By Keith K. Daellenbach | Contributing Writer
Have you ever wondered where wastewater goes when you flush the toilet? If that toilet is in Portland, it very likely ends up at the Columbia Boulevard Wastewater Treatment Plant on North Columbia Boulevard. This facility, operated by the City of Portland Bureau of Environmental Services (BES), continuously serves a population of 650,000 people.
Before 1952, raw sewage went directly into the Willamette River and Columbia Slough. Today, through nearly 75 years of incremental investment, the treatment plant processes an average of approximately 70 million gallons of wastewater per day. During periods of wet weather, the plant can process up to 450 million gallons per day. The big pipe system, a $1.4 billion investment (ca. 2011) that nearly eliminated combined sewer overflows, can hold 119 million gallons of sewage – sewage that would otherwise overflow into the Willamette River.
Large screens at the incoming Headworks Facility (built in 1996) filter out sticks, rocks, and litter, which are then trucked to a landfill. Next, the wastewater flows into an array of primary clarifiers, which are rectangular, concrete-lined pools, causing solids to settle as the wastewater movement slows. A bottomscraping mechanism removes sludge. Then, the liquid flows into aeration basins, where air is supplied. Trillions of microorganisms break down the organic material in an aerobic reaction, which creates byproducts of water and carbon dioxide. The wastewater then moves to the secondary clarifiers.
Circular clarifiers are the final step in separating fluids from solids. Alongside the eight existing secondary clarifiers, two new secondary clarifiers (each of which is 28-feet deep by 150-feet in diameter) were recently added to the plant and substantially increased its capacity. Wastewater from the aeration basins enters from the bottom center of these large clarifi ers and slows to a carefully designed fl uid speed, allowing remaining solids to separate for collection. These new clarifiers, located at the north end of the plant adjacent to the Columbia Slough, are part of a $515 million, seven-year upgrade aimed at increasing treatment capacity and reliability. The entire project is scheduled for completion in early 2027.
Given that the plant is in a low-lying area just above the water table on unstable soil, the potential for liquefaction during a Cascadia Subduction Zone earthquake is very high. Sophisticated construction methods were employed to construct a grid of concrete grout columns within the soil, extending nearly 70 feet down to provide a foundation designed to allow the new clarifi ers to remain operational following a signifi cant earthquake.
As the wastewater leaves the plant, it is treated with sodium hypochlorite to kill microorganisms. This disinfected effl uent is conveyed 2.3 miles north in a pipe to the Columbia River at river mile 105.5, where it is dechlorinated before entering the main stem of the Columbia River in approximately 43 feet of water off the north side of Hayden Island.
Waste also departs the plant as a biosolid. It is collected from the clarifiers, and, unlike the liquid effluent, is broken down in an anaerobic process that is largely free of oxygen. In ten large, above-ground, 90-foot to 120-foot diameter tanks called “digesters”, the sludge is heated to 95 degrees Fahrenheit, where microorganisms reduce the biomass content with methane and carbon dioxide byproducts. The solid, digested material is dewatered via fi lter presses and centrifuges, and then trucked 200 miles east, where it is applied to agricultural land in Eastern Oregon as a nutrientrich source of fertilizer to grow crops for animal feed. Nearly 100 percent of the renewable methane biogas byproduct is captured and added to NW Natural’s gas distribution network to serve as truck fuel eliminating the need for 1.3 million gallons of diesel valued at $3 million annually.
Investing in wastewater treatment plants replaces aging infrastructure, accommodates population growth with additional capacity, and responds to new regulations requiring enhanced treatment. Portland is famous for its pristine Bull Run drinking water supply near Mount Hood. At the other end of the water-use cycle, we are fortunate to have the Columbia Boulevard Wastewater Treatment Plant in North Portland. Through continual investment and upgrades, BES protects Portland’s public health and the environment.