When designing the 22.5 mgd Central Water Reclamation Facility in Cantonment, Fla., Baskerville-Donovan engineers specified oxidation ditches. The technology was familiar because the 8.2 mgd (design) Bayou Marcus sister facility had a four-stage biological nutrient removal (BNR) system.

WesTech Engineering won the bid for the project, which replaced the outdated and undersized Main Street Wastewater Plant. The firm supplied four five-stage 7.2 mgd OxyStream treatment trains and clarifier optimization packages. “We added the fifth stage — the reaeration zone — to help polish the water,” says lead operator and plant manager Kijafa Lee. “It adds a little more dissolved oxygen to the waste activated sludge to prevent anaerobic conditions that could release phosphorus in the secondary clarifiers.”

The facility began operation in August 2010. It uses 0.7 mgd of reclaimed water for washdown and feeds 3 to 6.5 mgd to Gulf Power and 6.3 mgd of reclaimed water to International Paper Co. for industrial purposes.

Advanced nutrient removal

The gravity-flow plant averages 14 mgd in winter and 16 mgd in summer, and it produces 15 tons of dry biosolids per day. Influent passes through two 40 mgd Step Screens (Huber Technology) and four Grit King tangential vortex grit chambers (Hydro International) before entering stage one of the reactor.

The anaerobic zone mixes influent with return activated sludge, allowing the mixed liquor to ferment. The process also promotes increased phosphorus uptake in the aerobic channels. In the second stage, a gate allows the nitrate recycle stream to combine with the mixed liquor in the pre-anoxic denitrification zone.

As the mixture enters stage three, slow-speed LANDY-7 surface aerators (WesTech Engineering) in the aerobic channels provide 3.8 pounds of oxygen per horsepower-hour to ensure complete oxidation of BOD and ammonia. Turning vanes (curved walls) at the ends of the channels minimize hydraulic losses, increase velocity and prevent solids from settling.

Liquid flowing from the channels enters stage four, the post-anoxic zone, where endogenous respiration removes remaining nitrate. From stage five, the reaeration zone, effluent enters the 125-foot-diameter clarifiers, each with an energy-dissipating inlet, flocculating feedwell, spiral rake blades and a sludge withdrawal ring that reduces blanket depth while maintaining high solids concentrations.

Water leaving the clarifiers flows through Rotamat ultra-fine rotary drum screens (Huber) and the chlorine system (Miox Corp) before being piped through 30-inch ductile iron mains to the industrial partners. Excess water is stored in a 60-acre, 70-million-gallon reservoir or sent to one of two spray fill irrigation zones.

The reactor’s control system uses probes that automatically adjust aerator power input to dissolved oxygen demand and optimizes performance by increasing or restricting aeration in specific zones. A programmable logic controller monitors all the equipment, and a touch-screen interface enables operators to make adjustments easily.

We make it, they take it

Reclaimed effluent must meet stringent permit limits. Even water affected during instrument recalibration for chlorine residual or pH is rejected. Two electronic valves automatically reject less than 1 million gallons if the chlorine residual drops below 1.2 ppm, turbidity spikes to 2.5 ppm or the pH drops below 6.0. Rejected effluent is tested in-house for suitability before operators return it to the process.

When the industrial partners exceed their normal water usage, the facility draws from two 2-million-gallon effluent storage tanks and one 4.7-million-gallon tank to meet demand. “If a hurricane left us without power, all the flow goes to two 11.25 mgd reject tanks,” says Lee. “Once power is restored, we can send water from the tanks back into the process.” Generators power the chlorine system during electrical outages.

The plant faced its first major weather challenge in June 2012, when 6.2 inches of rain fell over a weekend, sending more than 40 mgd to the headworks. “The nutrient values didn’t change, and other than increasing chlorine feed, the operators saw nothing that would affect the process,” says Lee. “The BNR performed very well.”

Efficient operation

The plant permit limits are 5 mg/L CBOD, 5 mg/L TSS, 3 mg/L total nitrogen and 1 mg/L phosphorous. Effluent averages 2.3 mg/L CBOD, 0.68 mg/L TSS, 1.38 mg/L total nitrogen and 0.19 mg/L phosphorous.

“Originally, we weren’t sure how much alum to inject because the feed is proportioned to flow,” says Lee. “We’re at 4.1 gph now. The DO set points vary from 1.5 ppm in fall to 2.0 ppm in summer.”

At the Main Street plant, everything was pumped, making it expensive to operate. At the new facility, gravity flow and flow-proportional dosing greatly reduce energy usage, as do variable-frequency drives on the two aerators and four motors. “We thought about using just three treatment trains in fall and winter, but it takes five or six days to bring one online,” says Lee. “That luxury isn’t available when we’re dealing with heavy rains or late-season hurricanes, so we run all four.”

The only issues on the system involved fiber and rag fragments from the grinder pumps in three lift stations slipping past the sides of the step screens and piling up on the mixing impellers. “That caused false alarms,” says utility operations plant manager Ray Yarbrough. “We’d turn off the mixers, run the impellers backward to dislodge the material, then reset the units.” Stiffening the sides of the screens helped trap fragments and has reduced resets to a minimum.

“It’s a simple-to-operate, very forgiving system with lots of redundancy and capacity,” says Lee. “We like it.”