July 2006, Vol. 18, No.7

Plant Profile

Rock Creek Advanced Wastewater Treatment Facility


Location: Hillsboro, Ore.
Startup date: 1978
Service population: 214,336
Number of employees: 27
Average flow: 32 mgd (121,120 m3/d)
Design flow: 200 mgd (757,000 m3/d)
Peak flow: 126 mgd (477,000 m3/d)
Annual operating cost, operations and maintenance: $6,425,000

The Clean Water Services Rock Creek Facility in Hillsboro, Ore., is more than meets the eye. With the capacity to treat 200 mgd (757,000 m3/d), this treatment facility is actually two plants in one, split between the east and west sides of the plant. In addition to a unique setup, management provides educational resources to plant staff in the form of The College of Clean Water — an onsite training facility for use by employees — which provides up-to-date information on wastewater treatment operations.

Rock Creek’s flow comes mostly from households and light industry, but it also receives flow from silicon chip and wafer plants. About 10% of the facility’s influent comes from industry.

While both the east and west sides of the facility share a common influent pumping station, headworks, and primary clarifiers, after these unit processes, the facility splits, with each side operating separately.

The two halves of the plant operate separately because the west side of the facility was part of the original plant in 1978. The plant expanded to the east side as the population grew and influent flow increased.

Treatment processes on the west side include a primary effluent pumping station, two aeration basins, four secondary clarifiers, two tertiary clarifiers, four filters, and two chlorine contact basins. On the east side, a primary effluent pumping station, three aeration basins, three secondary clarifiers, two direct filtration channels, four tertiary clarifiers, 10 filters, a chlorine contact basin, and a dechlorination channel make up the process train.

Raw wastewater is pumped from the influent pump station to a common header at the inlet to the screenings channels. Rock Creek has three screenings channels — one channel is rated for 100 mgd (378,500 m3/d) and the other two are rated at 50 mgd (190,000 m3/d) each. Automatically raked 3-mm fine screens remove debris.

Following screening, the flow enters a common header that distributes it to the primary clarifiers. Recycle-flows from solids processing and filter backwashes also enter this header. Flow is typically directed to one of three primary clarifiers, each with the capacity to treat flows of 50 mgd (190,000 m3/d).

The liquid stream from the primary clarifiers enters a distribution pipe that feeds one of two primary effluent pump stations.

From the split, the wastewater undergoes activated sludge treatment that removes phosphorus and ammonia. After aeration and secondary clarification, during nutrient removal season — May through November — the facility uses chemical addition to reduce phosphorus levels further.

The flows then pass through one of 14 gravity filters — four on the west side, 10 on the east side — to one of three chlorine contact basins — two on the west side, one on the east side. Liquid sodium hypochlorite is added for disinfection, and the flows recombine on the east side for dechlorination via sodium bisulfite addition.

Solids Treatment
Primary, waste activated, and chemical sludge from the east side nutrient removal process, as well as sludges from two of Clean Water Services’ other wastewater facilities, are blended in a feed thickening tank and pumped to four gravity belt thickeners. The gravity belt thickeners increase the solids content to between 5% and 7%. The thickened sludge is then fed to one of three active anaerobic digesters. Rock Creek has two modified egg-shaped digesters with a volume of approximately 1.4 million gal (5.3 million L) each and one fixed-lid unit with a volume of 660,000 gal (2.5 million L). The digesters provide an average of 20 days detention time.

Methane gas from the digestion process powers two engine generators, which typically produce 500 kW/h each. The gas also fires up to five boilers, as needed. Excess gas is burned through one of two waste-gas burners.

Digested biosolids are fed to a Humboldt centrifuge for dewatering. The centrifuge produces biosolids cake with 23% to 24% solids. A series of screw conveyors moves the cake into one of two hoppers. Trucks then load the biosolids out of the hoppers for land application.

Clean Water Services has a nationally recognized biosolids program. When available and weather permitting, biosolids from the Rock Creek Facility are applied to local farm sites in the counties surrounding the facility. Nearly half of the biosolids produced are applied locally. During wet months, biosolids are hauled to Madison Ranch near Hermiston, Ore., in the eastern part of the state.

To maintain its high performance levels, Rock Creek’s operations staff have to stay current on their training. To that end, Clean Water Services provides a variety of onsite personnel development classes for its employees. The College of Clean Water was created as a resource for all employees. The ability to provide training in-house enables the facility to customize the training to employees’ specific needs and schedules. The college offers courses in computer software and professional development.

The facility’s wastewater treatment department hires and develops operator trainees to fill many of its vacancies. Trainees participate in a modular training program that helps them understand the facility and develop their skills to advance from the trainee to the journeyman level. The facility also provides safety training to its employees on such topics as lock-out–tag-out procedures, confined space entry training, first aid, and cardio-pulmonary resuscitation.

Clean Water Services also fully supports the annual Water Environment School, a 3-day school for water and wastewater professionals held at Clackamas Community College in Oregon City, Ore.