The North End Wastewater Treatment Plant in Tacoma, Wash., was built in 1968. The Clean Water Act requirement for secondary treatment led the city on an exhausting search for alternatives to comply with the requirement and still fit all processes within the original footprint.
The requirement to add secondary treatment almost led to the demolition of all of the plant’s original structures and a $38 million pure-oxygen activated sludge process. But the plant’s operators and management refused to give up on the plant’s chemical treatment process and conceived of a physical/chemical process combined with a biological-filter polishing step to meet the required limits for a fraction of the cost.
Advanced primary treatment
Originally constructed in 1968 as a primary treatment facility, the North End plant is nestled in Mason Gulch — the steep wooded hillside where Mason Creek originates and deer and other wildlife that often frequent the plant live. To meet the Clean Water Act’s secondary treatment requirements, the city’s public works staff developed a chemical process using liquid aluminum sulfate and dry anionic polymer that achieved extraordinary results using the original primary plant design. While able to achieve the 30-mg/L limits regarding total suspended solids (TSS) and biochemical oxygen demand (BOD), the chemical treatment process was unable to consistently meet the Clean Water Act’s 85% BOD-removal requirement.
The chemical process had brought the plant to the threshold of compliance but lacked the ability to break down the soluble BOD that constituted more than 90% of the total effluent BOD. Undeterred, in 1988, led by now retired Assistant Public Works Director Bob Sparling, the city returned $19 million in federal grant money that had been earmarked for construction of the conventional plant. The city was determined and confident this alternative process would work while balancing economics with environmental regulations.
To meet the compliance requirements, plant staff turned to biofiltration to reduce the soluble BOD that the chemical process cannot. The staff was confident this marriage between chemical and biological treatment processes would provide the city with the best of both worlds.
To prove the concept, plant staff set up a small pilot-scale biofilter. However, convincing the regulators that this pairing could work took time. From 1986 to 1994, both the Washington State Department of Ecology and the U.S. Environmental Protection Agency remained skeptical about the new process. Only in 1995, after several years of negotiations, did the city finally gain approval for this alternative process.
Expanding the concept
In 1996, the year after the biofilter process was approved, the plant began a $10 million upgrade. One of the original anaerobic digesters was converted into the biofilter.
Today, the biofilter receives flow of 22,700 to 30,200 m3/d (6.0 to 8.0 mgd) through distributor arms moving at 90 revolutions per hour. The process goes into a backwash cycle from 2 a.m. to 6 a.m. During this period, the flow is dialed back to 19,000 m3/d (5.0 mgd) and revolutions are slowed to two per hour. The backwash strips excessive biological growth from the filter. The sloughed bacteria are sent to the grit tank upstream of the primary clarifiers.
The chemical process removes more than 90% of the TSS, while the biological treatment breaks down soluble BOD levels and addresses any compliance issues related to the 85% removal requirement.
During this upgrade, all of the plant’s original buildings were retained and refurbished. Two additional buildings were added, along with an odor-control tower and a distributed computerized control system. The plant’s second original anaerobic digester was converted to a solids holding tank. Solids from the holding tank are trucked to Tacoma’s Central Treatment Plant and processed into TAGRO, an award-winning biosolids product.
In the expanded plant, after screening and grit removal, alum is added to influent at the approximate rate of 0.215 L per m3/d (215 gal per mgd) of flow. As the flow changes, a computer-controlled system adjusts feed rates accordingly.
At flows in excess of 45,400 m3/d (12 mgd), polyaluminum chloride is introduced and the alum dose is decreased. This change is to prevent the alum, which creates an acidic solution, from lowering the plant effluent’s pH below 6.0, the plant’s permit limit.
Next, a liquid polymer solution is injected into the sedimentation tank center wells at a concentration of 0.4 mg/L to create floc that settles easily. Effluent from the sedimentation tank then passes to the biofilter for secondary treatment.
After biofiltration and disinfection, the plant discharges its secondary effluent to Commencement Bay, which flows into Puget Sound. Typical effluent TSS concentrations range from 2 to 8 mg/L, and BOD concentrations are 8 to 15 mg/L.
This unconventional treatment process eliminated the need for secondary clarifiers, which translated to big savings in construction costs. Eliminating clarifiers also made sense, since the plant sits in a confined location. Additionally, operators estimate that operational costs are about $300,000 less per year than a conventional system.
Now, city staff members are working to get the plant re-rated to permit increased influent loading. This increase will be needed to accommodate current and future development of Point Ruston, a 39-ha (97-ac) mixed residential and commercial development along the shores of Commencement Bay.
Meanwhile, the plant has been earning awards. North End has won 11 consecutive Platinum Peak Performance Awards from the National Association of Clean Water Agencies (Washington, D.C.). The plant also has won Outstanding Performance awards from the Washington State Department of Ecology. The performance awards are for meeting effluent criteria 100% of the time, as set forth in the plant’s permit. They require zero violations of any type, including reporting and monitoring. And the city was presented with a 1998 Exemplary State Local Award by the National Center for Public Productivity at Rutgers University (New Brunswick, N.J.) for implementing a treatment process that combines technological advances with simple systems to provide a cost-effective and environmentally friendly solution.
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