No Less Than the Best

A search for the optimum biosolids solution leads Exeter Township to an indirect thermal drying process that yields a Class A EQ product.
No Less Than the Best

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Costs for landfilling biosolids were escalating, and the staff at Exeter Township (Pa.) Water Quality Control knew it was time for a change.

Land application of liquid or cake product held little appeal to the board of supervisors of the Township of Exeter, owner of the wastewater treatment plant. They insisted that any beneficial use product be of the best possible quality.

Starting in 2005, Dale Miller, one of four operators of the 7.1 mgd (design) activated sludge plant, led a project to explore thermal biosolids drying. Today, the plant operates an indirect thermal dryer that yields a Class A EQ (exceptional quality) product, already finding great favor with local farmers, who pay $10 per ton for it.

The $6 million investment in the drying technology all but eliminated landfilling of biosolids and will be recovered in about 10 years through savings on tipping fees.

Just too costly

The Exeter WQC plant was built in 1968, and its biosolids had been dewatered on belt filter presses and landfilled since 1978.

"In recent years, we had seen a huge increase in landfill costs," says Miller, a plant team member for 15 years. "In 1992, it was $58.60 per ton; in 1996, it was $88.60; at present, it's $96.50. The price of landfilling had shot up dramatically, and that was the driving force behind this project.

"It had been many years since we applied biosolids to farmland. A few of our supervisors were steadfastly against that. They were worried about liability issues. We're in a fairly populated area and there are not very many farms in our township, so most of the product would have to go outside our borders."

There was early consensus that thermal drying was the best alternative, but that left the question of which drying technology to choose. The township hired Entech Engineering of Reading, Pa., to conduct a feasibility study. Meanwhile, a team led by Miller; H. David Miller, assistant plant superintendent; and Doris Heckman, an Entech representative and secretary/grant writer, began doing research. Two township supervisors met with the team periodically for updates.

Road trips

The research involved numerous "field trips," according to Miller: "I was all over the country looking at how different municipalities were doing thermal drying, what kind of equipment they were using, and what they were doing with their end product. I made quite a few trips from 2005 to 2007.

"I looked at a lot of equipment to get an idea of what I felt would work best for us. Our big question was whether to choose a continuous-feed dryer or a batch-feed dryer. We ended up with a batch system, and it seems to be working well."

In 2009, the township settled on drying technology from Fenton Environmental Technologies of Brownwood, Texas. The addition of the dryer dictated a change in the sludge dewatering process, from belt filter presses to two centrifuges (Alfa Laval Model G2-95), each rated at 2,500 pounds per hour.

"We found out from our research that it's much more economical to remove water mechanically than thermally," says Miller. "If you can produce a drier cake before it goes into the dryer, you're saving money all the way around. We did a quick payback analysis, looking at centrifuges as opposed to the belt presses.

"While centrifuges consume more electricity and use more polymer, they also create a much drier cake. We found that we would achieve about a two-year payback by installing the centrifuges and removing the belt filter presses." The centrifuges produce cake at 23 to 26 percent solids, versus 13 to 15 percent for the belt filter presses.

Getting it done

Entech handled project design. Construction of the dryer system began in February 2010 with Miller as construction manager, assisted by Entech personnel.

Ronca Construction of Allentown, Pa., built the facility. The Exeter WQC team began operating it in June 2011 after Fenton Environmental ran a three-month testing and shakedown operation. The process has a footprint of about 3,300 square feet. It's housed in an existing building with a small addition.

The process dries a mix of primary sludge and gravity-thickened waste- activated sludge. Both sludges are fed to two anaerobic digesters. Digested material is pumped to a 230,000-gallon holding tank that also receives liquid aerobically digested sludges from some neighboring municipalities.

From the holding tank, the material goes straight to the centrifuges, where polymer is added. Centrifuged cake is conveyed to a 75-cubic-yard hopper. To start drying of a batch of material, a gate opens, and a cake pump activates, augering material into a hopper at the front of the dryer. From there, material is pumped to the drying chamber. Each batch contains about 12 wet tons of cake, which yields just over two tons of dried material — a five-to-one reduction.

Drying takes about three hours, after which the product is discharged into a surge bin. From there, a blower delivers the granular biosolids to a storage silo, which holds about 40 dry tons.

Indirect heating

The dryer is fueled by natural gas. The drying chamber consists of a large drum. "It's an indirect dryer," says Miller. "It uses a thermal fluid, so there
is no direct flame-to-biosolids contact. An auger inside the drum rotates
back and forth, turning one way for 75 seconds, then stopping and reversing. It keeps moving the material back and forth inside this drum."

The thermal fluid, at about 390 degrees F, circulates through the outside of the drum and through the auger itself. Sensors measure the temperature of the biosolids, and when the material reaches 275 degrees F, it is discharged.

"The dryer opens up, in almost the same way as a clothes dryer at home," says Miller. "The auger turns in a clockwise direction and pushes the biosolids out. Then a screw conveyor delivers it to the surge bin. It takes about 10 minutes for the dryer to unload. If we're going to run another batch, the conveyor shuts off, the door closes, and the whole process repeats itself."

The end product, at 95 percent solids, is light brown and granular, with the consistency of coarse sand. A de-dusting agent (Dustrol 3003 made by ArrMaz Custom Chemicals) comes out of the silo as the product and goes into users' trucks.

The plant team moved proactively to avoid odor issues involving off-gas from the dryer. "A lot of the plants I had visited bubbled the off-gas up through their aeration tanks," say Miller. "That pretty well scrubbed the odors from it.

"At our plant, the aeration tank is a pretty long distance away from the dryer, but the sludge holding tank is right next to the dryer building. We decided to blow the off-gas into there for scrubbing. The tank already had a small odor-control unit on it. The process has worked quite well."

Sharing duties

Primary responsibility for the drying operation falls to Miller, along with operators James Bingaman, Aaron Franckowiak and Edward Pribish. They are overseen by H. David Miller and Paul Herb, treatment plant superintendent. The Exeter WQC staff also includes maintenance mechanics Dave Welgoss, Mark Maciejewski and Kenny Templin; assistant operators David Seltzer, Josh Koontz and John Dean; and collection system operators Pavel Babich and Kevin Dunn.

The drying process is fully automated — no new staff had to be hired for it. "You basically hit start and watch it from then on," says Miller. "It loads, dries, discharges, fills the silo and shuts down. Other than supervision, when everything is working properly, that's about all it is. We've had a few hiccups here and there, but we figured them out.

"Overall, in running for more than a year, our biggest problem has been power failures, which have nothing to do with the equipment manufacturer. We're pretty happy with the drying equipment, and the centrifuges as well."

To market, to market

The plant landfilled the product for a short time before receiving the state Department of Environmental Protection permits needed for land application. Product was distributed for free until the township received a state fertilizer resale permit, which required a nutrient analysis (the product contains 4 percent nitrogen, 5 percent phosphorus and no potassium).

Marketing to date has been simple and inexpensive. Material Matters, a consulting firm in Elizabethtown, Pa., helped the township develop a marketing strategy while assistant superintendent Miller developed a product logo. The product is simply called "Exeter Biosolids," since it could also be used as an energy source.

An ad in the Lancaster Farming magazine offering the product at $10 per ton brought numerous phone calls from farmers. Initially one farmer used most of the material, with a second farmer as a backup. The storage silo has an offload screw that can load dump trucks, tractor-trailers and other vehicles.

"We're eager to hear what kind of success our customer has had," says Miller. "It's a whole lot less expensive than commercial fertilizer — that's the reason he took it. We also have some turf farms in the area that are interested. I know the material works really well on grass. We've put some on patches of grass around the treatment plant, and we've found that it makes grass grow like crazy."

Community friendly

To introduce the drying process to the community, the plant held an open house and ribbon cutting on Sept. 14, 2011. It attracted 120 people. Last May, 220 members of the Eastern Section of the Pennsylvania Water Environment Association toured the facility.

Part of the idea behind the project, which received a $1 million grant from the DEP, was to extend its benefits to other communities by becoming a regional facility. "We're one of the bigger treatment plants in our area," says Miller. "We said to our neighbors, 'Here's a place you can bring your liquid sludge and we can process it, dry it and use it for land application.' We're trying to grow that business.

"We tried to get other communities to be involved before we went ahead with the project, but that was difficult because we didn't have enough concrete information. Their first question was, 'How much is it going to cost?' They weren't going to commit without knowing that, and I don't blame them one bit. It's evolving slowly."

Exeter WQC now accepts sludges from 10 other treatment plants, amounting to about 75,000 gallons per week, or about 20 percent of the total volume processed. "All in all, I'm very proud of our process," says Miller. "I'm happy that it has worked out so well."


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