Although membrane bioreactor systems are more expensive to operate than older technologies, they're a popular new-development option in arid areas, like the Southwest, where effluent requirements are strict, flows rarely peak, and the membranes are in almost constant use, says George Crawford, principal wastewater treatment engineer with consulting firm CH2M Hill, Denver. Cities farther north, which frequently handle high peak flows from storms and spring runoff, incorporate membranes into their treatment systems to treat dry weather flows.
King County, Seattle, where the world's largest MBR plant is scheduled to go online in 2010, will treat the first 39 mgd of inflow with membrane bioreactors, says Crawford. During peak flows, the excess will divert to chemically enhanced primary treatment, after which the two flows will come together—and still meet typical discharge standards for landscape and agricultural irrigation and wetland enhancement.
Membranes eliminate the need to manually test settling velocities and characteristics, but they require a proper programmable logic control design with redundancy as well as a robust communication network. Standby power is required so power outages don't shut down the entire system. As with any new technology, they increase the need for instrument technicians.
They're also more expensive to run. Day's electricity bill has gone from $500 to $4000 per month. But his plant's new 43x150-foot addition doubles the total possible sewer connections to almost 3000. The city has 5000 plots. If it maintains its current growth rate, it can increase capacity beyond 3000 simply by installing more membranes in its existing lagoons.
Day, along with engineers from Keller Associates in Meridian, Idaho, traveled to California, Utah, Georgia, and Kansas to explore options before choosing a membrane bioreactor system made by Japan's Kubota Corp. and distributed by Enviroquip Inc.
In Ohio, Riddell's team initially leaned toward using Orbal oxidation, a multi-channel ditch secondary biological treatment process developed by US Filter Envirex Inc., Waukesha, Wis. Further examination nixed the idea.
Delphos has a large food manufacturing industrial base, so its carbonaceous biochemical oxygen demand and total suspended solids loadings are extremely high. Because Orbal size is based on loadings, the new plant would have required an extremely large oxidation ditch, which would have led to problems when attempting to treat large-variation inflows and loadings.
A cost comparison revealed that a membrane bioreactor system would be less expensive to build, provide much more flexibility, and outperform Orbal oxidation in removing mercury. “By using membrane technology, we felt we were being proactive in terms of the EPA's ever-stringent requirements,” says Riddell. Delphos also chose a Kubota/Enviroquip system.
Adding to the existing plant wasn't an option. Most of the ground at the site was already being used for treatment, so parts of the plant would have needed to be taken offline while new treatment facilities were built. And there was nowhere to go. Like the city of Star, Delphos had grown up around its wastewater treatment plant. Parks surrounded it, and people had been complaining about odor for decades.
In the end, the city borrowed $32 million from the Ohio EPA's Division of Environmental and Financial Assistance to build a new treatment plant on a 15-acre site about 2 miles out of town. To pay off the loan, the city is increasing sewer rates 15% every year for four years ending in 2008.
“We sold it by making Ohio EPA out to be the ‘bad guys,'” says Riddell. “Residents have been very concerned about the increases, but they're easier to accept with us moving the plant out of town.”
The Star Sewer and Water District told long-time residents it wouldn't raise rates if they'd pass the bond necessary to get a loan from the Idaho Department of Environmental Quality. Because the loan is being used on technology that accommodates growth, new residents subsidize the addition through water and sewer line hookup fees, for which contractors pay $5150.
Day needed at least 1150 connections to begin using his new technology. By the time the membrane bioreactor went live on Jan. 1, 2006, he had 1600. “Growth is paying for this new addition,” says Day. “And I have lots of room for expansion.”Multitalented technology
Membrane bioreactors eliminate secondary and even tertiary filtration.
Think of a membrane bioreactor as an extremely powerful piece of cheesecloth that produces crystal-clear water from just about any type of effluent.
A membrane bioreactor is a biological treatment process (hence, “bio”) that combines traditional and new technologies in a single step: The traditional activated sludge process to remove soluble and particulate matter from wastewater, and membranes to separate the activated sludge from the treated effluent.
Membrane bioreactors use membranes to retain biological solids in the mixed liquor by withdrawing the treated water through an extremely small-pored membrane instead of settling in a secondary clarifier. The membranes therefore also perform the tertiary filtration function traditionally accomplished with granular media filtration.
As in any activated sludge process, the key to successful treatment is to maintain the required quantity of return activated sludge.
This is rarely a problem with a membrane bioreactor because their pores are minute—smaller even than the pores of filter papers used for laboratory analysis—so the separation of solids from liquids is essentially complete and all biological solids are retained in the process to use as return sludge or for wasting.
One caveat, though: Although membrane pore sizes are in the microfiltration or ultrafiltration range, they're not small enough to capture soluble organic compounds, metals, or trace contaminants such as pharmaceutical and personal care products, priority pollutants, or endocrine-disrupting compounds. A membrane bioreactor may adsorb or reduce such contaminants, but it can't directly filter these materials from wastewater.
— George Crawford, principal wastewater treatment engineer with consulting firm CH2M Hill, Cambridge, Mass.