The population of Marco Island, Fla., more than doubles, to about 38,000, during the winter season as tourists surge onto the island to escape cooler climates. Since 1972, a conventional wastewater treatment plant (WWTP) has served the growing community on this small 24-sq-mile island in Florida’s Ten Thousand Islands area. The WWTP has been expanded three times over the years, increasing treatment capacity to 3.5 mgd for the rising numbers of permanent residents and tourists. As growth continued, plans began for another plant expansion to further increase capacity as more residences receive sewer services and the number of septic systems on the island is reduced.
“A waterway runs to the east of the site, and development borders the other three sides,” said Bruce Weinstein, senior project manager at Marco Island. “An expansion with conventional technology would have been impossible because there’s no room to expand the plant footprint and additional land is not available. A compact MBR system was our best option for achieving our expansion goals.”
Most efficient solution
Marco Island Utilities staff worked with consulting engineering firm CDM to select a Z-MOD–X packaged membrane bioreactor (MBR) system, manufactured by ZENON Membrane Solutions, a part of GE Water & Process Technologies, to replace the existing conventional system and increase treatment capacity at the WWTP.
“The compact size of the system was one of the main reasons we selected MBR, but the system also brings many other benefits to Marco Island,” Weinstein said. “The MBR produces effluent that exceeds our discharge requirements, so we’re in an ideal position to respond to increasingly stringent discharge regulations. The MBR system will also provide high-quality reuse water, which will reduce our demand of potable supplies by creating a continuous, drought-proof supply for golf course and residential property irrigation.”
Customers will also be pleased with the price, Weinstein noted, adding that recycled water is a fraction of the cost of potable water—just $0.53 per 1,000 gal compared to $2.90 per 1,000 gal for potable water.
The pre-engineered, modular system will enable the city to achieve a build-out capacity of 5 mgd without increasing the plant footprint. Skid-mounted plug-and-play components were quickly delivered to the city, ensuring the project could be completed before the start of the 2006 tourist season and providing the flexibility the city needed to meet its construction objectives.
Continuous operation
The first phase of the MBR expansion was completed during the city’s annual low-flow period, in which an existing 1-mgd contact stabilization package plant was converted to a 3-mgd Modified Ludzack-Ettinger operating mode followed by the MBR system. The contact stabilization tank was converted to anoxic and aerobic tankage, and four 1-mgd, skid-mounted Z-MOD-X filtration trains were installed. The remaining two contact stabilization plants remained in service during the construction. Once the ZeeWeed MBR is fully operational in November 2006, it will receive most of the flow into the plant, and only a minimal flow will go to the two remaining contact stabilization plants.
The process
Wastewater will enter the plant through a 2-mm fine screen and then flow into a 0.5-mg equalization tank. From this tank, the wastewater will flow into the bioreactor, first for denitrification in the anoxic zone and then nitrification in the aerobic zone. After biological treatment, the mixed liquor from the aeration basin will be distributed among the four membrane trains.
Each membrane train can hold up to six cassettes of reinforced hollow fiber membranes. With a nominal pore size of 0.04 microns, the membranes will remove virtually all suspended particles from the mixed liquor, including harmful pathogens.
Because ZeeWeed MBR removes solids by filtration rather than settling, the process is more effective than conventional treatments, and can operate at much higher mixed liquor suspended solids (MLSS) concentrations. Marco Island will operate its bioreactor at an MLSS of 8,000 to 12,000 mg/L, compared to 3,000 to 5,000 mg/L for a conventional system, resulting in a much more compact plant footprint.
Membrane fibers will be automatically cleaned with a clean-in-place backpulsing process that forces permeate water back through the membranes. Aeration is also used to scour debris from the fibers, provide mixing within the process tank, and provide oxygen for the microorganisms.
When necessary, in-tank chemical cleaning can be automatically performed if membrane fouling reduces permeability below a specified performance level. The flexible four-train system gives operators the ability to schedule cleaning during periods of low demand. One train can be offline while the others continue operating to meet design capacity.
Treated effluent will flow into a chlorine contact tank and will then be pumped to two onsite 0.5-mg reuse water storage tanks. These tanks are already in use at the existing plants and are connected to high service pumps that provide high-quality reuse water for irrigation. During the rainy season when demand for irrigation water is low, reuse water goes to infiltration basins off the island or for deep-well injection.
“Our residents welcome the availability of recycled water,” Weinstein said. “We’re still expanding the system, and as we continue installing sewers, the volume of recycled water available will keep increasing.”
