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Sludge cake management can be a major headache for municipal wastewater treatment plant managers and operators. The sludge cake properties and volumes can be inconsistent. Truck loading can be hit or miss.
Operational expenses can get out of hand with frequent startup and shutdown of the systems and labor costs can skyrocket if the system requires more attention than planned (or overtime).
Not unlike today’s crowded highways, jam-ups and problems in conventional sludge cake transportation, typical storage and loading systems create costly delays and have serious ramifications back up the line.
A new solution
New gravity flow systems featuring hydraulically operated slide gates offer operational improvements.
These systems use the weight and thixatropic qualities of the sludge cake to establish and control the flow of sludge cake into the trucks from the storage hoppers. Metering is accomplished by hydraulically operated MSE Diamond Seal slide gates that control the flow to the trucks as a function of weight. In newer applications, truck loading can be automatically controlled.
Compared to conventional “live bottom” or screw systems, total costs can be reduced by 25% or more and the time required for loadout to the trucks can be cut significantly. The systems are supplied through a strategic alliance between MSE, San Rafael, Calif., and USFilter Asdor Products, Markham, Ontario.
Salt Lake City study
The Central Valley Water Reclamation Facility in Salt Lake City serves 50% of the residents of Salt Lake County and handles 60 million gallons of wastewater each day. It is located on a 158-acre site, with a footprint of 85 acres.
The plant used to operate belt filter presses to dewater sludge and conveyors to transport sludge cake to trucks that hauled it to disposal sites.
Six trucks were required to handle the sludge cake volume of 115 cubic yards a day, and loadout could take up to two hours or more, as the trucks had to be moved periodically to ensure they were loaded evenly. With no means of determining how much sludge cake was loaded into the trucks, drivers had to use their best judgment—a process that resulted in both overloaded and underloaded trucks.
An operator needed to be present during the loadout, and in the winter, truck loading was limited to daytime hours when the trucks could be unloaded immediately to prevent the load from freezing. If for any reason the dewatering facility had to be run for longer periods of time, the plant incurred overtime costs.
Because of these limitations on sludge processing, blending tanks were used as extra capacity for surges.
A new gravity system started up in the fall of 2002 and has operated successfully since then.
The dewatered sludge cake collection and transport system is designed around a PCP pump mounted on a load cell supported frame, with its inlet hopper at the end of the existing collection belt conveyor. As the individual belt presses are brought on and off line, the pump’s control system senses the variations in sludge cake rate and automatically adjusts the pumping rate.
The existing belt conveyor has been reversed so it delivers the sludge cake to the PCP inlet hopper rather than to the old truck loadout bays.
Dewatered sludge cake is distributed to three 160-cubic-yard storage silos. Each silo is mounted on load cells that weigh the contents of the silo and transmit a weight signal to the main control panel. The hydraulically actuated MSE Diamond Seal metering gates rapidly and accurately meter the stored sludge cake into the trucks.
A complete control system ensures even distribution of the sludge cake to the silos; automatic loadout of the trucks to specific, preprogrammed values; and the monitoring and controlling of all the system interlocks.
In the loadout bay, operators or truck drivers have the option to loadout manually, automatically or from the cab of the truck using a remote transmitter. The operators and drivers can see the silo contents and the amount being loaded into the trucks on a digital display.
Once the driver positions the truck and activates the loadout system, the metering gates open to a slow feed position to establish a small flow into the truck. This creates a soft landing for the dewatered sludge cake and minimizes splashing.
The metering gate then moves into an intermediate open position where the main part of the loadout occurs.
When the desired loadout weight approaches, the metering gate returns to the slow feed position until the load-out has been completed.
This final function enables the system to maintain the accuracy of the loadout process by avoiding the possibility of the system overshooting the predetermined weight.
The loadout of a 40,000-lb truck will take approximately 30 to 60 seconds with an accuracy of ±200 lb (± 0.5%).
The district estimated recovery of a capital cost expenditure of $2 million in less than two years, while improving the efficiency of the sludge digestion process and gas production.
On a present worth basis, the potential savings from just the capital equipment expenditures that were being anticipated by the district before the installation of the system are in excess of $25 million over a 20-year period.
The district was able to drop plans for six new belt presses—an estimated savings over a five-year period of $2.5 million.
An additional cost saving has been immediately realized by the ability to reduce the number of trucks from six to three, as well as cutting fuel, maintenance and personnel overtime costs.
Because the plant does not have to operate dewatering activities as a function of the trucking operation, dewatering now runs on a continuous, 24-hour basis. This results in improved digestion.
Gas production and sludge quality have improved and the possibility of upsets to the digestion operation have been minimized.