The Sewer Department of Lebanon, Tenn., operates an extended air-activated sludge treatment plant. Excess solids under aeration were cycling up, but the one-meter belt press could not run fast enough or long enough to dewater the inventory.
Superintendent Billy Dranes consulted Liquid Solids Separa-tions in Butler, Tenn., which recommended geotextile tubes as a more cost-effective dewatering technology than contracting for a mobile belt press.
Three 60- by 100-foot-long TitanTubes from Flint Industries Inc. brought the sludge inventory into the proper mixed liquor suspended solids range within two weeks. The dewatering process also achieved better dissolved oxygen profiles and lower sludge blankets, resulting in optimal sludge settling and higher flow rates through the plant.
Methods and materials
With one tube in reserve, city workers laid the other two on an existing concrete pad at the plant, then connected them to the building with 3-inch Schedule 40 PVC pipe. A header pumps sludge to one tube, and dewatering occurs at the other. Sludge was pumped continuously at 40 gpm during the day until levels reached the proper range for air requirements and sludge settling characteristics.
Based on laboratory results, two cationic emulsion polymers were selected for the dewatering project. “The first was the captive polymer we use in our belt press,” says Dranes. “It has a medium to high charge with a high molecular weight. The second polymer was selected for field evaluation. It has a medium to high charge with a low molecular weight.”
The captive polymer proved superior because the floc did not shear down and lose conditioning. If polymer conditioning is incorrect, Dranes notes, sludge will clog the geotextile fabric and prevent dewatering.
How it works
The conditioned slurry is pumped into the tubes, which retain the floc solids, but allow water to seep through the porous fabric. “One major benefit of tube technology is that the dewatering surface area is hundreds of times larger than on a belt press,” says Dranes. “If necessary, we can dewater at up to 600 gpm.”
After 45 to 60 days, the dewatered sludge is dry enough for removal. Dryness is checked by inserting a Sludge Judge through the port where the slurry was introduced. Tubes are opened, and the 120 tons of dry material, as well as remnants of the tube, are transported to a landfill.
The project cost less than $9,000 including setup, personnel training, consulting fees, TitanTubes, and piping. Lebanon personnel provided the labor. “Contracting for a mobile belt press and crew would have cost $10,000 per week for a minimum of three weeks,” says Dranes.
The plant is dewatering 24 hours per day without odors or vectors. The tubes also can be used for hazardous waste containment, or provide a low-cost dredging and dewatering option for bio-treatment lagoons.
