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For utilities, disposing of 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 too expensive and intensive to be a viable alternative, and some sites are not big enough for any sand-drying beds.
Such was the case for Resolve Systems, a privately owned utility that serves the Whisperwood community located northeast of New Orleans near Slidell, La.
The system has approximately 1,600 connections, most of which are residential. When first constructed in 1970, the plant was sized to treat an average daily flow of 100,000 gal per day (gpd).
At initial design, hauling of liquid sludge made sense, because this was the simplest 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 sludge from the digester a few times a month was the best alternative, so no real thought was given to using any other method of solids handling.
But as the saying goes, the only thing that remains the same is change.
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 gal.
Even though the plant itself had the capacity to keep up with recurring 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, because personnel had to run not only the plant, but the entire water distribution and wastewater collection systems as well.
Don Cunningham of Water Tech Corp., a manufacturer’s sales representative in the area, came up with a solution. Cunningham, who had been working with the Resolve Systems utility for many years and was familiar with their situation, presented the concept of installing a Wedgewater Filter Bed system to the chief operator at the time, George Franklin.
“The system would not require the room of sand drying beds, and could fit in the limited area the plant site had to work with,” Cunningham said. “Operation and maintenance was easier than sand beds, and the installation would cost much less than regular mechanical dewatering. Franklin liked the idea, and the utility decided to install the beds.”
“The system provides a major 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.
All parties agreed with the idea, and the decision was made to move forward with the dewatering bed system.
Gravity Flow Systems Southwest, Inc. worked with the utility to design the filter bed installation. The goal was to handle the most plant-generated solids while fitting the beds into the limited available area.
The installation went online in the summer of 2004, and consisted of two separate beds, with a total area of just under 2,200 sq ft.
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 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 manufacturer was even able to show the utility how converting unused tankage into an additional digester would further decrease solids, without constructing any added infrastructure. The filter bed system addressed all the challenges the utility had faced.
“The appeal of the solution included not only the ease of the polyurethane drying tile system, but the ability to truck more sludge and much less water,” Franklin said. “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. An immediate benefit is not having the hassle of scheduling trucks multiple times a week because 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.
“The system offers the customer quality construction, ease of integration into many existing plant systems and exceptional performance,” Cunningham said. “Many customers are so pleased with the Wedgewater bed performance that they choose to expand their sludge drying capability with additional beds.”