where partially treated wastewater flows into primary clarifiers so that remaining solids sink to the bottom, becoming “sludge” and are removed.  Pollutants such as grease, oil and other floatable substances are skimmed off the top.


Secondary Treatment
The next stop is the aeration basins where tiny microorganisms live and feed on the incoming waste. Air is continually pumped into the water to keep the microorganisms active.


Water now flows into secondary clarifiers where the water flow is allowed to slow down.  Final bits of solids sink to the bottom and are removed or returned to aeration basins for further processing.


The final stage in the wastewater treatment process is disinfection.  During this process, treated wastewater is disinfected with chlorine to kill any remaining bacteria such as E. coli, Streptococcus, Salmonella, Shigella, and Giarida lamblia, which can be harmful to humans and wildlife.  The chlorine is neutralized before the clean water or “effluent” is released into the Willamette River.


Treating the Waste in Wastewater


Pollutants that settle to the bottom of the clarifier, called “sludge,” and those that are skimmed from the top, such as grease, are pumped to large holding tanks called “digesters.”  The digesters process the sludge in an oxygen-free (anaerobic) environment.  The microbes in the waste produce methane and carbon dioxide, or digester gas. The digester gas can be burned as fuel to generate electricity at the plant.  This process creates a nutrient-rich organic product called biosolids.

Odor Control
The Tri-City Service District prides itself on being a “good neighbor;” therefore, significant investment has been made to address odor control at the plant.  Because odor associated with untreated wastewater can be unpleasant, all significant odor sources at the plant are covered.

Membrane Bioreactor
Operating parallel to the activated sludge process described above, the membrane bioreactor serves as the secondary treatment for the parallel process.  Hollow membrane fibers with millions of microscopic pores separate particles and bacteria from the treated water, resulting in Class A effluent suitable for reuse. On account of this industry-leading technology, the Tri-City Water Pollution Control Plant has been called a Model of Sustainability by the industry.

The wastewater treatment process protects public health and the environment.  If any of these processes are compromised, the result could alter affects of the entire treatment process, resulting in a negative impact to our local environment and human health.  You can help protect our natural resources by being aware of what you flush and rinse down drains.

Keep wipes out of the pipes 
Did you know that flushing items such as cleaning and baby wipes can cause damage to our sewer system? Many things routinely flushed down the toilet or washed down the drain, including those sold as “flushable” can cause expensive maintenance problems and sewer back-ups in your home, the public sewer system and in the environment. The following are examples of what should not be flushed or washed down the drain:

  • Baby and cleaning wipes
  • Paper towels
  • Cotton balls, swabs and pads
  • Facial wipes and tissue
  • Feminine Products including the applicators
  • Unused medications
  • Chemicals
  • Fats, Oils and Grease (FOG)

Thank you for protecting public health and the environment in Clackamas County.