The Municipal Utilities Board of the City of Albertville evaluated numerous options before settling on a long-term biosolids technology.

Between landfilling 30 years ago and indirect closed-loop thermal drying today, the utility tried land application of Class B biosolids and explored thermophilic digestion, lime stabilization and various direct drying technologies.

Elden Chumley, general manager and CEO, calls the Alabama utility’s BIO-SCRU drying system (Therma-Flite) a clean and economical process that yields Class A biosolids with a variety of potential applications.

“Nearby landfills were no longer in operation and land application sites were limited,” says Chumley. “We had to start looking for the next phase. We asked, ‘As the city grows, what is going to be our option?’ We looked at drying technologies for reductions in volume, handling costs and regulatory restrictions. Today, our end product is 90 percent dry and meets the Exceptional Quality requirements. It has done very well in the marketplace. Further treatment of the material makes it easier to find end uses for it.”

The process yields about 104 dry tons per month. Most of the material is applied to hay fields; some is used for landscape fertilization.

Quality effluent

Albertville, in northeast Alabama, has a diverse industrial base that includes poultry processing, automotive and paper manufacturing, plastics extrusion and wood products. It is also, in Chumley’s words, “the fire hydrant capital of the world” — Mueller Systems operates a large plant there.

Albertville’s 11.5 mgd (design) wastewater treatment plant uses a conventional activated sludge process to treat an average 7 mgd flow that is about half industrial. Effluent discharges to Turkey Creek, which ultimately flows into Short Creek (a Tennessee River tributary) upstream of Albertville’s drinking water intake.

For that reason, and because Turkey Creek is classified as a fish and wildlife stream, effluent quality is paramount. The plant consistently meets permit limits of 5 mg/L BOD, 30 mg/L TSS and 1.2 mg/L ammonia. Final effluent is disinfected with chlorine and dechlorinated with sulfur dioxide.

Primary and waste activated sludges are anaerobically digested and dewatered to 16 to 17 percent solids on a belt press (Alfa Laval Ashbrook Simon-Hartley) before thermal drying.

Process of elimination

Albertville’s biosolids journey goes back to the mid-1980s, when sludges were dewatered on drying beds, manually removed and hauled to a city-owned landfill. In the early 1990s, that landfill was converted to construction and demolition waste only. The utility had to seek alternatives because private landfills charged tipping fees and were farther away, increasing hauling costs.

“We decided in 1994 to land-apply Class B biosolids,” says Chumley. “We met all the Class B qualifications because of the heating process in the digesters. We began applying biosolids to hay sites for cattle operations.” A third party hauled and applied the dewatered cake.

Land application worked reasonably well, but wasn’t ideal. Sites had to be permitted. Farmers had to keep livestock off treated ground for specified periods. Because the closest suitable farms were 13 to 20 miles from the treatment plant, trucking costs were significant. “When fuel costs got up to $4 a gallon at various times, we were spending $160,000 to $200,000 a year in handling costs,” Chumley says.

There were also seasonal issues. In wet conditions, when application sites were not available, material had to be landfilled and tipping fees paid. “Eventually we built a covered storage area so we could hold product until farmland was available,” says Chumley.

Stepping up in class

In the mid to late 1990s, Albertville began to investigate producing Exceptional Quality material. Thermophilic digestion turned out not to be feasible because the existing digesters were not designed for sustained higher temperatures — new digesters would need to be built.

“We studied dryers and actually took bids on one,” Chumley says. “The capital costs at that time were prohibitive. Between that and the price of natural gas to run the dryer, it didn’t make sense versus what we had been doing. Instead of $200,000 a year in handling costs we were facing up to half a million dollars for operations, plus the capital cost. Therefore, we had to shelve that project.”

A study of lime stabilization in the early 2000s found that the treatment plant lacked adequate space, and the issues of limited farmland and high hauling cost remained. The search ultimately led to the drying technology in place today. “We kept looking for a dryer we could run economically, and we found it in the Therma-Flite dryer,” says Chumley. Krebs Engineering of Birmingham was the consultant on the project; the dryer went online in 2014.

Automated process

The BIO-SCRU dryer system is fully automated; all operating parameters are regulated by a programmable logic control (PLC) system. The technology is designed to dry digested as well as undigested primary and waste activated sludges, as well as mixtures of those types.

The heat for drying comes from thermal fluid circulated through twin screw rotors, flighting and the outer jacket of the drying chamber in a closed-loop path. Drying proceeds under slight negative pressure in a sealed chamber. A lock box and cooling screw maintain the seal at the discharge. An integrated scrubber/condenser removes vaporized water and captures particulate. The system’s self-clearing design continuously removes buildup from the heat-transfer surfaces. A series of cutters keep large clumps from building up and passing through the system.

“We liked the thermal fluid concept,” says Dale Williams, wastewater plant manager.

“Once we heat the fluid, it recirculates, and we just use energy to maintain the temperature. That makes the dryer more efficient. It also has a smaller footprint so that we could put it in an existing space.”

SEEPEX pumps deliver 17 percent solids cake into hoppers from which the drying unit is fed. The thermal fluid circulates at about 540 degrees F; the product itself is heated to about 220 degrees F for a detention time of three to four hours, depending on how much feedstock material is being processed. At discharge, the material passes into a water-jacketed cooling screw that lowers the temperature to about 100 degrees F, and then is air-cooled in two screw conveyors (SPIRAC).

Targeting efficiency

The dryer can operate in a batch mode, but the Albertville team prefers to run it continuously for two to three days at a time to avoid the inefficiency that goes with repeatedly starting up and cooling down. Operator interaction is limited. “The biggest thing is to maintain a constant feed rate and constant temperatures to ensure a 90 percent solids product,” says Williams.

The boiler (The Fulton Cos.) inside the dryer system can burn both pipeline natural gas and digester biogas; the Albertville team uses both and is looking to expand the biogas share.

“We don’t have enough biogas to fully run the dryer,” says John Wright, wastewater superintendent.

“At present, we use about 70 percent natural gas and 30 percent biogas.”

They are exploring ways to feed the boiler a blend of natural gas and biogas. That would eliminate the short production delays that go with switching from one fuel to the other.

Meanwhile, they have added two methane storage tanks and now have three. “When we had only one tank, we would build up a fuel supply and use it, and then we had to wait to rebuild the supply from the digesters before we fed biogas again,” Wright says. “The additional storage tanks will give us longer runtimes on biogas. Now we can fill one tank while we’re pulling out of another.”

Appealing product

The dryer’s end product has the consistency of coarse sand and is dark gray. Demand has been immediate and strong. It’s sold in large bags that hold nearly a ton of material; the price is $15 per bag. The primary customer is a farmer who picks up the product at the treatment plant and applies it to hay land. This has eliminated the treatment plant’s hauling costs.

“We have a verbal commitment with him for a year,” Chumley says. “This is our first year of full dryer operation, and we’re in a trial and error mode with marketing. The product has a high nitrogen content. The farmers in our area understand its value. We’re not having any difficulty selling it. We’ve talked to other farmers who want to apply it straight to farmland. We’ve talked to potential customers who want to blend it with other fertilizers. We’re not sure what direction we’ll go in the long term, but it’s great to have a number of options.

“We’re evaluating the price we sell the product for. I believe the people we’re dealing with now, and others who are interested, would be willing to pay a little more. After we get this first year under our belt, we’ll have more information, and our prices may change.”

Substantial savings

The dryer was part of a $6 million plant upgrade project that also included a PISTA Grit system (Smith & Loveless) at the headworks, an improved digester mixing system, and the two new biogas storage tanks and low-pressure gas delivery system.

Chumley estimates payback on the dryer itself at about seven years, counting elimination of $200,000 in annual hauling costs and revenue from product sales. The dryer was designed to handle Albertville’s biosolids volume while running at half capacity. That means potential exists to dry material from neighboring communities.

Chumley expressed gratitude to Therma-Flite representatives who helped with installation and startup, and to plant team members. “Therma-Flite had staff members here for the first 45 days,” he says. “Our people were right there beside them and received valuable training.

“Once our staff received training from the dryer technical team, they rose to the occasion. The production of Class A biosolids adds some extra work to our process. The team really stepped up to the plate because it was the best thing for the future of our community.”

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It’s In the Bag

There are many ways to load and transport dried biosolids products. The Municipal Utilities Board of the City of Albertville uses bags — great big ones.

“We’re using what we call super sacks,” says John Wright, wastewater superintendent.

“They’re breathable bags often used to haul sand, gravel and other products. Each bag holds about 1,600 to 1,800 pounds of our material.”

The bags, purchased from Joe M. Almand in Atlanta, Georgia, have straps at the four corners that can be used for handling. The treatment plant team engineered a hookup and auger system to fill the bags. “We’re set up with two chutes,” says Wright. “When one bag is full, we can pull a chain lever and switch over to start filling the other bag, while the full bag is removed and replaced with an empty bag for another run. We change bags about every two hours. We use a forklift to load the filled bags onto truck trailers for hauling.”

It’s possible to blow dried biosolids into storage silos or load it directly into trucks. Albertville chose to use the bags largely for dust control: The plant is not equipped to spray the product with oil or water for dust mitigation.

The bags do pose logistical issues for farmers at the land application sites: The bags have to be opened and the product transferred to broadcast spreaders. “In the future, we may come up with a different mechanism where we could load directly into a spreader,” says Wright. “As of now, that’s a capital investment we’re not prepared to make.”