The Florida panhandle is booming, especially scenic Walton County. Located half way between Pensacola and Panama City, the county borders the Gulf of Mexico, includes large areas of public land and claims much of Choctawhatchee Bay. Its setting has made the county the sixth fastest growing county in Florida, attracting many new residential and recreational developments and their associated commercial businesses.
Also booming are the challenges for Regional Utilities of Walton County, operated by Florida Community Services Corporation. The utility runs the water supply system, sanitary sewer collection system and the three activated sludge wastewater treatment plants that serve the county’s southern region.
“Our permanent population is about 25,000” said Dave Marell, director of water and wastewater for Regional Utilities. “But it can easily triple during the month of Spring break and again the weeks around the 4th of July. During the summer there are at least 40,000 people down here constantly.”
Now due to the growing population, the flow rates to the three wastewater treatment plants have permanently increased by 15% each year in recent years. With these greatly increased wastewater flows the utility has struggled to keep up with drying and disposing of its digested sludge. After considering all of its options, Regional Utilities turned to USFilter for an easily implemented solution.
Service Area and Facilities
Regional Utilities provides water and wastewater services to an area of Walton County that borders the Gulf, extends about 4 miles north to Choctawhatchee Bay and the inter-coastal canal and is about 30 miles wide. The entire service area is on a coastal plain and the water table is only a few feet below the surface. The utility maintains more than 50 miles of collection system and nearly 400 lift stations.
Wastewater treatment is provided by three activated sludge plants. The first plant, acquired in the 1980’s from the Sandestin resort on the western edge of the county, is a 2.0-mgd conventional, activated sludge plant. The second plant, built in 1993 at Point Washington, was recently upgraded to a 2.0-mgd sequential batch reactor (SBR). The third plant located at Seacrest, also a 0.5 mgd SBR began operation in 2000.
A SBR plant performs flow equalization, biological treatment and secondary clarification sequentially in a single reactor tank. It works on a reaction time basis as opposed to a conventional plant that achieves treatment as the waste flows from one tank to the next. SBR plants produce high quality treatment for a wide range of waste flows and loads. It also biologically removes phosphorus and nitrogen. As a result of its single tank treatment, an SBR plant takes up much less space than a conventional secondary plant. “Down here a smaller footprint is a great thing to have,” said Marell.
The three Regional Utilities plants are interconnected so flows can be controlled to distribute peak loads or reduce specific plant loadings during maintenance cycles. Finally, the three plants are somewhat unusual because their effluent is used entirely and exclusively for irrigation. No effluent is discharged to natural water courses.
The Sludge Dewatering Challenge
Walton County’s continual growth places a large burden on the treatment plants, especially their sludge drying beds. “We worked hard during the winter to make space in the digesters at all three plants,” said Marell. “We wanted to have the digesters empty and the drying beds clean when Spring break began. Then we would keep our fingers crossed that we didn’t get too much rain. A couple of days of rain would put us way behind.”
Why not add more drying beds? Regional Utilities looked at that option but decided against it for several reasons. First, “Cleaning sand beds is labor intensive,” said Marell. “You mostly clean them by hand.” Second, rain is a continual problem especially in the summer. “It’s hard to account for all our rainfall when sizing drying beds,” he said.
“But the main consideration,” Marell said, “was simply the amount of land you had to take up for more drying beds. At Sandestin we would have had to build 15 to 20 more beds to keep up. Down here an acre of land can cost $6 million.” The Sandestin plant, for example, abuts expensive homes, 72 holes of golf and dozens of tennis courts.
They also considered land applying their sludge. “The company that land applies our dry solids offered to land apply our liquid solids,” said Marell. “They dispose of the sludge 80 miles from here. You’d be paying them to haul 98% water. From a cost standpoint that’s not very feasible.” After looking at all of the options Regional Utilities found their answer. “One centrifuge and its building takes up less space than half of one drying bed,” said Marell.
The Sludge Dewatering Answer
As part of their upgrade of the Point Washington plant Regional Utilities ordered the first of three skid-mounted decanter centrifuge dewatering systems originally from USFilter. These units were completely prefabricated and pre-engineered for operation. After initial set up, the centrifuge automatically adjusts to achieve maximum cake solids.
Regional Utilities had looked at several different decanter centrifuges but chose USFilter because, “We wanted the whole package,” said Marell. “We’ve got a lot of their equipment and we like the service we get from them. We wanted a unit that would work well and if we have a problem with it we know USFilter will take care of us.” The skid mounted decanter centrifuge systems appealed to Regional Utilities for another reason. Due to the high water table the reactor tanks of all three treatment plants are built on the ground. So, “It is real simple for us to hook up these centrifuges,” said Marell. “We use the hydraulic pressure of the digesters to push the sludge to the centrifuges.”
During the Point Washington upgrade, “We could no longer keep up with the solids at our Sandestin plant,” said Marell. “We were closing in on permit limitations.” To help them respond quickly to that growing problem, USFilter provided a skid already in-house that could be used for a quick delivery. Within six months of placing the Point Washington order the centrifuge was adapted, installed and functioning at Sandestin.
That first decanter centrifuge has operates 3-4 days a week for 8-10 hours at a time. “We feed the sludge between 65 to 75-gpm,” said Marell. “We’re able to achieve 18% dry solids. It couldn’t be easier. It’s all automated. The operators turn it on and walk away. They’ll come back about every hour just to check on it. Other than that it runs itself. They turn it off manually and then the unit goes into automatic back wash and shuts down itself.”
A second decanter centrifuge has begun operating at the Point Washington plant and a third one is on order for Seacrest. “Sand beds are definitely a thing of the past,” said Marell. “The centrifuges weren’t a luxury. They were a necessity. With our flows we didn’t have a choice. We had to have something to dewater quickly.”
All three decanter centrifuges have now been operating for several years with an excellent performance record.
Eric Mc Attee