MBR Package Plant Ends Sewer Moratorium in Growing Community
A sewer moratorium brought development to a virtual standstill in Huntsville, TN, during the fall of 2002.
A sewer moratorium brought development to a virtual standstill in Huntsville, TN, during the fall of 2002. Under the state-enforced moratorium new sewer hookups were prohibited until the town could solve overflow and washout problems at its conventional oxidation ditch wastewater treatment (WWTP) plant. Constructed in 1989, the 150,000 gpd WWTP was consistently meeting the treated effluent limits of its discharge permit. But with average day flows at almost 85 percent 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 Town of Huntsville doubled treatment capacity and ended a state-enforced sewer moratorium by retrofitting its conventional oxidation ditch WWTP with a MBR system.
With its environmental and economic health at risk, the Town of Huntsville’s Board of Mayor and Aldermen began investigating options to upgrade treatment capacity and effluent quality at its WWTP. They selected an advanced membrane bioreactor (MBR) package plant that doubled treatment capacity to 300,000 gpd and can consistently deliver high quality effluent to the receiving rivers.
The 110,000 gallon oxidation ditch became the new equalization basin, and the larger volume dramatically increased the plant's ability to handle large volume flows during high use or wet weather.
As Tennessee’s first wastewater treatment plant to use MBR technology, Huntsville’s facility is becoming known for it’s unique design and cost-effective approach - features that earned the consulting and engineering firm of Jordan Jones and Goulding (JJG) the 2005 Excellence Award from the American Council of Engineering Companies.
Protecting a Treasure
The New River is at the headwaters for Tennessee’s Big South Fork National River, which was heavily affected throughout the last century by acid mine drainage and excessive sediment from logging and poor road construction. Today the river and the surrounding lands are protected by the National Parks Service (NPS), which works closely with the state and the Environmental Protection Agency (EPA) to monitor and limit pollutants entering the watershed from point and non-point sources.
“The state and the NPS have been working hard to help the Big South Fork National River and Recreation Area recover from the ecological damage and to ensure that visitors to the recreation area can enjoy clean swimmable and fishable water,” said Wesley Riggins, Alderman for the Town of Huntsville. “It was very important for us in Huntsville to know that our wastewater effluent was high quality and was helping to restore the health of this wonderful waterway. We wanted something that we could all be proud of.”
Working with JJG, the Town of Huntsville began searching for a solution for the WWTP upgrade. Since the existing WWTP was still relatively new, the town wanted to incorporate as much of the plant as possible to minimize cost and eliminate the need to locate a new site. After completing a preliminary engineering report, JJG initially recommended expanding the existing WWTP with conventional technology, but later recommended ZeeWeed® MBR technology from Zenon Membrane Solutions, a part of GE Water & Process Technologies.
ZeeWeed® MBR was recommended for several reasons including declining system costs, ease of operation, high-quality effluent, and a compact process footprint. Moreover, with a steep slope at one side of the existing WWTP, and a nature trail on another side, site expansions using conventional technology would be difficult.
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.
“We toured several wastewater treatment plants, each operating with different types of technology,” Riggins said. “The ZeeWeed® MBR plant was the most impressive technology viewed during our site visits and we selected it for Huntsville because we felt that it was the best system to meet our needs.”
The retrofit of the Huntsville WWTP began in October 2003, with the construction of a new influent line and new headworks. A new equipment building was also constructed for the blowers, pumps and control equipment for the MBR system.
Work continued without affecting operation of the existing plant; however, the equalization basins were taken out of service since they would be retrofitted to contain the new membrane cassettes. A 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 gallon oxidation ditch would become the new equalization basin, and the larger volume would dramatically increase the plant’s ability to handle large volume flows during high use or wet weather. By November 2004, the newly upgraded Huntsville WWTP was fully operational.
“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.”
With a total cost of just over $2 million, the project was funded 60 percent by the United States Department of Agriculture/Rural Development Fund, 30 percent by the East Tennessee Development District, and 10 percent by the town’s sewer customers.
“The system is exceeding our expectations and is attracting quite a bit of interest from communities facing similar situations,” Mayor Potter said. “Since our plant has become operational we’ve already hosted 24 groups of visitors from as far away as Jamaica.”
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 that 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. The membranes, which resemble long strands of spaghetti, are actually hollow, plastic fibers that are filled with billions of microscopic pores. The pores have an average diameter of 0.04 microns.
Thousands of membrane fibers are bundled together and suspended in large cassettes that are connected to collection pipes. The cassettes are immersed directly into the mixed liquor of the process tank and a slight vacuum is all that is required to draw water into the membrane fibers.
By using a slight vacuum, the membrane system at Huntsville WWTP is significantly different than many other membrane systems available today. Whereas many systems encase fibers into pressurized cartridges and force water from the inside of the membrane to the outside, the ultrafiltration system in Huntsville immerses the fibers directly into the process tanks and draws water from the outside to the inside of the membrane. This results in a system that is smaller and requires fewer chemicals for cleaning.
Since the system removes solids by filtration rather than settling, the process is much more effective than conventional treatment systems, and can operate at a much higher mixed liquor suspended solids (MLSS) concentrations. The Huntsville WWTP operates it’s bioreactor at an MLSS of 10,000 mg/L, compared to 3,000 to 5,000 mg/L for a conventional system.
Automated Operation, Cleaning
“The automated operation and reliability of the new ZeeWeed® MBR has dramatically reduced the amount of time we spend on plant maintenance and monitoring,” said Chesty Strunk, Operator at the Huntsville WWTP. “Remote monitoring systems keep watch over the plant and can automatically notify an on-call operator if any parameters are out of line. We also have the benefit of automated cleaning processes that pretty much run themselves.”
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. Aeration of the membranes is also used to scour debris from the fibers and provides mixing within the process tank to maintain solids in suspension.
When necessary, chemical cleaning can also be 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 cleaning processes means that we are using about 50 percent less chemicals with the ZeeWeed® MBR than we were with our old oxidation ditch,” King said. “We are 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 $1400 per month.”
About the Authors:
J.C. Lan, P.E., is Vice President, Director of Wastewater Treatment, at Jordan, Jones & Goulding. He may be contacted at Tel: 770-455-8555 or via e-mail: firstname.lastname@example.org. Roland Lamoca is Director, Eastern Region, for Zenon Membrane Solutions, a part of GE Water & Process Technologies. He may be contacted at Tel: 905-465-3030 or via e-mail: email@example.com.