A streamlined method of dewatering digested wastewater sludge is helping cut costs for one rural Virginia water authority. The self-contained mobile belt press system is housed in a conventional trailer that can be easily transported from one treatment site to another. According to Bar Delk, general manager of the Louisa County Water Authority (LCWA), Louisa, Va., the new unit eliminated the need to expand the capacity of the Authority's sand bed drying system.
A political subdivision of the county, LCWA operated a 200,000 gpd Regional Sewage Treatment Plant (RSTP), which is owned equally by the Town of Louisa and the County of Louisa. Each governmental entity has access to 100,000 gpd of its capacity. The original user base in 1984 was composed of 1,000 residents and two users in an industrial park. Since then, the base has grown to include four large schools, ten users in the industrial park and the town's 1,500 residents.
The Previous System
In the past, after wastewater had been treated in the facility's 25,000-gal aerobic digester, the sludge was dewatered on drying beds prior to land applications as fertilizer. Delk stated that this method was the limiting factor in expanding LCWA's capacity when the adjacent Town of Mineral, Va., decided to contract its sewage treatment to the Authority. The Town could not accommodate the increased demand and meet stringent clean water regulations at the same time. In addition, its sewer customers could not afford the cost of building a new treatment plant.
Therefore, the County of Louisa agreed to an expansion of the RSTP to 400,000 gpd (with the Town of Louisa and the County of Louisa each having 200,000 gpd). The expansion also included the treatment of septage from private contractors. Furthermore, the Authority was to build a new digester as well as increase the capacity of its existing one.
"We talked about adding more drying beds, about covering the beds and the additional labor to shovel the dried sludge onto dump trucks. The treatment of solids previously was always a problem because we never had the capacity to get rid of them as fast as we would have liked," Delk said.
Solids would fill up the facility's digester to the point where LCWA personnel would have nowhere to dry them. The situation only worsened during wet times of the year.
After LCWA began researching alternatives to expanding its capacity, Delk spoke with an official of the Virginia Department of Corrections and learned of a portable sludge press that the Department was moving from prison to prison to treat wastewater without having to build a new treatment facility at each site.
The New System
LCWA contacted the mobile system's manufacturer, Or-Tec, Inc., Bedford, Ohio, and they sent engineers to the RSTP to make an evaluation of its sludge. After the tests, Or-Tec customized its HP-1000 belt press system to the Authority's specifications. The system was piped and wired on a stainless steel skid and installed on a trailer.
According to Delk, installation was simple. The unit was hauled by a sports utility vehicle from Ohio to the RSTP and positioned beside the digester. The LCWA simply extended an electrical connection to the trailer and it was ready to operate. No piping was necessary.
In operation, the HP-1000's sludge pump draws waste-activated sludge from the digester through a flexible hose to the grinder that macerates any large objects in the 1.5 to 2 percent solids sludge prior to reaching the first PC pump. This pump transfers the sludge to the upwardly sloping belt press where gravity dewatering begins. The sludge is dosed with polymer and flocculated, after which it is conveyed through a wash station where water is removed by pressure rollers. The belt then returns through a wash station to begin the process again.
Delk pointed out several benefits that the LCWA has received over its earlier drying bed methods. First, the $92,000 expenditure for the HP-1000 was less than the cost to build an entirely new structure housing the same type of dewatering equipment for processing sludge at its old 200,000 gpd rate.
At the same time, the new unit costs no more that it would have cost to add a sufficient number of drying beds to handle its old production rate. This solution would have also required hiring more labor. The quality of labor, Delk noted, has improved with the new unit. "It's now mental labor (overseeing the belt press) compared to the physical labor of shoveling dried sludge onto trucks," he said.
In addition, Delk said that the unit has operated maintenance free since its installation. "For our situation, the mobile belt press system was the right decision. It easily paid for itself after it was put on stream."