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Environmental Services

Wastewater Treatment Plant

The City’s treatment plant treats an average of 12 million gallons daily and serves approximately 129,000 customers in Gresham, Fairview, and Wood Village. 

  • Energy Net Zero
  • How the Plant Works
  • Educational Tours
Energy Net Zero

In 2015, the treatment plant reached energy net zero. The plant now produces more energy than it uses, saving the City an estimated $500,000 a year in electricity costs.

Fats, oils, and grease (FOG) are trucked to the plant from local food service establishments. The City collects a tipping fee for receiving and recycling this waste. Adding FOG to the digestion process increases biogas production.

 The biogas produced is treated to remove contaminants and moisture and fed into two powerful cogen engines that convert biogas into heat and electricity. 

How the Plant Works

Liquid treatment

1. Preliminary treatment 

Wastewater enters the treatment plant, flows through screens with quarter-inch spacing, which removes large objects that could damage equipment. Small particles, called Grit, typically composed of sand, gravel, and other heavy solid materials, are then removed in a large conical tank where, with mixing, the grit settles to the bottom of the cone and the water is able to pass to primary treatment.

2. Primary treatment

Solids that make it to primary treatment are settleable or suspended in nature. The settleable particles are forced to settle out of solution with reduced velocity through large tanks called primary clarifiers. The settled solids are called primary sludge and are removed, treated, and converted to biogas to help create energy to power the treatment plant.

3. Aeration

As suspended solids are not treated in primary treatment, their removal involves secondary treatment to convert them into a settleable form. This is accomplished by adding air with Aeration Basins. Aeration is a process of pumping air into tanks which promotes microbial growth in wastewater. The oxygen helps the bacteria break down suspended organic matter, converting it into a settleable form, called Flocs which are removed with Secondary Clarification.

4. Secondary clarification 

The wastewater from the aeration basin is slowed down and the Flocs generated are settled to the bottom of the tank and either returned to the aeration basins to keep the cultivated microbes in solution to continue treatment, or removed and further processed via solids treatment.

5. Disinfection

The wastewater is then disinfected with sodium hypochlorite to remove any disease-causing organisms and ensure that the water leaving the plant meets the water quality standards set by the Environmental Protection Agency (EPA) and the Oregon Department of Environmental Quality (DEQ).

6. Plant Effluent

Following the treatment, the water is discharged to the Columbia River.

Solid treatment 

1. Sludge thickening

Solids produced in primary and secondary treatment require additional treatment before beneficial reuse through the Biosolids Program. The first step is to thicken the sludge to maximize retention times in the digesters. 
The WWTP accomplishes this with equipment designed to drain water from the sludge while retaining a majority of the solids.

2. Digestion

Thickened sludge and FOG are pumped into large tanks called Anaerobic Digesters. Anaerobic means without air and digestion is another process that cultivates microbial growth to break down the organic compounds in the sludge. The two major outputs from this process are biosolids and biogas.

3. Sludge dewatering

Solids processed through digestion are less concentrated than the thickened sludge fed into digestion. To save on hauling costs, these solids are dewatered prior to distribution through the Biosolids Program. 

4. Biosolids Program

Solids processed through digestion are treated to Class B per EPA standards and contain beneficial nutrients for various crops. Instead of disposing of these solids in landfills, the City, with coordination and approval of DEQ and farm-owners, hauls these solids for beneficial reuse through land application. 

Educational Tours

Schedule a tour of the Wastewater Treatment Plant 

20015 NE Sandy Blvd.
503-618-3450

  • Take a 45-minute guided tour through each step of the treatment process.
  • Learn how the solar array and biogas generation allow the plant to be energy net-zero.

Tour the Columbia Slough Regional Stormwater Treatment Facilities 

  • Take a 45-minute self-guided tour through this restored parcel of land.
  • School groups can check out a backpack of educational materials full of activities such as a scavenger hunt.

More information and educational resources.

  • FOG Tanks
  • Large solar array with trees in the distance.

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  • Gresham’s Wastewater Treatment Plant is the first in the Pacific Northwest to reach energy net zero status, meaning the plant produces at least as much energy as it uses. We celebrated the achievement with the next generation of great minds on Earth Day 2015.

  • Fats, oils, and grease collected by regional restaurants and food service establishments come in on trucks from six haulers – about 12,000 gallons a day.  
    FOG is fed into digesters and the natural byproduct, biogas, is captured, treated, and converted into heat and electricity.

  • Gresham earns up to $350,000 a year from fats, oils, and grease collection.

  • The treatment plant got to net zero two ways. It made it’s operations and equipment more efficient (cutting energy consumption), but mostly it got to net zero by producing renewable energy on-site. This is the fats, oils, and grease receiving station.

  • The co-generators are powerful gas engines that convert the biogas into heat and electricity – enough to heat the plant and produce 6.3 million kWh of electricity a year. Gresham saves $500,000 a year on electricity.

  • A solar array made up of 1,904 panels operates 365 days a year, producing 440,000 kWh a year, or 7% of the renewable power produced at the treatment plant. Clean energy is sent to the electrical grid and accounted against plant energy use through net-metering. Excess energy produced is managed by PGE to supply low-rate power to economically-stressed households throughout PGE’s service area.

  • Gresham’s plant is one of only a handful in the United States to achieve net zero status. Discover the ingenuity and collaborative spirit that made possible an environmental achievement at the City – engineering the Pacific Northwest’s first energy net zero wastewater treatment plant. Achieving net zero status means that the plant makes about the same amount of electricity as it consumes in a year, saving tax dollars and protecting the environment.