Dehydration System Works Well for Smaller Plants
A new biosolids drying process using hot oil is being put to the test at several wastewater treatment plants in the United States. The sludge dehydration system may be an economical solution for smaller plants to produce Class A biosolids and reduce their landfill or incineration costs.
The SludgeMaster® IRC natural-gas-powered dryer, created by U.S. Filter/Davis, dehydrates sludge with heat, producing a Class A product which complies with Federal Regulations 40CFR Part 503, which establishes pathogen and vector attraction standards for waste material.
"We have several projects working that plan to use the dryer. I believe that its going to be a good unit for smaller treatment plants, and possibly some mid-size installations," said Gary Shimp of Black & Veatch.
In the system, wet sludge is fed from a feed hopper into the dehydration chamber by a transfer screw. A flight auger and the dehydration cylinder move in opposite directions, agitating and turning the biosolids to achieve maximum contact between the material and the surfaces of the chamber, which is heated by burners. To increase the chambers efficiency, oil heated to 450 degrees F circulates through the auger and is recirculated through the system. An expansion tank provides a buffer.
The dehydration chamber is divided into three temperature zones to control the drying process. The first zone, the temperature is maintained to 650 degrees F. The temperature drops to 600 degrees F in the second zone and approximately 550 degrees F in the third.
The system is modular in design to allow for expansion. The largest model in the SludgeMaster line, the M500, is 27 feet long, 13 feet wide and 10 feet six inches high. It can process 50 wet tons in 24 hours, generating 5,850 BTUs/hr. and consuming 55 kW/hr. The smallest unit in the line can handle 8 wet tons per day and consumes 33 kW/hr.
The system is fully enclosed to control dust and odors associated with the drying process.
"We pull a vacuum on the tube that the solids are in and it draws off the vapor and gases. That goes to a condenser and separates the vapors back into a liquid," said Dave Lowry, Product Manager of U.S. Filter/Davis. The gases that are left might be transported to an aeration basin in the plant. "It is basically a clean exhaust, which is not the case youre going to have with a direct fire heater, where youve got the burner blowing right at the sludge or in the same air mass as the sludge," Lowry said.
The system is designed to help operators avoid manual handling of the biosolids during dryer operation.
"You put the sludge in the hopper in one end and you convey it into a silo on the other. Nobody has to handle it or touch it, you dont move it with a backhoe or front-end loader," Lowry said.
Black & Veatch is involved in five installations of the unit in the U.S. One of the planned installations is in Pensacola, FL, where the ultimate design is for four dryers. Three are to be installed in the first stage. The facility is a 25 mgd plant, the largest known plant with the SludgeMaster on line, Shimp said.
"This will be the first multi-dryer plant that Im aware of. Were having to develop some of the materials handling systems to accompany that, given that we have more than a single dryer. The dryer is designed as a modular unit, but when we have three of them, we have to design systems that enable the three units to work together."
Once the dryers are in place and working, the plant will be able to stop or reduce incineration of its biosolids.
New Jersey Installation
The 5 mgd Mount Holly Municipal Utilities Authority, N.J., treatment plant produces Class A biosolids with a Model 500 Sludgemaster installed in late 1997. The dryer wasnt fully functional at first.
New Jersey Installation
"Some of the bugs that showed up werent really worked out until April or May," said Executive Director William G. Dunn.
New Jersey Installation
U.S. Filter/Davis claims the unit can typically reduce 2,000 lbs. of wet sludge at 18 percent solids into 500 lbs. of dry solids at 90 percent. Mt. Hollys results are similar. It turns 28 to 30 tons of pressed sludge into 7 to 8 tons of finished product, Dunn estimates.
New Jersey Installation
"We put in 20 percent wet material in the head of the system, and it comes out in the mid-to-low 90 percent dry from the other end," Dunn said. With disposal costs between $40 and $50 per ton, that translates into a savings of $800 to $1000 per batch. He hopes to make a deal with a fertilizer company to sell the dried material, further reducing the disposal costs.
New Jersey Installation
The plant cannot indiscriminately give away the treated biosolids because the state of New Jersey Department of Environmental Protection requires that only licensed entities receive the biosolids.
New Jersey Installation
"U.S. Filter still has some computer work and some interconnects to do. The unit works rather well. Weve had some nuts and bolts problems, but when youre only the third machine like it thats been built, that to some degree is expected," he said.
New Jersey Installation
"Were comfortable with it, recognizing that we still dont have the interfaces that we need."
New Jersey Installation
Wastewater treatment plants in New Jersey have to keep a continuous record of the temperature of the material in the process discharge, so that they comply with Jerseys regulation that it is at least 90 percent dry, Dunn said. The plant could also benefit from a monitoring system that records a wide range of events for early troubleshooting or later diagnosis of a problem.
Odors
Mt. Hollys dryer has been generating some odor that cant be handled by the scrubber. Dunn describes it as a burnt odor
Odors
"Weve addressed that by doing more scrubbing area. It is something that I think that U.S. Filter needs to improve - the system for taking the off-gases from the sludge."
Odors
The plant installed the scrubber in an existing building, retrofitting to make space as needed. But the tight space may have caused the scrubber to be less effective at removing odors, Dunn said.
Odors
"This dryer, perhaps better than some, has a lot of features that minimize odor, but there can be potential for odor with sludge drying, theres no doubt about that," Shimp said. The nature of the biosolids product that is going in will impact the odors encountered. "This is an enclosed drying system with a fairly small gas stream that would have to be treated for odors," he added.
Odors
Mt. Hollys scrubber, which uses water, also is releasing some smoke into the air. It does not appear to contain any hazardous substances, but the operators would like to reduce the visual impact. To help the situation, the exhaust is being discharged to another tank containing carbon.
Odors
Black & Veatch is designing a special scrubber system for the Pensacola plant because the design includes three dryers.
Odors
"For the instance of Pensacola, where we are drying a more odorous sludge to begin with, we are going to begin using thermal destruction methods for treating the off-gases," Shimp said.
Texture
The dryer produces a granular rather than a pelletized product, which may not be what some operators are expecting.
Texture
"I think customers for the dryer need to be aware that not all dried products are created equal in terms of handleability and their marketability," Shimp said.
Texture
"This is a fairly simple dryer and the product out of the dryer tends to be granular - sometimes a little on the floury side if the material is overdried, and for some users that might not meet their goals. But for installations where they are prepared to produce a granular product, I think this dryer is well-conceived and should be affordable for mid-size plants."
Texture
Some people might also be concerned with the dusting potential from using the material.
Texture
"The user is going to have to use methods of handling and applying the material that are consistent with the product," Shimp said. "If you tried to broadcast this, you would get a big cloud of dust. So you need to use appropriate spreading equipment, which would be drop-spreaders."