Squeeze Play

A rotary fan press helps a small treatment plant in Virginia relieve biosolids dewatering capacity issues in a cost-efficient manner

Wastewater entering the Mt. Jackson (Va.) Sewage Treat-ment Plant was so high in nutrients that director of utilities Gale Netz wasted the 78,000-gallon digester daily. However, the six 25-foot-square drying beds often lost the struggle to keep up.

A 700,000 gpd (design) treatment plant with sequencing batch reactor basins and dewatering technology was in the planning stages, but until it opened in November 2008, Netz needed an economical solution to his problem.

While Netz was on duty with the National Guard in Afghanistan, town manager Charles Moore and the temporary plant operator toured three treatment plants to see belt and rotary fan presses. Impressed with the latter, they contacted Matt Winschel, the representative for Prime Solution Inc. in Richmond.

Winschel loaned the facility an older rotary fan press until the new plant could install the Prime RFP36D-SK 36-inch dual-chamber press on skids. In 2008, the old Mt. Jackson plant generated 176 tons of biosolids. By April 2009, the new facility had already surpassed that number, but the new press easily kept pace.

Stress and press

The new plant averages 280,000 gpd from 850 residential and 50 commercial customers. Mt. Jackson has 16 miles of sewer mains and three lift stations.

Netz could fill the old drying beds in one day, but turnaround was 21 to 30 days in summer, and 60 to 90 days during the rainy months of winter. Biosolids were hauled to a landfill 10 miles away by dump truck. “We made at least one trip a week, often more,” says Netz. “Every time the truck left the yard, it cost $500. It still does.”

Netz returned from Afghanistan just before the loaner press arrived. “Charlie took me to see the belt and rotary presses at the other plants,” he says. “From what I saw, the rotary fan press was the only way to go.”

Winschel trained Netz and assistant Johnny Raines to run the dual-chamber 36-inch rotary fan press loaner. “It was pretty simple,” says Netz. “If I can understand it, anybody can.”

Netz and Raines mounted the loaner on a trailer. In cold weather, they pulled it from the maintenance shop to the old treatment plant because liquid inside the press froze if they left it outside. In summer, they parked the trailer next to the digester tank, attached the hoses, and started the unit’s 70-gpm pumps. The men dewatered daily, running the pumps at 35 gpm.

Feed sludge, entering two rectangular channels, slowly moves between two parallel revolving stainless steel filter plates. Filtrate passes through these slotted, wedge wire plates as the flocculated material advances. Dewatering continues until the material reaches a tapered restriction zone at the outlet of the press. An adjustable, two-piece pneumatic restriction plate squeezes the material into the final dry cake.

Slick operation

Cake exiting the back of the press landed in a borrowed grain elevator that deposited it into the dump truck parked beneath it. Although a clever and smooth operation, it wasn’t cost effective because of the power requirements.

“Nevertheless, I was impressed with the unit,” says Netz. “It really hummed but was more sensitive than the new model, and one of us had to be there every moment. If something went wrong, it didn’t shut itself down. The new press does, enabling us to drive by once an hour to check on it.”

The men converted one drying bed to a storage area and built a roof over it. “We stored cake all winter until it stopped raining and we could haul again,” says Netz. “With only five drying beds, though, we had some very intense days when it was too cold to dewater, the beds were full, and the biosolids kept coming.”

The new treatment plant opened on schedule, with the Prime RFP36D-SK 36-inch dual-chamber rotary fan press a permanent fixture attached to the digesters. Netz wastes sludge to the digesters daily and starts dewatering when they are full. “With the amount of sludge we generate, we dewater every day,” he says. During an 8- or 9-hour day, the press processes 19,000 to 20,000 gallons of feed sludge, producing cake as high as 30 to 35 percent solids.

“Those are fantastic numbers for aerobically digested sludge,” says Netz. “Our waste activated sludge runs around 0.80 percent solids.” The continuous dewatering process is totally enclosed, eliminating odor and reducing corrosive exposure to nearby equipment.

The self-contained press needs little attention after Netz sets its operational adjustments using a touchscreen with icons to control the programmable semi-automated self-clean cycle and the pneumatic-operated pinch valves with magnetic flow meters that equalize flow for maximum efficiency. Other icons control gear ratios for variable speed and emergency stops of the press, water, and polymer.

“The press enabled us to go from manual labor to no labor,” says Netz. “That’s another reason why we love it.” Netz also knows that should regulations change and reduce the plant’s BOD discharge permit level from its present 45 mg/l, it won’t be a problem. The liquid from dewatering has zero to 5 mg/l BOD.


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