Wastewater treatment process

We use physical, biological, and chemical treatment to clean wastewater to some of the highest standards in the nation. The cleaned wastewater is then released into the Tualatin River.

Learn more about how wastewater treatment works from the Water Environment Federation, or continue reading to learn what makes our process unique!

The collection system

More than 800 miles of sanitary sewer collection lines crisscross Washington County. Where possible, gravity carries the wastewater to one of four treatment plants in the county. Where we run out of gravity, pump stations are installed which "give the sewage a lift" and turn the transmission task back over to gravity.

Clean Water Services operates and maintains 41 pump stations in our service district. It is essential that the community's sewage be continually moved away from homes and industries. Should a pump station fail for any reason, it is possible for flows to back up in the system and overflow into either neighborhoods or creeks in the area. Backup generators are installed at most pump stations to ensure uninterrupted service during electrical emergencies. A crew dedicated to keeping these facilities running in top shape, provides preventative and reactive maintenance.

Odor control

Clean Water Services is committed to being a good neighbor. We back up this commitment with substantial investments in odor control at our four wastewater treatment facilities.

Odor has historically been an unwelcome associate of wastewater treatment. To combat this, all significant odor sources are covered or contained. Air from potential odor sources is captured and treated using chemical scrubbers to neutralize odor-producing compounds.

Biosolids handling

Reusing biosolidsClean Water Services' four wastewater treatment facilities recycle more than 31 dry tons of safe, nutrient-rich organic material from the millions of wastewater we clean each day. This recycling process results in the production of biosolids. In fact, the more we succeed in cleaning our water, the more biosolids we can produce and recycle for beneficial use.

The solids that settle out during primary and secondary treatment, are routed to digesters—large cylindrical tanks kept mixed and heated to body temperature. The solids are treated for about a month, undergoing anaerobic digestion, that is digestion by bacteria that need no air. In their metabolism, methane gases are given off which you will often see burning in a torch-like device in any wastewater plant.

The nutrient-rich biosolids are dewatered and recycled onto local farms and rangeland in Eastern Oregon as a soil amendment.

Preliminary treatment (Screen)

Preliminary treatment, or screening, is the first step in cleaning wastewater. Before the raw sewage is introduced into our plants, large contaminants such as rocks, rags, toys, and golf balls must be removed. Remember, the definition of sewage includes anything that can be forced down a 4-inch hole. This may include, and has included such things as dentures, wedding rings, pieces of two-by-four. Such items could plug pumps, fill treatment channels, and damage moving equipment parts. Mechanically cleaned barscreens accomplish this task, removing the offensive material, and depositing it into dumpsters for disposal at a landfill. Following screening we use small mechanically mixed basins to remove any grit and sand-like material before the wastewater is routed to primary treatment.

Primary treatment

In primary treatment, the incoming flow is slowed in large tanks which allow the dirt, gravel, and other heavier-than-water components of the waste stream to settle out. Grease, oil, and other floatables are also removed here. Rotating arms simultaneously remove the settled solids from the bottom and the separated floatables from the top. Both pollutants are pumped into large heated holding silos, called digesters. Odor, resulting mainly from the formation of Hydrogen Sulfide in the incoming sewage lines, escapes the solution at all points of turbulence. These first tanks, or primary clarifiers as they are called, are covered and kept under a constant vacuum, with the gases removed routed through odor reduction equipment.

The flow leaving this process is markedly cleaner than the contaminated flow that first appeared at the head of the plant. It is as clean as much of our nation's wastewater was upon discharge to rivers, bays, and oceans, not so long ago. There is still however, about 30 percent of the original suspended solids that were in the plant's influent, and about 70 percent of the original Biochemical Oxygen Demand or BOD, (a measurement of the strength of the pollution). The removal of this final fraction is the job of the secondary treatment process.

Secondary treatment

What follows next in the plant process is closely akin to sourdough cooking whereby the cook carefully maintains the environment of the sourdough organisms to ensure their survival and availability when he needs them for food preparation. By carefully manipulating the environment, food supply, and population dynamics of the microorganisms in their plant, operators maintain a healthy, manageable biomass, which in turn is happy to feed on incoming waste loads. Since this is a living, breathing, process, it is important not to introduce toxic substances, which can kill these environmental volunteers. Clean Water Services' Source Control Division closely monitors industries in our community to ensure industrial wastes are within the tolerance range of the microorganisms.

After gorging themselves on incoming waste, the microorganisms are introduced into settling tanks where they can process their meal. After settling out in these clarifiers, one of two routes is taken. Some will be reintroduced to the feast, while others are sent to be thickened, digested, dewatered, and ultimately trucked to farms across Oregon as a soil amendment.

The cleaned wastewater then flows to a third—or tertiary—phase of treatment.

More than 98 percent of the nation's wastewater treatment facilities only provide secondary treatment. However, the slow-flowing Tualatin River demands that Clean Water Services provides advanced tertiary treatment.

Tertiary treatment

Cleaning your wastewaterBoth the Durham and Rock Creek facilities provide advanced tertiary treatment to remove phosphorous and further reduce BOD and suspended solids loading in order to protect the Tualatin River.

The Tualatin River has proved sensitive to algae blooms in the summer which in turn deplete oxygen levels. By reducing phosphorus to among the lowest levels in the nation (0.07 parts per million) we discourage the growth of the algae. Historically, the addition of alum and lime has been the treatment of choice for such intensive removal rates. A newer chemical-saving method now in use at the Durham facility uses manipulation of bacterial lifecycles to capture and remove phosphates from the process. Planned modifications to the Rock Creek facility to enable similar process dynamics there should result in a significant savings in the total chemical budget.

Tertiary treatment is the same process used by drinking water plants to clean raw water for municipal and industrial drinking water.


The final step in the wastewater treatment process is disinfection.

Our Durham and Rock Creek Advanced Wastewater Treatment Facilities use Sodium Hypochlorite (~12% Bleach) dosed at a controlled level to kill any remaining bacteria. Instrumentation monitors the chlorine residual attained, with an eye toward providing the minimal dosage for complete disinfection. By monitoring a slight excess of dosage, (residual), we ensure that all reactive material in the effluent has been exposed to the disinfecting agent.

Thirty years ago, a chlorine residual of 2 to 3 mg/l was considered good insurance for a proper disinfection of a treatment plant stream. Stream health, however, is not enhanced by the introduction of any chlorine residual into the environment. For this reason, Sodium BiSulfite is now dosed in sufficient quantities to remove residual chlorine before discharge.

At our two smaller plants, Hillsboro and Forest Grove, disinfection is accomplished by means of ultraviolet rays. The cleaned wastewater flow is routed through banks of UV lights, whose intensity is controlled to minimize electrical consumption while maximizing bacterial kill. This system has no need for neutralizing the disinfecting agent before discharge.

Cleaned wastewater released from Clean Water Services' treatment facilities compare favorably to federal drinking water standards and improve the health of the Tualatin River.

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