About the Wastewater Treatment Process

Blue close up photograph of microorganisms
Wastewater treatment recovers resources from the water we use in our homes, such as in sinks and toilets. Recovering the resources from that “used” water helps keep our community’s rivers healthy and creates renewable resources for energy and agriculture.
On this page
From Environmental Services Clean Rivers Education

Most of Portland’s wastewater travels through a series of pipes and pump stations to arrive at the Columbia Boulevard Wastewater Treatment Plant in North Portland. At the plant, it undergoes a transformative process that includes a little bit of biology, chemistry, and physics along with trillions of tiny microorganisms (which we call bugs) who are not too picky about what they eat. Gradually, the process recovers valuable resources that can create renewable energy and return nutrients to the soil. At the end, the treated water is sent to the Columbia River and back into the water cycle. And all that happens in about 24 hours from the time you’ve flushed your toilet or washed your dishes.   

First, What is Wastewater? 

Wastewater is all the “used” water that flows out of your kitchen sink, dishwasher, toilet, bathtub, shower, washing machine, or any drain in your home or business. This water can carry with it lots of different things depending on what you’ve used it for, like shampoo from the shower, laundry soap from washing your clothes, tiny bits of food from washing your dishes, and of course, the stuff that goes down the toilet. 

What Happens After the Flush 

Portland has a vast network of pipes and pump stations that move wastewater to a treatment plant, where that transformation from waste to resource begins. Learn more about Portland’s pipe network. On average, more than 70 million gallons of wastewater go through this process each day. 

Step One: Primary Treatment 

Incoming: The Influent Channel 

By the time it reaches the treatment plant, the wastewater is moving fast. It flows into the plant through a channel that can carry up to 300 million gallons per day (mgd). It brings with it all kinds of leaves, tree branches, garbage, and other debris.  

First Stop is the Headworks 

Photograph of the interior of the Headworks area of the plant with four large metal structures in a concrete industrial space.

In the Headworks, large metal screens filter out the large things like sticks, rocks, and litter. Trucks take that debris to the landfill. The water continues on to the primary clarifiers.

Time to Slow Down in the Primary Clarifiers 

Photograph of the primary clarifiers, sever large vats of water separated into rectangular pools with a catwalk in the center. In the background are other facilities of the wastewater plant and trees in the background.

The primary clarifiers are large tanks where the water slows down to let solids sink to the bottom or float to the top. These solids — called scum (or “floatable”) and sludge (or “settleable”) — are sent to the digesters to start their own journey. The water flows next to secondary treatment. 

Next Step: Secondary Treatment  

Photograph of aeration basins, large tanks of water in a concrete basin surrounded by grass, trees, and open skies. There are catwalks in between the tanks so for engineers to monitor the treatment process.

The water starts secondary treatment by flowing into aeration basins. These are large, bubbling tanks filled with trillions of microorganisms—these are the bugs that are not too picky about what they eat. Here, the water gets mixed up, and the bugs eat the tiny particles in the water. The bugs are mostly  “aerobic” organisms, meaning they need oxygen to survive. To keep them happy and healthy, air is pumped into the tanks using large blowers.  

Photograph of the secondary clarifier, a large tank of water with a long metal skimmer arm sitting at the surface.

After the aeration basins, the water flows into the secondary clarifiers and again things slow down. We give the water time to settle so that solids (mostly the microorganisms from the aeration basins) can sink to the bottom or float to the top where they are scraped up and returned to the aeration basins or sent to the digesters to follow the other solids. 

Out to the Columbia River 

Water from the secondary clarifiers picks up speed again as it is piped to the Columbia River. Along the way, it’s disinfected with sodium hypochlorite to kill more bacteria. It is then dechlorinated to water quality standards at the outfalls. Finally, the water flows out to join the river flowing west to the Pacific Ocean.

Fulfilling the Clean Water Act and Protecting our Rivers 

The City of Portland makes sure the water that flows out from the treatment plants meets the standards set in the federal Clean Water Act. Our treatment plants have National Pollution Discharge Elimination System permits, or NPDES. These permits set what is acceptable for “effluent,” the treated water flowing out of the plants. Environmental Services tests our effluent regularly to ensure we are meeting the requirements of our permits and protecting our rivers. 

What Happens to the Solids?

Photograph of the exterior of the digester tanks, two large cylindrical facilities.

The scum and sludge from the primary clarifiers are sent to huge cylindrical tanks called digesters. The digesters heat the solids to a minimum of 95 degrees Fahrenheit while “anaerobic” (meaning without oxygen) microorganisms (or bugs) get to work decomposing the solids. Together, these processes destroy pathogens and reduce odors.

As the bugs breakdown the solids, biogas is produced. This gas is collected in the digesters and sent to another process at the plant where it is “refined” or cleaned to produce renewable energy.

The solids have a longer stay at the plant. They are typically in the digesters for about a month, where they start to transform into “biosolids.” Learn more about biosolids.

24 Hours a Day, 7 Days a Week

We generate sewage every minute of every day. Around the clock, plant operators manage, monitor, and adjust flows going through the plant to recover the water and other valuable resources found in the city’s wastewater. To keep the plant operating every minute of every day, special teams fix problems like clogged pumps, blocked drains, and power outages. That keeps the plant working in all conditions to protect the health of our community.  

What You Flush Matters 

And that’s where you come in. To protect our pipes, pump stations, and even the tiny bugs at the treatment plant, we all need to be careful about what we put down our drains or flush down our toilets. Learn what's okay and not okay to flush or put down the sink