Crunch Time

Passive dewatering helps an Idaho city cope with an emergency, control odors and reduce its waste activated sludge inventory

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When mechanical failures disabled two digesters at the Idaho Falls (Idaho) Wastewater Treatment Plant, sludge had to be transferred to an 18-million-gallon storage lagoon. There, partially digested material floated to the surface, formed a scum layer, and decomposed under the summer sun.


“The odor was immediately evident downwind from the lagoon,” says supervisor David Smith. The staff had to deal with the odor and find a way to skim off and control the solids.

Research convinced Smith that a Geotube® container from TenCate Geosynthetics — a flexible tube made of high-strength, permeable, engineered textile — could contain and dewater the high-moisture-content sludge.


He ordered a 60- by 100-foot container from sales agent Jim Bridges of Clearwater Dewatering in Nampa, Idaho. The emergency application proved so successful that Smith ordered five more tubes, and made dewatering an integral part of the treatment process.


Preparation minimal

The 17 mgd (design) treatment plant has an average flow of 11 mgd from 26,000 customers, including seven industrial accounts. Effluent discharges to the Snake River.


The plant processes 70,000 gpd of combined primary and thickened waste activated sludge (WAS). The digesters have a constant inflow and outflow with a minimum 15-day sludge retention time. After the 38-year-old tanks were washed, plant staff saw that welds had failed at the stainless steel pipes that lead to the methane gas mixers. A mechanical firm repaired the piping in two weeks.


After the digester failure, a top priority was controlling the odor from the lagoon. Smith contacted WaterSolve LLC in Grand Rapids, Mich. Company representative Randy Wilcox, P.E., sent Solve 351WS odor-control chemical, which the plant staff mixed with water and sprayed over the lagoon. “It was a magic silver bullet,” says Smith.


Meanwhile, Mike Broering, WaterSolve project manager, arrived to help set up the geotextile container and program a WaterSolve WSLP-2400 E-10 polymer mixing and injection system, which the plant rented.

Dewatering began the same day because the plant already had the required drainage system. “Our old, unused lagoons/drying beds have sloped asphalt areas that drain into two secondary aeration basins,” says Smith. “We just stretched out the bag on the asphalt and were ready to go.”


Operators lowered one valve on the sludge storage lagoon to catch the floating scum layer, which a trailer-mounted 4-inch pump sent through the feed hose to the polymer make-down system. The system injected polymer into the sludge, mixed it, then pumped the chemically conditioned solution into the Geotube container until it reached its maximum 7.5-foot height. As the bag filled, workers sprayed its surface with the odor-control chemical.


Clear, decanted water immediately drained through pores in the textile, which retained more than 99 percent of the solids. When Smith saw how effective the process was, he ordered two more containers and a WSLP-2400 F-10 progressive cavity polymer make-down unit to replace the rented one.


Almost foolproof

“The containers are easy to use, and the feed system almost foolproof,” says Smith. “Mike instructed an employee in its operation, and he caught on right away.”


Over winter, solids continued to consolidate as residual water vapor escaped. Volume reduction in the containers can be as high as 90 percent, according to the manufacturer.


In November 2009, operators opened the bags and took core samples of the material, which contained 10 to 11 percent solids. “Before we had the containers, we land-applied sludge at 2 to 4 percent solids, so 10 percent is a big change,” says Smith. “We’re thrilled because it will reduce our transportation and land application expenses.”


Using 3,500-gallon tanker trucks, drivers haul about 70,000 gpd to farms 10 to 15 miles away before spring planting and after harvest. “Our problem is not enough tankers and not enough daylight hours to meet the demand of the agricultural community,” says Smith. “Dewatering will eliminate a lot of truckloads and make it easier to stockpile the dried biosolids at some farms for spreading later.”


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