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Facility expands its conventional activated sludge system with MBR to meet demands
In the upper northwest corner of Oregon, Clackamas County is surrounded by frontier wilderness and scenic beauty—it is home to the end of the Oregon Trail, the soaring peaks of Mt. Hood, the largest waterfall in the Pacific Northwest (Willamette Falls) and some of the country’s most renowned fishing holes on the Willamette River. Protecting the natural environment and the health of its communities were principal priorities of the county’s Tri-City Water Pollution Control Plant’s (WPCP) expansion project.
Earlier this decade, rapid growth, tighter regulations and aging infrastructure significantly strained Clackamas County’s wastewater systems. The over-capacity Kellogg Creek WPCP was leasing capacity from the Tri-City plant, which was also nearing its operational limits. District planners focused efforts to expand the Tri-City WPCP’s conventional activated sludge system with new treatment technology designed to meet the current and future demands of both districts.
To increase capacity at the Tri-City WPCP, located in the Portland suburb of Oregon City, a membrane bioreactor (MBR) was brought online in February 2011 in parallel to the existing conventional system. The project is a success story of many facets, but perhaps the most significant is the collaborative effort of the community and contractors working together, merging old and new technologies.
Voice of the People
The two-year, $90-million construction project commenced after six years of public discussion. Following several public surveys, countless community meetings and input from two advisory committees, it was clear that sustainability—social, environmental and economic—was paramount.
Not only did the plant expansion need to fit within a small footprint—the property is confined by the Clackamas River to the north, Clackamette Cove to the west, a landfill to the south and a major freeway to the east—but it also needed to enhance the neighborhood. With a multimillion-dollar residential development planned nearby, it was critical that the facility looks attractive.
To the citizens of Clackamas County, the most important elements of the project were open space for public access, links to trails and protection of the region’s water quality. The end result offered these advantages and more, including living roofs and interactive exhibits in and around the facility.
The MBR Solution
Advanced MBR treatment technology answered the challenges of the Tri-City WPCP project, including limited space, odor control, and new and stringent discharge and reuse limits. Working in parallel with the plant’s existing conventional activated sludge facility, GE’s ZeeWeed 500 MBR was utilized, resulting in clean, high-quality effluent that meets the state of Oregon’s Class A reuse standards. Untreated water is split between the two systems, which work together to meet a 10 biological oxygen demand/10 total suspended solids effluent, improving overall plant effluent quality while also providing a source of reuse water for the surrounding area.
The Tri-City WPCP’s new MBR process is completely enclosed, offering an aesthetically pleasing commercial look that also shields the old plant from the surrounding new residential development. The MBR technology’s reduced footprint eliminated the need to construct future facilities in the landfill and provided maximum open space for public recreation—a benefit that will extend into the future as the facility further expands to its ultimate plant capacity of 30 million gal per day (mgd). Additionally, the compact components of the membrane technology minimize the size of the structures, so potential odors can be contained, collected and treated.
The Tri-City WPCP is the Northwest’s largest MBR treatment system running parallel with a conventional treatment system. Expectations for the project’s success were met: compact design, reduced land and construction expenses, enhanced operational efficiency, mitigated noise and odors, and high-quality effluent. It is what was not expected, however, that provided even more value.
Since the addition of the MBR, increased nitrification has been observed from the conventional system, which was a non-nitrifying facility that is now removing more than 60% of ammonia nitrogen from the wastewater at two- to three-day sludge retention times (SRTs)—an important realization given ever more stringent ammonia discharge regulations. The combined effluent of the MBR and conventional system is achieving more than 75% ammonia removal. With a future anticipated effluent ammonia limit for the facility, this enhanced performance eliminated the need for a diffuser modification, avoiding an estimated additional $3 million in capital costs that would have been required to upgrade the existing diffuser.
After a detailed evaluation of the processes, it was determined that the filtrate from the gravity belt thickeners thickening the waste sludge from the MBR system contains nitrifying bacteria that is recycled to the conventional treatment process. There is enough nitrifying bacteria in the filtrate to seed the conventional system with a continuous source of nitrifiers, allowing partial nitrification at extremely short SRTs.
Weathering the Storm
The parallel operation of the conventional activated sludge system and the MBR also is ideal in high-flow situations, a benefit tested annually during the Northwest’s wet season. The combined facilities received their first extended exposure to high rain flows in January 2012, requiring the MBR system to perform beyond expectations by sustaining the design peak hour flow of 10 mgd for more than five days, taking a lot of burden off the conventional system. The MBR system successfully processed flows beyond the design, significantly reducing stress on the conventional system without detriment to the membranes. Both of the plant’s treatment trains maintained operation and met all effluent requirements during this extreme high-flow event.
The upgraded Tri-City WPCP is a model for sustainable storm water management and treatment. Any rain that falls on the plant site is contained. More than 20,000 sq ft of green roof help curtail rainwater runoff in combination with storage under the pervious and impervious streets and a storm water retention and infiltration basin. No rainwater has left the site since the system began operation, even when the facility experienced a five-day/24-hour storm event.
One of the county’s most successful and highly regarded capital improvement projects, the Tri-City WPCP expansion not only received praise from the public, but also from environmental and water industry award programs. It is a focal point of the community, enhancing the area’s natural beauty while protecting land values and paving the way for future development throughout Clackamas County. The MBR facility also provides Clackamas County Water Environment Services with a safeguard against future, more stringent water quality regulation while providing the ability to treat increased wastewater flows through phased expansions with the addition of more MBR tanks to meet the long-term growth needs of the county.