The Process

At our state-of-the-art treatment facilities, we clean 65 million gallons of wastewater each day to some of the highest standards in the nation.

After water is used inside by over 600,000 customers in urban Washington County, it's collected and routed to one of our four treatment facilities—Durham, Rock Creek, Hillsboro and Forest Grove. Then we start the job of recovering resources and cleaning the used water before returning it to the Tualatin River. Learn more about the process below or about the resources we recover as part of the process. 

Explore our interactive One Water Cycle website to follow the many paths water can take from the rain, through pipe systems, treatment facilities and back to the environment to be used again.

Want to see the process first-hand? Request a tour at one of our treatment facilities.

Pipes, pumps get water to plants

Underground in Washington County more than 800 miles of sewer pipes and 40 pump stations carry wastewater to one of our four treatment facilities. Just like when rain flows to streams and rivers, we use gravity to move the water wherever we can. Sometimes water needs a lift from pump stations to get over a hill and let gravity take over. 


Before we start cleaning the water, large debris and garbage like cleaning wipes, straws and q-tips are taken out and sent to the landfill. After the screen we slow the water down enough to allow heavy material like gravel, grit and sand to settle out before water moves on to primary treatment. 


Water is pumped into large tanks where it becomes nearly stagnant, or hardly moves at all. This allows anything denser than water to settle (like human waste and toilet paper) and anything lighter than water to float (fat, oils and grease). Rotating arms push these solids to pumps, that send materials to our solids handling facilities for resource recovery.


The aeration basin is home to tens of thousands of pounds of naturally-occurring microorganisms, which you could find living in pristine mountain streams or healthy soils. The microorganisms are happy to consume and metabolize the organic material they find.

Much of the cost of cleaning wastewater is in making sure their environment, food supply and population are healthy and productive. This requires a lot of energy to give them the dissolved oxygen they need. After metabolizing their food, the mix of microorganisms and water overflow into secondary clarifiers where they are separated by gravity. To save energy we only keep enough microorganisms to get the job done, which means constantly monitoring the population and sending the extra to solids handling.    

More than 98 percent of the nation's wastewater treatment facilities only provide secondary treatment. However, the slow, sensitive Tualatin River needs very clean water to be healthy.


The next step is a lot like the process to cleaning drinking water. There are still some very small solids and dissolved phosphorous that we need to remove. Alum and polymer are added to the water and become a solid that fine particulates stick to. As the mass gets bigger, it also gets heavier and sinks to the bottom of the clarifiers before moving onto solids handling.  


We use several feet of finely graded sand that water passes through to remove leftover chemical solids that didn't settle in the previous step and very fine particulates. 


We add a small amount of chlorine (less than in a swimming pool) to kill any pathogens. Before sending water to the Tualatin River (or reusing it for irrigation) we make sure it is safe for wildlife and people by removing the remaining chlorine with sodium bisulfite. The cleaned water is returned to the Tualatin River to be used again. 

The Tualatin River is the only river in Washington County.

The Tualatin River

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