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An onsite packaged membrane bioreactor plant provides comprehensive benefits for a national treasure.
The only practical wastewater treatment option for isolated or unserviced sites used to be a septic system. Not anymore. When an aging septic system’s performance at Frank Lloyd Wright’s Fallingwater deteriorated, design engineers determined that an advanced membrane bioreactor (MBR) ultrafiltration system would better manage wastewater treatment at this highly acclaimed, architectural treasure.
The new Z-MOD packaged MBR plant, made by Oakville, ON–based Zenon Membrane Solutions, part of GE Water & Process Technologies, is working so well that it is now part of the Mill Run, PA, attraction. As part of the tour, Fallingwater guides tell visitors about the MBR and how it helps protect the site and the sensitive Bear Run watershed for future generations, says Lynda Waggoner, vice president and director of Fallingwater, owned by the Pittsburg-based Western Pennsylvania Conservancy (WPC). The challenging and high-profile project also earned consulting, engineering, and construction firm CH2M Hill the prestigious National Design-Build Award for Water for projects less than $15 million in 2005. Presented by the Design-Build Institute of America, the award recognizes the company’s innovative and groundbreaking approach to building the environmentally sound wastewater treatment system, based on a zero-discharge concept.
Fallingwater personnel weren’t so proud of the site’s wastewater treatment system in the late 1990s, Waggoner notes. The failing septic system had begun discharging wastewater into Bear Run—the environmentally sensitive stream over which the architectural masterpiece is cantilevered—and the holding tank they’d installed was only a temporary solution. Not only was there a great need for a new wastewater treatment system, but Fallingwater administrators also wanted to replace the site’s composting toilets with a more aesthetically pleasing but still environmentally sound technology. The ideal wastewater treatment system would protect the creek, handle the variable flows produced by tourist traffic, require minimal oversight, produce high-quality water for onsite reuse, and be virtually unnoticeable.
“We were looking to deal with sewage comprehensively on the entire site,” Waggoner says. “We wanted zero discharge to the stream, because it’s an exceptional value stream. …We wanted to be a model of good stewardship for the stream. … We wanted to use the facility actively as an element of interpretation of the site, to let people know how we dealt with wastewater on the site. We were also looking for a system that had a track record. It had to be reliable.”
With this in mind, Fallingwater staff turned to Englewood, CO–based CH2M Hill for a solution. The team initially considered a mound septic system as a replacement, but WPC rejected the idea because another septic system could not achieve the environmental and water management goals that the WPC wanted for the site. Instead, CH2M Hill engineers turned to advanced membrane bioreactor technology to provide a highly efficient, compact, and cost-effective solution for the job.
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| The entire wastewater treatment system is enclosed in a building designed to blend in with the surrounding area. |
GE Z-MOD MBR systems combine activated sludge with reinforced, hollow-fiber ultrafiltration membranes. The membranes are immersed directly into the mixed liquor and draw treated effluent into the fibers via a gentle suction, leaving suspended solids and pathogens behind in the tank. Because liquids and solids are separated via filtration rather than settling, MBRs can operate at mixed-liquor suspended solids concentrations between 8,000 and 15,000 milligrams per liter—much higher than the MLSS levels in a traditional activated sludge reactor—and produce high-quality effluent no matter what the sludge’s settling characteristics are. MBRs offer an extremely compact footprint and can completely treat wastewater in one or two steps. The systems are capable of producing effluent with less than 2 milligrams per liter of biochemical oxygen demand, less than 3 milligrams per liter of total nitrogen, less than 0.05 milligram per liter of phosphorus, and less than 2 milligrams per liter of total suspended solids.
This high-quality effluent means that Fallingwater will continuously meet the regulatory requirements of its water-quality management permit. However, even though MBR technology is a proven performer for challenging municipal and industrial applications of all sizes, the Pennsylvania Department of Environmental Protection (DEP) still wanted to keep a watchful eye on the plant. According to Donald Leone, chief of sewage and facilities planning at the DEP, Fallingwater is subject to more stringent performance standards and monitoring to ensure that the protected Bear Run watershed remains in pristine condition.
“Usually about three years of monitoring is required to show that the system is operating properly,” Leone says. Otherwise, the permitting process is similar to what a conventional system would undergo. One advantage of the reuse system, Leone says, was that Fallingwater was spared the time and cost of obtaining a National Pollutant Discharge Elimination System (NPDES) permit since the effluent from the MBR is being reused onsite and is not discharged into a receiving body.
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| Membrane bioreactor systems are highly automated, and most small systems do not require full-time operators. |
While a septic system would have also eliminated the need for an NPDES permit, a large earth mound would have been required for the drainfield due to the rocky terrain in the area. Moreover, the septic system would not satisfy Fallingwater’s environmental stewardship objectives, and it would not provide high-quality effluent that could be safely reused onsite.
Overall, the MBR system offered many benefits that septic or conventional activated sludge systems could not provide. The Z-MOD system costs less to build and operate than traditional activated sludge systems because it is smaller, does not require primary and secondary clarifiers, and needs much less operating equipment. Its modular design is flexible and needs as little as 20% of the space that a conventional wastewater treatment plant requires.
A typical MBR system includes pretreatment for trash removal, membrane cassettes, a bioreactor, permeate pumps, blowers for process and membrane scouring, and clean-in-place equipment for membrane maintenance. The Fallingwater MBR has also added activated carbon adsorption to remove dissolved organics and a UV disinfection system to inactivate any remaining pathogens.
Overall, the project was economical to construct, even though contractors had to bore through rock to lay the pipe, according to Paul Whitener, the Fallingwater project manager at CH2M Hill. “We were lucky,” he says. “The conservancy didn’t put any undue restrictions on us; they were fantastic to work with. ZENON basically shrank its municipal-scale ZeeWeed MBR process down into one tank for this site, and although the site is rocky, contractors were able to lay the [collection system] pipe using standard construction equipment.”
As expected, the project team had to construct, test, and commission the system without affecting the visiting public or harming any of the structures onsite, Whitener notes. “We were able to do much of the work in the winter—even though the construction conditions are a little bit tougher—during normal hours because the visitation cycle was at its lower point. But during the spring and fall, we had to work at night.”
Also, the project team “basically had to bring all materials in and all equipment in through a series of old maintenance roads, Whitener recalls. “We weren’t permitted to come in through the public side of the Fallingwater site, and we had to construct without destroying trees, with minimal disturbance to the property.”
Installing the collection system was a little difficult, Whitener says, because the various collection points were widespread—the Frank Lloyd Wright house, the gardener’s cottage, the cutting garden, etc.—and each required a new collection point that had to be constructed without disrupting the public or the properties themselves. A below-grade pumping station was also installed at each collection point to transport the wastewater to the treatment plant. Constructing the treatment facility itself was simpler, Whitener notes, because it was located away from the public areas, masked by trees, and accessible by some old maintenance roads. Contractors could work on it during the day. However, “some of the initial testing and commissioning of the system had to occur during non-visitation hours,” he says. The treatment facility also includes a carbon adsorption system, to remove dissolved organics, and a UV system for final disinfection.
The Z-MOD membrane bioreactor has been online since November 2003. “Installation went really smoothly,” Waggoner says. “CH2M Hill was a great project manager for the site. The impact wasn’t nearly as big as it could have been and as they typically are on a site as complex as this one.”
The entire 33.5-cubic-meter-density (8,840-gallon) wastewater treatment system is enclosed in a 167-square-meter (1,800-square-foot) building designed to blend in with the surrounding area. Its discreet design, low odor, and quiet operation makes it barely noticeable to the site’s 140,000 annual visitors.
The tertiary-quality effluent is reused as flush water in the Visitors’ Pavilion bathrooms and for landscape irrigation through a 0.6-acre subsurface drip-irrigation system and an 800-square-foot surface drip-irrigation system.
MBRs are highly automated. Most small systems do not require full-time operators, because system performance can be monitored remotely. This system is managed by the site’s chief of maintenance and one certified treatment plant operator, who spends an hour or two a week maintaining it, Waggoner says. “We didn’t want it to be high maintenance, and it hasn’t been. There have been a couple of occasions where the counts have been a little high, but we’ve been able to adjust it very quickly.”
To minimize fouling, the membrane fibers can be back-pulsed with permeate for about a minute and then put back into service. If membrane fouling is more significant, the membranes can be back-pulsed with sodium hypochlorite to dislodge particles from the pores and membrane surface. Meanwhile, aeration blowers at the bottom of the membranes produce bubbles that travel up through the fibers and scour debris from surface. The permeate is back-pulsed through the fibers to flush out the cleaning solution, and treatment resumes.
Overall, “it’s worked beautifully,” Waggoner says. “We love the idea that we’ve been able to maximize the reuse of wastewater and minimize the use of potable water.”
Herschell Winfrey is product manager for ZENON Membrane Solutions.
OW - March/April 2007 |
