The Athens Utilities Board is in a position where any product marketer would like to be: demand that exceeds supply.

Since 2006, the agency in Athens, Tennessee, has marketed its AthenaGro Class A dried biosolids to farmers, landscapers, and homeowners as a state-registered fertilizer. In 2017, the utility’s Oostanaula Creek Wastewater Treatment Plant produced 533 dry tons of the material using its 13-year-old thermal drying process.

“We have a waiting list of customers,” says Greg Hayes, facility manager. “A lot of times we don’t make it through the whole list in one year. We don’t have enough product.” The utility delivers bulk product to farmers at a cost of $50 per ton. In addition, homeowners can purchase AthenaGro at $5 per 40-pound bag.

It’s in line with the utility’s policy to “develop and implement the most economically sound and environmentally beneficial biosolids recycling options” for solids generated at the Oostanaula Creek and North Mouse Creek treatment plants. The utility is developing an environmental management system based on the National Biosolids Partnership’s Code of Good Practice.

The biosolids program received a 2017 Beneficial Use of Biosolids Award from the Kentucky-Tennessee Water Environment Association, and the 2017 Government Recycler of the Year award from Tennessee Recycling Coalition.

Legacy of excellence

Awards and recognition are nothing new to the Athens Utilities Board. The North Mouse Creek treatment plant has received 21 consecutive Operational Excellence Awards from the Kentucky-Tennessee Water Environment Association, and the Oostanaula Creek facility has earned that honor 11 consecutive times.

The city of Athens is divided into two drainage areas. For many years, wastewater from the North Mouse Creek drainage area was pumped to the Oostanaula Creek plant. The North Mouse Creek plant (1.2 mgd design, 1 mgd average) was built in 1995. Waste activated sludge from that facility is trucked to the Oostanaula Creek plant for processing.

Both plants are oxidation ditch extended aeration facilities with UV disinfection. The Oostanaula Creek plant (6 mgd design, 2.3 mgd average) has tertiary filters. That plant was built in the 1960s and upgraded most recently in 2005. Until the upgrade, Hayes notes, Oostanaula Creek “was not an award-winning plant.” 

That’s because the plant received irregular flows of high-strength wastewater from a dairy, causing capacity issues and frequently upsetting the anaerobic digesters. The oxidation ditch and new aerobic digesters cured the problem, and so did the dairy’s cooperation. “They are a much better customer now,” Hayes says. “They equalize their flow, they pretreat, and they monitor their wastewater.”

Efficient solids handling

Influent to the Oostanaula Creek plant first passes through a coarse bar screen (Hycor). Four 100 hp influent pumps (Patterson Pump) then deliver it to two fine screens (Parkson Corp.). A Parshall flume directs the flow to a grit and grease removal system (Schreiber).

The water then is treated in a pair of 3-million-gallon three-ring oxidation ditches with a jet aeration system (Fluidyne) fed by turbo blowers (Turblex). Then it’s on to four 700,000-gallon clarifiers (WesTech Engineering), the cloth disc tertiary filters (Aqua-Aerobic Systems), and the UV system (TrojanUV) before discharge to Oostanaula Creek.

Thickened waste activated sludge from both treatment plants is delivered to one of two aerobic digesters aerated by three positive displacement blowers (Ventuse). The digested material passes over a 2-meter belt filter press that yields cake at 13 to 20 percent solids.

Since the late 1980s, the Oostanaula Creek plant had used a lime stabilization process and sold the product to farmers. That worked well, according to Russell Coleman, lead operator, but it had a large footprint. “We had to quit that process to make room for the new construction in 2005,” he says. “At that point we were going straight to the landfill with dewatered material off the press.”

A better way

That lasted only until the plant upgrade was complete and a natural-gas-fueled Fenton SludgeMASTER dryer (RDP Technologies) was commissioned in 2006. Cake coming off the press falls into a hopper and is delivered to the dryer, which heats the material indirectly by way of heat transfer oil at about 450 degrees F inside piping and a rotary mixing system. “Biosolids come out in the upper-90 percents solids,” Coleman says.

The Athens team chose the technology after considerable research. “We visited a number of wastewater plants that were using dryers — eastern Tennessee, northern Georgia, South Carolina,” Hayes says. “We made several visits to look at different equipment.”

Coleman recalls, “We like both the operation and the maintenance of the batch dryer process. That’s why we decided on this brand. It seemed simpler. We fill it up with material, run a load, and it discharges. It’s automated with a lot of safety backup. Once we get the temperature up, the PLC runs the show. It loads for about an hour and dries for a little over two hours. Then it discharges and we repeat the process.”

All the operators rotate on duty running the dewatering press and the dryer. Each batch yields about 1 dry ton of material. The dryer typically operates three days a week. Loads are delivered to farms in a utility-owned dump truck. Farmers can rent a ground-driven spreader for $50 to apply the material, used mainly on hay and pasture land.

Bagged material is sold directly to customers. “We sell it from our main lobby, where people walk in to pay their bills,” says Craig Brymer, superintendent of water and wastewater. “We have a pallet there, and they pay for it at the cashier. We load it up for them and they take it home and use it.”

A registered product

AthenaGro was registered as a fertilizer by Tennessee Department of Agriculture. Coleman says, “We guarantee a nutrient analysis of 3-3-0.” (That’s 3 percent nitrogen, 3 percent phosphorus and zero potassium.) “But in the last quarter of 2017, for total nitrogen, we had 7.23 percent, and phosphorus was 6.52 percent. So for nitrogen, users are getting 145 pounds per ton, and for phosphorus, 130 pounds per ton.”

Coleman, who uses the product on his own deer farm, notes that farmers appreciate the fertilizer’s slow nutrient release: “It takes a little bit to get them to turn away from the co-op products, but the ones who use it and see the benefits, those are the ones who keep coming back. Especially when fertilizer prices go up, our waiting list gets longer.”

Marketing wasn’t difficult. “From experience with the lime stabilization process, we knew we could convince the local farmers if we let a few of them try it and relied on word-of-mouth,” Hayes says. That has mostly proven to be true, although the utility at first did some media advertising and other promotion.

Brymer recalls, “Wayne Scarbrough, our communications person, helped us put out a few press releases about the product. We had signs made and put them in our drive-thru. We would take out ads in the newspaper showing our label. It was just regular marketing like you would see for any other product. We did it for the bags more than for the bulk material.”

Hayes adds, “It is so well-accepted that at this point we don’t do very much promotion. We could probably increase our bag sales if we did a little more of that, but we always have more demand than supply for the bulk product. Our overall goal is to move the product out, so we don’t want to spend more money on it than we need to.” Total revenue from product sales is about $25,000 per year.

Simple to maintain

Meanwhile, operators have found the drying technology simple to maintain and keep online. “The main concern is to make sure no foreign objects are introduced to the process,” Coleman says. “A loose bolt, for example, would wreak havoc inside the dryer.”

“On the maintenance side, there are some large and expensive bearings that would take a long time to repair. We have an automatic lubrication system, but in addition, every six months or every year at the minimum, we pack those bearings with grease.”

Brymer notes that the dryer’s operating cost is comparable to the expense of landfilling biosolids. The entire dewatering and drying operation cost $83,800 in 2017, including natural gas, labor, and electricity. The gas cost was $55,500.

In the end, Coleman credits the operations team for the success of the drying technology and the biosolids program as a whole. The team includes Troy McDermott, operator II; Jody Derrick, John Hurst, Stan Young, Boone Walker and Josh Dove, all Grade IV operators; Scott King, Grade II operator; Frank Jarvis, biosolids transportation and preventive maintenance specialist; and Randy Millsaps and Jeremy Parsons, maintenance specialists. 

“They’re the ones who make this work,” Coleman says. “It’s a team all the way through. Without the money from upper management, we couldn’t do what we do. But even if we had all the money in the world, without the right operators who have their heart and soul in it, we would never get a truly good product.”

Driving down costs

A surplus of digester space helps the team at the Athens Utilities Board hold down the cost of producing AthenaGro fertilizer. The Oostanaula Creek Wastewater Treatment Plant has a pair of 1-million-gallon aerobic digesters.

“We usually only run one,” says Russell Coleman, lead operator. “That way if we have a problem with any of the biosolids portion of our process, we have plenty of room for temporary storage and we don’t have to go to landfill.”

Greg Hayes, facility manager, adds, “The second digester also gives us the ability to take an interruptible rate from our natural gas utility.” If the utility interrupts the gas supply, taking the dryer out of service (and that has happened twice during the winter heating season), the second digester can store material until service is restored.

Before the interruptible rate took effect, it cost about $250 per ton to dry the material. Now it costs $157. Coleman notes that some of the savings come from operating the belt filter press efficiently and thus sending higher-solids material to the dryer: “Every little bit we can eke out of the press really cuts the cost of drying down.”