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Membrane bioreactor (MBR) package plants are providing affordable, simple-to-use advanced wastewater treatment options for small communities that face the challenge of balancing environmental and regulatory responsibilities with budgetary limitations. Such was the case for the town of Huntsville, a growing community of 1,200 in east Tennessee.
In the fall of 2002, a state-enforced sewer moratorium prohibited new sewer hookups until the town could solve overflow and washout problems at its conventional oxidation ditch wastewater treatment plant (WWTP).
Constructed in 1989, the 150,000 gpd WWTP was consistently meeting the treated effluent limits of its discharge permit. But with average daily flows at almost 85% of capacity, the plant was unable to handle increased flows during wet weather or periods of high demand, which threatened the New River and the environmentally sensitive Big South Fork National River and Recreation Area.
The solution involved an innovative and affordable retrofit of the facility with an advanced ZeeWeed MBR package plant that doubled treatment capacity to 300,000 gpd and consistently delivered high quality effluent to the receiving rivers.
MBR retrofits WWTP
Working with consulting and engineering firm Jordan, Jones & Goulding, the town of Huntsville selected MBR technology for several reasons including declining system costs, ease of operation, high quality effluent discharge and a compact process footprint. With a steep slope at one side of the plant and a nature trail on another side, membranes were the only way to expand the plant within the existing footprint.
The retrofit of the Huntsville WWTP began in October 2003, with the construction of a new influent line and headworks. A new equipment building was also constructed for the blowers, pumps and control equipment of the ZeeWeed MBR system.
Work continued without affecting the operation of the existing plant; however, the equalization basins were taken out of service because they would be retrofitted to contain the new ZeeWeed membrane cassettes. A new concrete wall was poured in the center of the equalization basin to divide it into two separate membrane trains, and piping was added to accommodate the membranes.
The 110,000 gal oxidation ditch would become the new equalization basin. The larger volume would dramatically increase the plant’s ability to handle large volume flows during high use or wet weather. Construction was completed in the fall of 2004, and the new Huntsville WWTP was fully operational in December 2004.
“We’re very proud of the new wastewater system in Huntsville,” said George Potter, mayor of Huntsville. “Through a combination of grants, loans and town money we were able to cost-effectively complete the upgrade without any increase in wastewater costs to our ratepayers. We couldn’t have done this if it wasn’t for the MBR technology and the ingenious way it was retrofitted into our existing plant.”
How the MBR works
Incoming wastewater flow first passes through a 50-mm coarse bar screen to remove large debris, followed by a 2-mm continuous-belt band screen that removes finer debris. After screening, a high-flow diversion weir ensures the incoming flow does not exceed the treatment capacity of the MBR. Flows greater than 0.5 mgd are diverted to an equalization basin, while the balance of the flow is equally split between the two process trains of the bioreactor.
Two parallel trains of anoxic and aerobic tanks make up the bioreactor and operate independently of each other to enhance process optimization and simplify cleaning processes. ZeeWeed membranes are immersed directly into the aerobic zone of the bioreactor and filter treated effluent from the mixed liquor.
A slight vacuum is all that is required to draw water into the membrane fibers and filter out impurities.
Because the MBR removes solids by filtration rather than settling, the process is much more effective than conventional treatments and can operate at much higher mixed liquor suspended solids (MLSS) concentrations.
The Huntsville WWTP operates its bioreactor at an MLSS of 10,000 mg/L, compared to 3,000 to 5,000 mg/L for a conventional system, resulting in a relatively smaller plant footprint.
Membrane fibers are automatically cleaned with a clean-in-place backpulsing process that forces permeate water back through the membranes. This dislodges any particles that may adhere to the membranes. When necessary, in-tank cleaning can also be automatically performed if membrane fouling reduces permeability below a specified performance level. The flexible two-train system gives operators the ability to schedule cleaning during periods of low demand, when one train can be offline, while the other continues operating.
The efficiency of the system and the automated processes has reduced chemical usage by 50% over Huntsville’s previous WWTP. The plant is also using the high quality effluent for wash water at the fine screens and washdown water in the sludge press, eliminating the use of treated municipal potable water for this purpose and saving about $1,400 per month.
Effluent from the plant exceeds discharge requirements specified by their National Pollution Discharge Elimination System (NPDES) permit (Table 1). Several businesses, residences and an elementary school have already been connected to a sewer line to the east, and the town expects to add new customers each year.
“As we grow, we’re proud to know that we’re putting the best water that we can back into the river,” said Mayor Potter. “We’re also confident that as NPDES standards continue to become more stringent, our MBR system will be able to handle virtually any discharge requirements for the foreseeable future.”