Special System Solves Surplus Sludge Situation

The Charlotte-Mecklenburg Utility Department of North Carolina is using a mobile dewatering system to treat and remove sludge from its overloaded McDowell Creek wastewater treatment plant while a fixed-plant addition is being considered.

The utility has contracted the project to system developer/owner Bio-Nomics Services, Inc., Charlotte, N.C. One of the company's self-contained trailer-mounted belt press systems, working alone as an on-site total processing unit, has been dewatering the required 40,000 to 80,000 gallons per day of excess semi-treated sludge.

As currently being applied, the single mobile system, at well under $1,000 per day, is saving Charlotte-Mecklenburg at least 55 percent - and up to 85 percent - of the $2,000 to $6,000 daily costs of other methods considered for handling the excess sludge.

The situation that developed at Charlotte-Mecklenburg is familiar throughout the United States. From 1985 to 1995, the Charlotte metropolitan area grew by approximately 200,000 people. The McDowell Creek plant had reached the point where it had to handle far more solids than anticipated.

To take care of the overload, utility officials and engineers considered hauling excess liquids to another treatment plant, adding drying beds, or incineration. All three of those alternatives were cost-prohibitive.

The initial short-term contract with Bio-Nomics proved to utility management that their excess sludge could be economically dewatered by the temporary system while designs were completed for a permanent installation.

The System

The mobile system, now operating under a long-term contract, includes a filter press, pumps, generator, piping, conveyor, additive tank and controls - plus auxiliary equipment - all located on a out-of-the-way, leveled, L-shaped 40 x 140 foot plot.

A 21,000 gallon rectangular storage tank, set to the right of the mobile plant, provides a supply of clean water for polymer mixing and belt washing. Filtrate is pumped as needed to a 15,000 gallon clarification tank, set to the left of the mobile plant. Distance between permanent and mobile facilities has proved no problem. Filtrate is fed to the plant through 3,000 linear feet of 10 in. polyethylene piping laid on the ground by the contractor. A booster pump increases flow pressure when needed.

Dewatering proceeds independently of the main plant's personnel and equipment. This process draws sludge from plant piping.

The wet sludge is conditioned by polymer drawn from tanks mounted on the mobile plant; this mix, in turn, passes through the plant's 2-meter stainless-steel press. Dewatered solids travel by the unit's central collection conveyor up an elevating conveyor to top-load trailer dump haul units. Residual liquid, up to 100,000 gallons per day, is returned to the main plant's waste sludge holding basin. Flow is through a separate 10-inch polyethylene line, ground-laid by the contractor.

One man handles the entire operation. Truck drivers are requested by mobile radio only when their trailers are ready for removal.

Power for all plant functions is provided by a contractor-owned diesel-powered 65 kw portable electric generator, set adjacent to the mobile plant. Controls, meters and monitoring equipment are self-contained within the plant's design.

Frequent laboratory tests by both the Utility Department and Bio-Nomics Services show the sludge, incoming at 3 percent to 4 percent solids, is reduced by the mobile plant to a goal of 13 percent to 20 percent.

"We did a lot of studying before we decided on this system of producing biosolids," says the utility's plant supervisor, Pete Goins. "Our investigation of every option we knew of - plus our experience since - proves to us that we picked the most cost-efficient one. We had to make no capital investment. The Bio-Nomics Services' unit integrated seamlessly into our main plant operations."

"The contracting firm has continued to do the job for which it was hired, with minimal involvement by utility managers and working personnel, and continuing high quality," he said.