Sludge baggers work for small treatment plant

Written by Craig, Larry E.

Not many wastewater plants have to deal with too little sludge. But, when attempting to justify the purchase of sludge dewatering equipment, the Granville Township Wastewater Treatment Plant (WWTP), Pa., faced exactly that problem.

The WWTP treated an average of 130,000 gpd in 1992, using two se-quencing batch re-actors, two aerobic di-gesters and chlorine disinfection with discharge to the Juniata River. All of the sludge from the di-gesters was dewatered using drying beds.

Looking for an alternative to the labor-intensive beds, the Granville plant began to evaluate dewatering technologies, such as filter presses, centri-fuges, vacuum filters and thermal techniques. But even the smallest version of each of these conventional equipment alternatives was maintenance and energy intensive for a small plant and provided unneeded excess capacity.

After evaluating an alternative, Granville Township realized its answer was in the bag — literally. A sludge bagger system was the low-flow solution to the dewatering dilemma.

“The township procured the equipment by direct bid,” says plant Chief Operator Tim Tressler. “Plant personnel then erected a block building on the existing drying bed and installed everything themselves. The entire project, including the bagger, cost less than $50,000.”

The sludge bagger system is cost-effective because gravity does all the dewatering work. An influent slurry pump and a polymer feed system are the only energy-consuming components required.

After conditioning, the sludge is introduced into a gravity drainage plenum with an open top for easy cleaning and maintenance. The slurry is then continuously dewatered without mechanical shaking or auxiliary pressurization while it works its way into one of the bag chambers. As the bags begin to fill, water penetrates through the woven material and collects for return to the head of the plant. One worker can then remove full bags from the unit using a fork dolly.

The bags of dewatered sludge, typically 10 to 15 percent dry solids at this point, may then be disposed of or stacked on pallets for further dewatering. When the bags are stacked in an outdoor location, natural evaporation from within the bags may increase the solids content to as much as 40 to 60 percent.

The bags represent the largest single operating cost. The non-woven polypropylene bags originally selected for the Granville plant cost nearly $3 each. Assuming 60 bags are processed each week, that translates into an annual operating cost of $9,360. To reduce this cost, GTMA personnel switched to woven polypropylene bags that have the same capacity but cost about 50 percent less.

Ultimately, Granville landfills the bagged sludge. By allowing evaporation to reduce the weight of each bag to about 25 pounds, the GTMA is able to dispose of 80 bags per ton. This equates to 9,300 gallons of the original digester liquid sludge.

Bagger systems are available in 3-, 6- and 12-bag combinations, and each bag has a volume of approximately 22 gallons. Each operating cycle roughly consists of a 2-hour fill period, followed by two hours to four hours for additional drainage. The resulting processing capability is about 20 pounds of dry solids per bag per cycle.

Sludge bagger units are shop-wired and skid-mounted for easy installation. Operation is less labor intensive than drying beds, and bags are reusable in some cases.