May 2007, Vol. 19, No.5

Plant Profile

Warrenton, Ore.


Warrenton Wastewater Treatment Plant Location: Warrenton, Ore.
Startup date: Jan. 1, 2007
Service population: 8000
Number of employees: 2.5
Design flow: 2.3 mgd (8700 m3/d)
Average flow: 1.0 mgd (3800 m3/d) November–April; 0.70 mgd (2700 m3/d) May–October
Peak flow: 4.7 mgd (17,800 m3/d)
Annual operating cost: $240,000 (estimated) 

The Warrenton (Ore.) Wastewater Treatment Plant is brand new. In fact, it initiated full operation Jan. 2, although the plant has been utilized since June 28, 2006.

The sequencing batch reactor (SBR) plant was constructed on top of an existing facultative sewer lagoon site. The site conditions prohibited deep excavations, so the three basins for the SBRs are shallow, at only 12.5 ft (3.8 m), with a top operating water level of 8 ft to 10.5 ft (2.4 m to 3.2 m). The basins are 94 ft × 120 ft (29 m × 37 m) and have a 2.5:1 slope. They were designed and constructed like a diaphragm, so they can flex a bit without damaging the structures. The steep slope and flexible walls require that a close eye be kept on settling, or solids could be decanted inadvertently.

The plant also required using horizontal drilling to install a 5300-ft (1600-m) outfall that reaches into the shipping channel of the Columbia River, which flows into the Pacific Ocean soon thereafter. The 18-in. (450-mm) steel outfall pipe was installed by a separate contractor that specializes in horizontal directional boring. The pipe extends 4400 ft (1340 m) underneath the river. Because the outfall pipe is steel and protrudes into salty water, it also includes a cathodic protection system.
In addition to the treatment plant, the city has allowed a local seafood processor to discharge into the outfall. This has enabled the processor to cease discharges into the Skipinon River, which had degraded in water quality.

Treatment Process
The city’s collection system feeds the plant influent through three force mains. Headworks treatment includes fine-screening and grit removal. Biological treatment is performed via three separate SBR package plant basins. Each basin has a diffused-air system, two 7.5-hp (5.6-kW) down-force surface mixers, decanters capable of variable discharge rates, and waste pumps. Five 50-hp (37-kW) blowers with variable-frequency drives on a single manifold provide aeration for all three SBRs and are controlled by in-line dissolved-oxygen meters. The SBRs remove biochemical oxygen demand and nitrogen in the same basin utilizing alternating aeration and mixing regimens.
After treated wastewater decants from the SBR basin, it enters a three-module ultraviolet (UV) disinfection system. The UV system has automatic wipers and an air-scrub system to keep the bulbs clean and ensure proper dosage. The three-module design gives the plant flow-pacing capabilities, activating only the necessary number of bulbs based on flow.

Following disinfection, the treated effluent is pumped through the outfall pipe. Three 32-hp (24-kW) pumps are connected to a proportional-integral-derivative (PID)
As part of the upgrade, an 18-in. (450-mm) steel outfall pipe was installed by horizontal directional boring. The pipe extends 4400 ft (1340 m) underneath the Columbia River. (Photo credit: Eric N. Sharpsteen)
controller, which ensures proper discharges as outfall head pressure changes due to tides or the discharges from the seafood processor.

Also, at the effluent pump station, a 200-gal (760-L) fish tank serves as a bioassay to test the holistic effects of the wastewater on animals. The tank contains bass, crappie, bluegill, turtles, snails, and clams. Treated effluent is circulated through the tank, and the condition of the animals is monitored.

Effluent typically contains less than 5 ppm biochemical oxygen demand, less than 10 ppm total suspended solids, fewer than 15 colonies of Escherichia coli per 100 mL of water, and ammonia–nitrogen levels lower than 0.6 ppm.

Solids from the SBR tanks are sent to one of two facultative waste activated sludge lagoons with four 7.5-hp (5.6-kW) surface aspirating mixers. When the plant was built, part of one of the existing lagoons was partitioned to form these lagoons. One lagoon at a time will be filled and allowed to sit until the solids reach Class A requirements. Each lagoon has an estimated capacity of 4 years at design loadings. After reaching Class A status, the material will be land-applied.

Management Style
While the plant is publicly owned, the city contracts its operation to a private company, Ecology Associate Advocate LCC (Naches, Wash). Three staff members operate the plant, and each is equally responsible for operations and maintenance, laboratory, and administrative tasks. Duties are broken up into task blocks throughout the day. Each operator pitches in to get the task block completed. When the daily work is complete, individuals have the opportunity to focus on special projects or infrequent tasks. The plant is staffed 10 h/d Monday through Friday and at least 2 h/d during weekends or holidays.

The majority of the plant’s flow comes from domestic and commercial sources during the tourist season from May through October. From November through April, significant inflow and infiltration increases the flow by 0.3 mgd (1140 m3/d).

During the plant’s first wet season, the region was hit by terrible flooding, and the plant received 2.6 mgd (9800 m3/d) for 2 consecutive days during a power outage. By relying on the plant’s generator and spending late nights crunching data and tweaking equipment, the operators sustained full treatment. Operators are confident that as long as all of the equipment is functional, the facility can exceed its design flow and maintain full treatment.