Twenty-first century technology is gaining momentum in the staid business of wastewater treatment. Since the federal Water Pollution Control Act of 1972 fueled the buildout of the nation’s current crop of wastewater treatment plants, municipalities have mostly relied on conventional activated-sludge processes to meet secondary-treatment standards. That is changing now as vendors and engineers tweak membrane systems originally developed to treat drinking water for the more corrosive environment of wastewater. The results are positive, now often producing effluent that greatly exceeds federal standards for secondary treatment at roughly the same cost.
MBRs, or membrane bioreactor systems, combine either flat-plate or hollow-strand plastic membranes spotted with tiny pores and a conventional activated-sludge biological system. Cassettes or banks of membranes are submerged in CAS tanks where air is bubbled through to promote oxidation and mixing. Wastewater is sucked through the membranes’ pores, which are small enough to filter out not only suspended solids but bacteria and viruses. The resulting effluent can be used for nonpotable purposes.
Increasingly sophisticated owners are looking to the technology as it is scaled up and costs come down. “It is kind of like the I-Pod of the industry,” says Marie Pellegrin, HDR’s MBR technical leader in Kansas City. “If you’ve got one, you have a state-of-the-art system.”
The technology will reach new levels in 2011, when King County’s 36-million-gallon-per-day Brightwater plant comes on line north of Seattle. It will be the largest wastewater treatment plant in the U.S. fitted with an MBR system. But designers had to first ensure the system could be scaled up—most existing municipal systems are for smaller plants under 2 mgd—and be cost-effective.
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Wastewater is sucked through hollow membranes.
For King County, the numbers worked for an MBR system to treat the initial 39-mdg baseload. Washington state’s Dept. of Ecology, however, requires new plants to provide full secondary treatment to peak flows. That requirement would have been an MBR deal-killer, says Stan Hummel, the county’s wastewater capital-projects supervisor.
Instead, his team proved that splitting 130-mgd peak wet-weather flows between the MBR and chemically enhanced primary treatment and then blending the two streams still produced an effluent exceeding secondary-treatment standards. Brightwater will discharge about 1 million fewer pounds per year of suspended solids and BOD to Puget Sound than a conventional activated-sludge system (see chart). “Federal and state agencies turned around our permit quickly,” says Hummel.
Reduced capital costs—secondary aeration tanks were not needed—balanced the MBR system’s greater energy costs. The system’s ability to more effectively remove heavy metals, endocrine-disrupting compounds, parasites and bacteria also sets the county up to meet potentially tougher discharge limits in the future, Hummel says.
Vendors have been very aggressive in reducing energy needs and improving membrane quality, key items in driving an annual wastewater market now estimated at about $1.3 billion, says Dawn Kristoff, executive director the Water & Wastewater Equipment Manufacturers Association. “But the number of plants and mgd is growing exponentially,” says Pellegrin.
Paul Schuler, vice president of GE Water and Process Technologies, which in purchased membrane manufacturer Zenon Environmental Inc., says the U.S. municipal market is growing about 3% to 5% per year. Pellegrin and others say energy costs have been reduced by half since the first MBR systems came on line in the 1990s. Membrane life cycles have increased to about 10 years, and cleaning methods have improved, reducing the amount of chemicals needed. Backpulsing is commonly used to remove buildup on membranes in the tank.
MBR systems also are highly automated, an advantage for satellite plants in remote areas but a challenge for owners who must retrain operators and ensure a constant power supply. “You can’t let them lose power,” says Patrick Burke, Brightwater project manager for CH2M Hill Cos., which designed the plant.
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Membrane cassette fits snugly into bioreactor tank.
As systems increase in sophistication, owners must pay close attention to warranties and repairs and include them in bid documents. “More and more owners are putting guarantees for annual repair services and [membrane] replacement into the bid documents,” says George Crawford, CH2M Hill’s principal wastewater treatment engineer, in Toronto.
At Brightwater, “we were scaling up for the largest MBR in the world,” says John Komorita, King County design engineer. “A single process train is 3.9 mgd. One basin is larger than most [existing] plants. So we coordinated membrane procurement with the design process. The owner must decide what to put in the vendor scope of supply or the general contractor package.”
For instance, because air blowers are essential for membrane performance, the county included blower procurement in the vendor package. But a series of 10-mgd pumps needed to quickly drain or fill basins was put into the general contractor package. Zenon was awarded the membrane package in 2006.
“There is a definite risk in scaling up to this size,” says Hummel. “Our designers felt it was scalable.”
Brightwater’s status won’t last long: A 60-mgd plant now is being planned for Dubai, comparable to the overall scale of development in the Mideast emirate.