High Capacity dewatering system reduces digester maintenance

Two high capacity 72-inch Kason Corporation "Cross-Flo" sloped-deck sieves screen out indigestible solids from influent at Elk Pinch Public Service District.

Installation of a high capacity dewatering screen to the activated sludge secondary treatment units on the influent line at Elk Pinch Public Service District, Elkview, West Virginia, has reduced costly maintenance. According to the plant manager, Bud Wilson, the average time between shutdowns for cleaning the treatment process has lengthened by 50 percent.

The Elk Pinch facility has two treatment units able to process 1,000 gallons per minute of sewage at peak flow delivered through an 18 inch force main from the served community. The treatment units receive a percentage of indigestible solids into the waste stream which accumulate in the bottom of the treatment units. These solids consist of plastic, rubber, wood, cloth, and a variety of other troublesome materials.

Screen out Materials

"To increase the production time from each of the treatment units," reports Mr. Wilson, "I had to locate heavy duty dewatering equipment that could screen out these indigestible materials before they entered the treatment units." Reviewing the published data on similar high capacity dewatering applications in other industries, his search led to a rugged, stationary screening device featuring a manually adjustable, sloped screening deck built to withstand the impact of various solids without suffering screen damage.

Stainless steel, profile wedge wire screens remove plastic, rubber, wood, cloth, and other troublesome solids at high rates.

Referred to as a "Cross-Flo" sieve, this stainless steel, corrosion-resistant screening machine, manufactured by Kason Corporation, was being used for separating a wide range of abrasive materials, including sand, bark, foundry slag, and similar solids, from high flows. Wilson saw the potential for using it to reduce treatment unit downtime in his sewer system.

Working with Paul Naberhaus, the regional representative for Kason, the decision was made to install two 72 inch Cross-Flo sieves fitted with stainless steel, profile wedge wire screens, and modified to incorporate an automatic high pressure spray. Wilson's foresight proved correct. As a result of the installation, treatment unit downtime has been cost effectively reduced, along with appreciable labor savings.

How the dewatering system works

The continuous sewage stream enters the head box of the Cross-Flo unit through a 12 inch inlet. As the head box fills, the stream overflows onto an acceleration ramp, which helps to orient the solids as the stream builds velocity. The stream passes over the profile wires, which present slotted openings running perpendicular to the flow. The clarified liquid is accelerated through the screen, and the oversized particles are discharged at the lower end of the screening deck. Simple, manual operation of the deck slope allows direct control of the moisture content of the solids waste material.

An 18 inch force main feeds municipal waste containing indigestible solids into the head box of the Cross-Flo sieves through a 12 inch inlet.

The ability to adjust the angle of the screening deck extends the useful application range of the screen, and simultaneously determines the percentage of solids removal. With the same screen opening between the wires, the steeper the slope, the finer the particle sizes that will be removed; the flatter the slope, the coarser the particle size and the drier the dewatered material. Selection of the profile wire spacing and the slope assures optimum production with the desired degree of clarification.

Elk Pinch Public Service District is located on the Elk River, 10 miles north of Charleston, WV. It is a quiet, fast-growing area of single family homes, with a 750 gpm average and 1700 gpm peak sewage flow. The Kason Cross-Flo dewatering system is engineered to accommodate the anticipated community growth for years to come while satisfying the environmental concerns. It is Bud Wilson's belief that his installation will prove a model for similar communities. He would be glad to share his experience with his colleagues.