Think of the Clayton County (Ga.) WaterAuthority as a huge irrigation system. Now think of its William B. Casey Water Reclamation Facility as the watering can.
One hundred percent of Casey plant effluent is reused, most of it to replenish the Pates Creek watershed through an indirect potable reuse system consisting of more than 4,000 acres of constructed wetlands. The rest is put to work at the plant itself, irrigating the odor control biofilters, washing down belt filter presses, and mixing polymers.
“We’re big proponents of reuse, and we have been for years,” says Jim Poff, water reclamation department manager. “We opened the nation’s largest municipal land application system in the 1970s, and in 1990 we received an EPA award for best operated and maintained land application facility in the nation.
“For several reasons, we converted to constructed wetlands in 2004. Since then, we’ve actively participated with other agencies on natural system research projects, studying such things as the fate of trace organics from personal care products.”
A unique situation
Located in the Upper Piedmont Region of Georgia, Clayton County has no large rivers or natural impoundments to supply drinking water for its 270,000 residents. The area is underlain by dense bedrock that holds very little water. Groundwater quality and quantity vary widely.
The situation is challenging, but the county has viewed it as an opportunity to take the lead in water reclamation. “Our original land application system was one of the largest in the U.S., containing more than 300 miles of distribution pipes, 250 valves, 20,000 sprinkler heads and a pump station,” says Poff. “It was located on property owned by the authority.”
The system was effective, but it was labor-intensive and consumed a lot of energy. It also required 100 acres per million gallons of applied water, so its capacity was severely limited.
“After an exhaustive search, and with assistance from CH2M HILL, our engineering firm, we decided on a much less consumptive constructed wetlands approach,” Poff says. “Wetlands continued our practice of using natural treatment systems. They also afforded more efficient recharge, lowered maintenance and operations costs, and increased treatment capacity without having to purchase additional land.”
The new wetland system requires only 15 acres per million gallons of applied water, so it easily accommodates flow increases to the current level of 15 mgd. The system aligns perfectly with state mandates to maximize water return, and it represents a sustainable water source in all types of weather. A marshland and bird sanctuary offer additional benefits to the community.
While the constructed wetlands are an effective polishing step, what really counts is at the treatment plant. Chris Hamilton, plant supervisor, outlines the treatment sequence at the Casey facility.
“We’re an advanced secondary plant, with the ability to choose between two operational modes: the Modified Ludzak Ettinger (MLE) activated sludge process or the A2/O (nutrient removal) process,” Hamilton says. MLE systems incorporate an anoxic zone ahead of the aerobic zone. A recycle loop returns nitrified flow to the anoxic zone to facilitate denitrification.
The A2/O process incorporates an anaerobic zone ahead of the anoxic zone to promote the release of phosphorus. Subsequently, the luxury uptake of phosphorus is achieved in the oxic phase. Turblex blowers power the aeration processes.
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Ahead of the biological process, the Casey plant employs preliminary treatment using a pair of Parkson fine screens and a Smith & Loveless grit removal unit, followed by primary clarification in three Envirex Polychem clarifiers (Siemens). After biological treatment, wastewater is clarified in three 160-foot-diameter Walker Process circular basins. Sodium hypochlorite provides disinfection.
“We can feed ferric sulfate in the secondary clarifiers to achieve phosphorus removal, but instead we rely on luxury uptake to remove phosphorus biologically,” Hamilton says. “Ideally, our goal is to reduce the use of chemicals.” The plant is also equipped to add sodium hydroxide for alkalinity and pH control if necessary, but the plant’s ability to regain some alkalinity in the denitrification zone minimizes the need for that chemical.
The Casey plant recycles its biosolids, as well. After thickening and blending, an on-site pelletizing plant produces a Class A product for land application. Under a five-year rolling contract, the pellets are sold to a broker who trucks the material to Florida for use in agriculture and horticulture.
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“We get $7 a ton for the pellets,” says Hamilton. “We use a front-end loader to move the pellets from our covered storage area to trucks. We’ve found that’s cheaper than building a hopper and gate system.” The plant produces about 5,000 tons of pellets a year and ships two to three semi-truck loads a week. A separate dedicated pelletizing staff operates the facility 24 hours a day, six days a week.
As proud as the facility’s staff is of its recycling record, it’s just as conscious of the need to be a good neighbor by controlling odors. In line with its goal of minimizing chemical treatment, the plant uses four biofilters with Bay Products media. Three units are stationed at the solids thickening and blending locations and at a raw sewage pump station on the site of the old treatment plant next door. A central unit serves the new plant.
Each biofilter contains manufactured rock media on which microbes grow. “At first we were using wood chips, but the process called for changing out the media once every three years,” says Hamilton. “The manufactured media doesn’t need changing for at least 10 years. We have neighbors right across the street, so we take odor control seriously here. We don’t want anything crossing the fence line.”
The Casey plant also strives to be a quality employer, promoting continuous training of its staff members. “We’re proactive on training, and we use an online training website (Company College by BizLibrary) that our employees really like,” Hamilton says.
The website is a virtual college, offering more than 700 courses that include such topics as interviewing and reception skills, networking, healthy work habits, marketing, and stress management. “Each course takes anywhere from 15 to 60 minutes to complete,” says Hamilton. “Employees are required to complete safety courses, and they receive certificates of completion when they’ve successfully finished a certain course. The instruction is free to the employees, and the authority pays the course provider a small fee for each participant.”
The authority also provides in-house training to all employees, ranging from classroom computer and management classes to certified defensive driving and CPR/first aid classes.
Of course the ultimate aim of management and staff at the Casey facility is to meet the discharge permit and produce a high-quality effluent that can be reclaimed to help replenish the community’s drinking water resource. Effluent values from 2010 reflect impressive reductions in BOD and suspended solids, as well as effective nitrogen and phosphorus control.
The treated effluent is pumped some five miles away to the wetland area, where it is introduced at the upstream end of the system and moves across clay-lined wetland cells of varying depths or zones by gravitational flow. The marsh zones of each cell range from six to 12 inches of water depth and are planted with species selected both to enhance treatment and promote biological diversity.
“Wetland plant species thrive in saturated soils, and we selected ours on their ability to prosper in the local climate,” says Poff. “Some of these species flower and provide great aesthetic value as well. Alternating zones of pickerel weed, arrowhead, cutgrass, and bulrush, to name a few, were planted on a three-foot center-to-center pattern. Slope vegetation is a Bermuda grass mixture selected for high tolerance to heat and drought conditions.”
Control structures on the outlet end of each cell regulate cell depth. Deep zones from three to six feet in depth are provided at the beginning, middle and end of each treatment cell to prevent short-circuiting, enhance mixing, and promote uniform treatment.
The wetlands are designed to provide an additional 10 to 20 percent of conventional pollutant removal. Residence time is five days, and the design loading rate is 1.0 mgd per 15 acres of wetland.
Poff calculates costs at an average of $1.73 per gallon for capital construction of the 17.4 mgd of wetland treatment capacity. That does not include the cost of land the authority already owned. He also reports a reduction of 13 staffing positions through attrition since 2001, and an annual reduction in energy requirements of 5.3 GWh, enough electricity to power 407 Clayton County homes for one year.
“Constructed wetlands have proven to be an excellent treatment barrier for any water utility,” he says. “We’re very proud of our accomplishments. As we like to say, ‘It’s raining every day in Clayton County.’”
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