Water Treatment Process

Water Treatment Process

an aerial view of the rock creek water resource recovery facility.

At our state-of-the-art treatment facilities, we clean over 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 water resource recovery facilities — Durham, Rock Creek, Hillsboro, and Forest Grove. Then we start the job of recovering natural resources and cleaning the used water before returning it to the Tualatin River. Learn more about the process below and in this story map, and about the resources we recover as part of the process. 

Explore the Virtual Treatment Process to learn more about how water gets cleaned and the interactive One Water Cycle website to follow the many paths water can take from the rain, through pipe systems and 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 1,900 miles of sewer pipes and 44 pump stations carry wastewater to one of our four water resource recovery facilities. Just like when rain flows to streams and rivers, we use gravity to move the water wherever we can. However, sometimes water needs a lift from pump stations to get over a hill and let gravity take over.


Before we start cleaning the water, we first screen it to remove large debris and garbage like cleaning wipes, straws, and cotton swabs — which are sent to the landfill. After screening, we remove heavy, not readily biodegradable objects such as sand, grit, and gravel. This “grit” is small enough to escape screening, but still heavy enough to easily separate from the rest of the flow.


Water is pumped into large tanks where it becomes quiescent, like water in a pond. This allows anything denser than water (like human waste and toilet paper) to settle and anything lighter than water (fat, oils, and grease) to float. Rotating arms push these solids to pumps that send materials to our solids handling facilities for resource recovery. The Forest Grove facility is the only Clean Water Services facility that does not use Primary Treatment.


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. We only keep enough microorganisms to get the job done, which means constantly monitoring the population and sending the extra to solids handling.


The next step is a lot like the process to produce drinking water. There are still some very small solids and dissolved phosphorus 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 a third set of clarifiers before moving on to solids handling. Like drinking water plants, we also filter the water. We use several feet of finely graded sand that water passes through to remove leftover chemical solids and other very fine particles that did not settle in the previous steps.


Disinfection is the process to inactivate or destroy harmful microorganisms (pathogens). At the Durham and Rock Creek facilities, we add a small amount of chlorine (less than in a swimming pool) to kill any pathogens. We then make sure it is safe for wildlife by removing the remaining chlorine with sodium bisulfite.

At the Forest Grove and Hillsboro facilities, we disinfect with ultraviolet (UV) light. The plant flow passes in front of UV light, which inactivates the bacteria by damaging them so they can no longer multiply.

After disinfection, the cleaned water can be reused again for irrigation or it can be returned to the Tualatin River, helping it to maintain a healthy flow year around.

Each of our facilities is designed to meet the needs of the communities it serves.