Divide and Conquer

A Florida Keys city uses five advanced treatment plants in an innovative arrangement that saves money while protecting critical water resources.
Divide and Conquer

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When funding wastewater treatment plants,

most cities try to get the biggest bang for their buck. In Marathon, Fla., it was more like getting the biggest bang for fifty cents.

Under orders to get rid of package plants and upgrade treatment, Marathon faced a potential bill of over $181 million for a new plant. Instead, the city and its design firm, Weiler Engineering, came up with a plan for five small treatment facilities that cost about $90 million, including new sanitary sewers. Staffing costs have been lowered, too.

At the same time, the city anticipated upcoming stormwater regulations and installed storm sewers and treatment swales as part of the overall project. "It was a perfect team effort," says Susan Thomas, project manager for the city. "The city, the financial and design team, and the contract operator (U.S. Water Services) came together. It was awesome."

Ted Yarboro, operations manager with U.S. Water, observes, "What Marathon has done is unique. Other cities may have spent time fighting regulations. The city has given us every tool we need to operate the facilities. It's been fun."

Advanced processes

If you drive from the Florida mainland to Key West, Marathon is exactly half way. And if you stop to use the washroom, the wastewater will travel through a new system of gravity, low-pressure grinder, or vacuum sewers to one of the five advanced wastewater treatment (AWT) plants. The collection system includes 127 pump stations and five vacuum stations, and about 243,000 linear feet of mains.

The plants that serve areas 3, 4, 6 and 7 of the community are small sequencing batch reactor (SBR) plants, with design flows from 200,000 to 400,000 gpd. Incoming wastewater passes through SWECO fine screens ahead of the SBRs, supplied by Siemens Water Technologies. In plant 3, a JWC Muffin Monster grinder is positioned ahead of the SBR to handle wastes from the city jail and hospital.

Siemens multimedia filters follow biological treatment, and the effluent is disinfected with Hach chlorinators. Chlorine and salinity are monitored continuously, and the high-quality treated water is returned to various parts of the community for irrigation and dust control. Any excess — or water exceeding 1 part per thousand salinity — is pumped down one of five injection wells.

The plant serving area 5 was the site of a small SBR installation once owned and operated by the Florida Keys Aqueduct Authority. After purchase by the city, treatment was expanded from 160,000 to 450,000 gpd and improved through membrane bioreactor (MBR) technology (Ovivo), without increasing the plant footprint. "We couldn't build up either because of the nearby airport," explains Weiler Engineering's Ed Castle.

Plant 5 includes coarse screens ahead of an influent equalization tank, fine screens, and then pre-anoxic, pre-aeration, and post-anoxic tanks before the membranes themselves. Dual chlorine contact chambers follow the MBR. A large detention area enhances nitrification.

High-quality water

Treatment results are dramatic: All plants meet a permit limit of 5 mg/L CBOD, 5 mg/L TSS, 3 mg/L total nitrogen, and 1 mg/L phosphorus. "During the winter (dry season), we'll probably be recycling 75 to 100 percent of the effluent," says Yarboro. "During the wet weather, not as much. We're still having people connect up to the system."

Biosolids from all five plants are aerobically digested, then dewatered on a mobile centrifuge (Centrisys) to about 22 percent solids. The digester sizes were doubled to give the plants more capacity. Cake is hauled to a mainland Florida landfill.

The dewatering strategy has saved considerable money, according to Castle: "Tipping fees are high. Previously, the city was hauling biosolids at 1.5 to 2 percent solids to the mainland. That's a lot of water. We've cut the volume of water by a factor of 10."

Despite the design economics, the price was considerable for this community of 10,000 (15,000 to 20,000 in the tourist season). Thomas says the city was in the right place at the right time for funding, qualifying for a $16 million grant from the American Recovery and Reinvestment Act of 2009, $11 million in state grants, additional money from South Florida for stormwater and reclaimed water treatment, and the state revolving loan fund. As a result, she says the assessment per user household was about $5,700 for both the plants and the collection system.

Locating operators

Since 2008, U.S. Water has operated and maintained the treatment plants and the collection system under contract with the city. The firm has five full-time operators — Alex Gajewski, Joe Scholz, Kevin O'Neill, Andy Diskin and Ron Turner — one at each plant on six-hour shifts.

Yarboro travels from plant to plant to coordinate and oversee the effort. "We also have four individuals dedicated to maintenance — two on the plants, and two on the collections system," he says.

He has overcome several challenges, one of which was locating operators in the first place. Since there weren't many operators among the resident population, "We imported most of our people, relocating them in Marathon," says Yarboro.

Startup was also challenging: "Flow would go from zero up to 40,000 gallons as we hooked up a whole subdivision or condominium community all at once," says Yarboro. The team keeps communication frequent — everyone appreciates the need to meet the permit consistently.

The system's new SCADA network (Siemens) keeps everyone connected and enables off-site monitoring and control. "I can operate, monitor and troubleshoot the system remotely from my living room," says Yarboro. He can also log into each plant at any time to check on operations or monitor ammonia levels, for example. "I can see things as soon as they happen," he says. "If we have a lightning strike, which is a serious problem for automated treatment facilities, we can switch to manual operations immediately."

Monitoring quality

Marathon also uses a ChemScan system (ASA Analytics) to monitor water quality. "If ammonia or nutrient levels are going up in the effluent, we can adjust our biological process," says Yarboro. His staff also monitors dissolved oxygen and ORP to determine the amount of air that needs to be supplied to the system, avoiding over-aeration.

Since startup of the various plants, the Marathon team has fine-tuned process operations to deal with local conditions and requirements. The carbon source for nitrification in the SBR plants is a case in point. "With a total nitrogen limit of 3 mg/L, we tried several things," Yarboro says. "We started out with methanol as the carbon source, then switched to acetic acid, sodium acetate and molasses." Finally, the operators settled on glycerin, a safer and more cost-effective product that has worked well.

Another wrinkle involves the design of the SBRs. Castle explains that while most SBRs are designed with two trains, the Marathon SBRs have three. "That way we can take a tank down to work on it and still have two in operation," he says. It helps to have three tanks available to handle higher flows during the rainy season. Then during the dry season, the team can take one down for cleaning and operate with two.

Tackling stormwater

This "thinking outside the box" has typified the Marathon project from the start. "With our water system already in place, and our new wastewater system mandated by the regulators, we anticipated the stormwater regulations that are to come and built both the sanitary and stormwater systems at once," says Thomas. That cut overall costs by more than 50 percent.

As a result, the community is prepared and equipped for a three-hour, 25-year storm event. Like the treated wastewater, stormwater is collected but does not enter the ocean; instead it goes into shallow injection wells and to infiltration trenches or swales for treatment.

If an event results in excessive flow through the plant, the SBRs are switched to storm mode, allowing the facility to process the wastewater faster than normal. "We let the solids settle to the bottom, and the flow runs through our filters, chlorine contact chambers, and into the injection wells," says Yarboro. "You wouldn't know the difference from fully treated effluent."

Last fall, Yarboro's team prepared for Tropical Storm Isaac, which dumped 18 inches of rain in four days — six inches in one five-hour period. "We had no spillage or overflow," he says, even though many older properties are connected to the new sewer system. Leaks at those properties are common sources of inflow and infiltration, and are being fixed on a continuous basis.

In 2008, the city received the EPA Region IV PISCES award for the forward-looking approach among state revolving funded projects. The city also has received Peak Performance Awards from the National Association of Clean Water Agencies for seven years running. "The city has moved proactively on wastewater and stormwater," says Yarboro. "It took charge and took the lead, and it deserves the recognition it has received for this effort."


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