A highly automated dewatering and transfer facility provides state-of-the-art biosolids processing for the Clark County Water Reclamation District
A new centrifuge dewatering facility is uniquein both design and operation and will help the Clark County (Nev.) Water Reclamation District meet future demands, produce higher-quality biosolids, simplify dewatering and truck loading operations, and reduce operating costs.
The $125 million dewatering project, constructed by MMC, Inc. went online in May 2010 to replace recessed-chamber filter presses, which were reaching their end of life and were operating at capacity. “The district wanted a process that could dewater 24 hours a day, seven days a week, with minimal operator attention,” says Bruce Johnson, project manager for the designer, CH2M HILL. “While the facility is complex to accommodate a variety of operational scenarios, it is actually quite simple to operate.”
At the heart of the new facility for the 150 mgd Las Vegas wastewater treatment plant are eight high-speed, 30-inch-diameter Andritz D7LL centrifuges that work much like the spin cycle on a clothes washer, according to Mark Binney, director of operations.
Water is separated from the solids as the centrifuge spins at 2,600 rpm. The water is returned to the headworks for further treatment, and the solids are stored for landfilling. The project is designed to process an average of 180 dry tons of biosolids per day with a peak capacity of 230 tons. Its unique features include:
• Automation to allow around-the-clock operation with minimal operational staff, and during unmanned shifts.
• Ability to dewater undigested primary and thickened waste activated sludge separately or blended.
• Polymer feed systems that allow for use of two different polymers at the same time, one for primary sludge and one for waste activated sludge.
• A fully automated and enclosed truck facility that loads a truck in about five minutes and minimizes the potential for spills.
• A two-stage odor-control system to minimize odors inside and outside.
Biosolids enter the centrifuges at 3 to 5 percent solids and come out at 24 to 38 percent solids, depending on whether they are separated or blended. “By separating the sludges, it is possible to reduce the amount of phosphorous sent back to the plant,” explains Johnson. “When the sludges are separated, primary sludge can be dewatered to 36 to 38 percent solids and waste activated sludge can be dewatered to 22 to 24 percent solids. When the sludges are blended, it is possible to get 26 to 28 percent solids.”
The old process could achieve about 26 percent solids, Binney notes, but it was much more labor intensive. The sludges were not treated separately, and trucks had to be loaded directly from the presses at the end of each cycle because there was no storage for dewatered cake.
Operators have been able to improve the new system so that the centrifuges can take a higher loading than anticipated — about 5,000 pounds per hour — while reducing polymer use to 7.5 pounds per ton, about half of what was expected.
With treatment plant flows of around 100 mgd, the biosolids facility can meet demand with only two of the eight centrifuges, creating even more flexibility, extending equipment life, and reducing power costs.
In addition to these improvements, says Johnson, “Operators have found that by blending the sludge, they can reduce the odors in the feed storage tanks. That eliminated the need to add ferric chloride and helps increase the loading rate to the centrifuges. At the same time, it was found that only a small amount of ferric chloride, 0.7 gpm, is required in the water returning to the headworks to tie up phosphorous.”
The dewatering facility’s two 376,000-gallon storage tanks, polymer and chemical mixing and feed systems, dewatered biosolids storage, the truck loading facility, and the odor-control system are all housed in a four-story building of contemporary design to blend into the surroundings. One challenge was the site, a mile from the treatment plant. A pump station transfers sludges to the dewatering facility.
Flexible and expandable
The system can operate automatically with some unique measurements, like total suspended solids in the sludge coming into the centrifuges and in the centrate going out. The measurements automatically trigger changes in the polymer feed to optimize the dewatering process even during unmanned operation. “When it comes to creating a good biosolids cake with minimal operator attention, it provides a great deal of flexibility,” says Binney.
Two different polymers can be added at any of four locations to optimize the process. A testing facility helps the staff evaluate new polymers by testing them on only one centrifuge.
Stronger odors are treated through a two-stage process of packed-bed biotowers and organic media biofilters. More diluted odors from the truck loading bays can be treated with the biofilters alone. “It’s very seldom that you smell odors outside the plant, and you smell them only in certain rooms inside the plant, such as in the truck loading bays during loading,” says Binney.
The enclosed truck loading facility is also unique: Johnson describes it as being “as simple and automated as possible.” A driver pulls into the bay, and the doors close automatically. The driver enters maximum load limits for the vehicle and is directed by signal lights to the hopper that is the most full.
The driver starts the loading process with a remote control and watches the truck fill on a monitor. When the load begins to reach the vehicle limit, loading is stopped and the total load amount is displayed. The door opens, and the truck is directed forward through an automatic tire wash to make sure no biosolids are tracked off site.
The improved dewatering process has reduced the volume trucked to the landfill by a local refuse hauler, and it also saves on tipping fees. Landfilling has been found to be the most economical disposition for biosolids in the Las Vegas area. The cost of hauling and landfilling is about $20 per wet ton.
In the future, as landfilling costs increase, it may become more feasible to reuse biosolids, such as for a soil amendment. The dewatering facility was laid out to minimize the impacts of adding a drying or pelletizing facility. The centrifuge feed tanks were designed so that they could be used as digesters for an energy recovery system.
The new dewatering facility provides a sound biosolids management solution today, with flexibility for even better things in the years ahead.