For countless smaller utilities in the U.S., disposing of ever-increasing amounts of wastewater treatment plant sludge can be a complicated challenge, as the options are limited by space, manpower and budget constraints.
Plants that have hauled liquid sludge for years are finding that the rising cost of this service is becoming prohibitive. Too many times, conventional methods of mechanical dewatering are just too expensive and intensive to be a viable alternative, and the site just is not big enough for sand drying beds.
Such was the case for Resolve Systems, a privately owned utility that serves the Whisperwood community located just off Interstate 12 near Slidell, Louisiana northeast of New Orleans.
The system currently has approximately 1,600 primarily residential connections. When first constructed in 1980, the plant was sized to treat an average daily flow of 100,000 gallons per day. At initial design, hauling of liquid sludge made sense, since this was the easiest and least expensive method for a small plant in this locale.
As connections were added, the plant expanded, using more of the property for process equipment, and consequently generating more sludge. Still, using tanker trucks to haul liquid sludge a few times a month from the digester was the best alternative, so no real thought was given to any other method of solids handling. But, as the old saying goes, the only thing that remains the same is change.
Too much sludge
Additional development in the area and the ensuing connections, as well as the rising cost of hauling brought the utility to a point where looking at other means of dealing with their sludge was prudent. The plant reached a point where it had to haul about five loads every week, with each load consisting of approximately 3,000 gallons.
Even though the plant itself had the capacity to keep up with occurring growth, much of the site had already been used for process equipment.
Furthermore, budget constraints limited the amount of resources available for sludge handling. Manpower restrictions prevented devoting much time to addressing the sludge issue, since personnel not only had the plant to run, but also the entire water distribution system and wastewater collection system. Don Cunningham of Watertech Corp., a manufacturer's sales representative in the area, came up with the solution.
Cunningham, who had been working with the utility for many years and who was familiar with their situation, presented the concept of installing a Wedgewater Filter Bed system to the plant owner Steve Ernst, and the plant superintendent at the time, George Franklin.
The filter bed system would not require near the room of sand drying beds, and could be made to fit in the limited area the plant site had to work with. Operation and maintenance was easier than sand beds, and the installation would cost much less than regular mechanical dewatering.
"The system provides a major added benefit of allowing the operators to keep the mixed liquor levels in the optimal range required to maintain the high effluent quality this plant has historically provided. They've always made really good effluent," Cunningham explained.
Filter bed installation
Gravity Flow Systems Southwest, Inc. worked with all parties to design the filter bed installation to handle the maximum amount of solids to be generated from the plant, and to fit the beds into the very limited area available.
The installation went online in the summer of 2004, and consisted of two separate beds, with a total area of just under 2,200 square feet. Although the bed sizes would require filling and cleaning up to twice per week to handle the sludge load at full plant capacity, the sludge cake was still dry enough to be lifted and transported.
Even then, only four to eight man-hours per week maximum would be spent in operation and maintenance. The bed system was far less demanding than normal mechanical dewatering systems with regard to operation, maintenance and power consumption. The utility was shown how to convert surplus tankage into an additional digester would further decrease solids, without constructing any added infrastructure.
"I recognized that the capacity of the filter bed system could drastically and easily reduce operating costs through efficiently dewatering the sludge," said plant owner Ernst. "The appeal of the solution was not only the ease of the polyurethane drying tile system, but the ability to truck more sludge and much less water. The overall operational and financial performance of my plant really improved."
The savings realized over hauling liquid sludge should pay for the new solids dewatering system in less than five years. The immediate benefits include no longer having the hassle of scheduling trucks multiple times per week, since sludge volume is dramatically decreased. Several beds-worth of dried cake solids fit easily in a roll-off container. Additionally, the utility has the peace of mind that comes with knowing that they are not paying for hauling large quantities of water, as well as realizing that their solids problems are solved for many years to come.
