Carnation Wastewater Treatment Plant, Membrane Bioreactor (MBR), King County, WA, USA

On 6th July 2006, Carnation took a major step towards overcoming the current moratorium on growth with the formal groundbreaking on the city’s new membrane bioreactor (MBR) plant. Scheduled to be operational early in 2008, the Carnation WWTP will provide tertiary level treatment and sidestep the current reliance of both residents and local business on ageing septic tanks, which had long been seen as an obstacle to further development.

The full scheme includes the treatment plant itself, a new conveyance pipeline and discharge, together with the installation of the City’s first centralised sewer system. The plant will initially have the capacity to treat up to 1,500m³ of wastewater per day, which could be expanded to treat around 1,800m³/d to meet projected growth.

The project costs have been estimated at $30m, with the membrane system costing around $900,000. To date $1.5m in federal funding has been forthcoming for the project and another $500,000 is expected to be provided from the current Environmental Protection Agency budget.

BACKGROUND

Historically, Carnation has relied on individual septic tanks and drain fields in the absence of a central sewerage system. In 1987, however, the number of these found to be inadequate coupled with soil surveys and the likelihood of the unprotected aquifer becoming contaminated led to major public health concerns.

The city council recognised that no attempt to bring about sustainable, long-term economic revitalisation would be possible without first addressing the lack of a centralised wastewater collection and treatment system. Consequently, in July 2002, after a series of engineering studies and considerable discussion, an agreement was reached between the city and King County. Under the terms of this, the county are to build the new wastewater treatment facility, while the city designs and builds a local sewer system.

As a result of this initiative, it is expected that some 90% of the potentially 'sewer-able' areas in the region, which were not served by a sewer system in 2000, would be connected by 2030, with 100% connection by 2050.

The new WWTP will protect the nearby Tolt and Snoqualmie Rivers and, as Carnation lies in an urban growth area defined by the state Growth Management Act, will play an important role in helping the city manage the increased demands made by the expected expansion. It has been predicted that the population will double within five years of the plant becoming operational.

THE PLANT

Two sites were considered for the plant – a city-owned property within the city limits and one to the south at Weckwerth. Though either would have been satisfactory, the city-owned site had several advantages, being larger, $2m cheaper to develop and offering better access options. Consequently, in December 2004, the city-owned site was formally selected.

The finished facility will occupy about two acres of the ten-acre site, which will also accommodate the new wastewater pump station to be built close to the treatment plant itself.

Although the new facility is relatively small – being only designed to serve the city of Carnation and its designated urban growth area – the water quality standards to be achieved remain stringent. This led to the plant being required to provide tertiary treatment in order to guarantee the high removal-levels demanded for suspended solids, nitrogen and phosphorous.

King County conducted trials on a number of potentially suitable systems for both this application and the larger Brightwater project, including reverse osmosis and fuzzy upflow filtration, before finally selecting MBR technology.

Influent arriving at the plant will undergo a conventional treatment regime comprising screens, clarifiers and grit removal. The final stage process, which employs the MBR ultra-filtration technology, contains five membrane cassettes, to provide the plant’s 1,800m³/d design capacity.

Systems from a number of providers were considered, with Zenon’s ZeeWeed MBR eventually being chosen for both this and the larger Brightwater project. The membranes lie immersed in the mixed liquor within an aeration vessel. A pump draws water through them, while intermittent airflow entering from the bottom of the module ensures that the surface of the fibres remains clean.

Highly effective removal of both soluble and particulate biodegradable material is achieved within the system as a result of the elevated biomass concentrations supported. The combination of the high quality effluent produced, together with proven operational reliability, were major factors in the adoption of this technology.

The reclaimed water will undergo final UV disinfection prior to discharge and the settled solids will be trucked off-site for disposal.

DISCHARGE ARRANGEMENTS

Three discharge options were considered – discharging to the Snoqualmie River, enhancing the wetlands in the Stillwater wildlife area and an upland discharge option – the treated water entering constructed basins and subsequently filtering into the ground.

It was initially decided to proceed with a river discharge option, to be located at Carnation Farm Road Bridge – this being the most cost-effective solution – the wetland alternative forming a second phase of the project. Construction of the wetland began in 2007, its design focusing on enhancing native plantings and controlling reed canary grass.

During the plant's start-up, treated water will be discharged via the river outfall, with the wetland becoming the primary discharge point for reclaimed water later in 2008. The river outfall will remain operational, serving as a backup to be used only under exceptional circumstances.

The plant will produce Class A reclaimed water, the highest quality of the four classes identified by Washington State and suitable for irrigation or non-potable industrial processes. It is hoped that some of the treated water may ultimately be made available for irrigation, particularly in the summer and its potential use on park or agricultural land is under evaluation.

KEY PLAYERS

King County is responsible for the design, construction and operation of the new treatment plant; the city of Carnation is designing and building the allied local wastewater collection system. The plant design team comprises Carollo Engineers and Michael Willis Architects. The 2004/2005 water conservation and demand analysis was done by Aquacraft, the Pacific Institute and Carollo. Zenon are to supply the membrane systems.

King County is working in partnership with Ducks Unlimited, a non-profit group dedicated to wetland conservation, to design the wetland discharge project.

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