Savings in the Mix

The Gallatin treatment plant turns to lime stabilization to cut biosolids costs significantly and deliver a beneficial product to local farmers.
Savings in the Mix
The Gallatin plant uses drum screens manufactured by Parkson Corp.

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The Gallatin (Tenn.) Wastewater Treatment

Facility used to pay farmers 2.5 cents per gallon — up to $25,000 per month — to land-apply liquid Class B biosolids. Today, the plant produces a lime-stabilized Class A biosolids product that farmers can take away at no charge — and in the future may purchase by the ton.

It's all the result of a major upgrade that doubled the plant's design capacity to 12 mgd and added a lime stabilization building. Although the biosolids facility went online only last February, the product is already gaining favor with farmers and with local residents who use it on their gardens and lawns.

"Because we're just getting started, right now about 75 percent of our biosolids is going to landfill," says Brandon Traughber, chief wastewater plant operator. "We're just waiting for farmers to say, 'Hey, why go 40 or 50 miles down the road to buy lime to raise my soil pH when I can go to the treatment plant and get a product that does the same thing — and has nutrient value, too?'

"The mayor [Jo Ann Graves] has said we're going to give it away for the first year just to see what the demand will be. If the farmers really want it once they start using it, then we may be able to generate a little revenue. If we can charge, say, $10 per ton, then we can almost recover the cost for the lime kiln dust we have to buy."

Built for the future

Gallatin, county seat of Summer County, is home to about 30,000. The flow to the treatment plant is about 90 percent residential. Inflow and infiltration cause flow spikes in the winter months; the city is investing about $1 million a year in I&I control.

The original plant was built in 1970 and received a major upgrade in 1983. The new facility was built on the same site. "We were right at the old plant's capacity," says Traughber. "It was rated at 5.5 mgd, and our average flows were right around 5.5 to 6 mgd. We had to build for growth in the city."

The old plant had square-tank aeration basins with surface aerators. The new one, online since January 2011, has an oxidation ditch (WesTech) with anoxic and aerated zones. "We're on a big enough receiving stream [the Cumberland River] so that we don't have an ammonia limit yet, but the state and EPA require us to monitor for it on a quarterly basis. So, anticipating that we'll probably have an ammonia limit in the future, we decided to put a system in place to allow for ammonia removal."

Traughber's fully cross-trained operations team includes Bobby Tucker and Gary Henson, wastewater operator II; Wayne Thompson, wastewater operator I; Leslie Gammons, Matt Wilson and Terry Fultz, wastewater attendants; and Kevin Byrd, wastewater assistant.

Digesters in series

On the solids side, the Gallatin team made the move to Class A largely to get beyond the site monitoring requirements associated with Class B material. Some 85,000 gallons per day of activated sludge is wasted from the oxidation ditch at 0.5 to 0.75 percent solids.

The plant has six digesters with a combined 3-million-gallon capacity. Numbers 1, 2 and 3 are kept empty and held in reserve for emergencies. Waste activated sludge goes straight to the Number 4 digester, aerated continuously during a five-day detention time. It then gravity feeds into the Number 5 digester for daily aeration (about 12 hours), followed by settling and decanting.

Thickened material (1.5 to 1.75 percent solids) is then pumped to the Number 6 digester, which essentially serves as a holding tank. Digested solids first pass through an inline Muffin Monster grinder (JWC Environmental) and then to a polymer injection system (Fluid Dynamics). The material is then fed to one of three centrifuges (Andritz Separation), each with 220 gpm capacity, that dewater it to 23 to 24 percent solids.

The resulting cake can be emptied into trailers and sent to landfill or fed into the lime stabilization process, supplied by Alka-Tech.

Making product

"Material comes out of the centrifuge discharges into an auger that sends it over to the lime stabilization building," says Traughber. "It fills a hopper there, and once the hopper gets to a certain level, the system kicks into automatic. It augers biosolids out of the bin and sends it to a mixer that adds lime kiln dust.

"The mixing process is automated — we can adjust the mix by way of the SCADA system (M/R Systems). Currently, the auger pulls out about 600 pounds per minute, to which we add 200 pounds of lime kiln dust. We run the process about six to eight hours a day, generating about 30 tons of material."

The mixture is transferred to four bunkers inside a sheet metal building. There it stands for 72 hours at a pH above 12 to comply with federal standards for Class A material. "We have to make sure the temperature stays above 52 degrees C (126 degrees F) for at least 12 hours," Traughber says.

"We have a temperature probe in each bunker. We put that probe into the pile, and with the SCADA computer we trend that temperature over a 72-hour period. I wanted those temperature probes so that if the state ever had any question about whether we sustained the necessary temperature, we'd be able to pull up the chart and say, 'OK, what date do you want?' " Proper pH is verified by laboratory testing.

Quality material

Heat from the exothermic reaction with the lime drives off substantial moisture, so that the final product contains about 65 percent solids and has the consistency of cornmeal or flour. A small Kubota tractor with a chain-and-flight conveyor is used to load customers' incoming trucks or trailers with material for transport.

It's easy to see why the Gallatin team prefers to maximize lime stabilization in the future. "A load of centrifuged cake that goes to landfill costs us about $900," says Traughber. "If we take that same amount, add about $400 of lime kiln dust and treat it down to Class A, we can save about $500. If we run the process four days a week, we can save $2,000. If we do that over the course of a year, we're saving some real money."

"We'd like to get to 100 percent lime-stabilized biosolids and do away with landfilling — it costs us more, and it doesn't benefit anybody. If we lime stabilize, we save money, and local farmers get the benefit."

And the benefits are substantial for the area's high-clay, low-alkalinity soils. "In this part of the country, everybody has to lime their fields," says Traughber. "They're having to pay $10 to $15 a ton for lime."

Customers now, besides homeowners taking small amounts, are mainly fescue farmers who can apply material between cuttings of hay. Farmers have taken as much as 40 to 45 tons at a time. The material contains on average about 1.0 percent nitrogen and 0.5 percent potassium.

Getting the word out

So far, Gallatin has relied mainly on word-of-mouth marketing, although Gallatin Utilities included a letter about the material in residents' water bills and information is available on the city website.

"We've also have talked with the county agriculture extension office, the local Natural Resources Conservation Service, and the University of Tennessee Extension office," says Traughber. The idea is that advisors in those offices will let farmers know about the products as an alternative to commercial lime.

"It just takes time," he says. "At the time we started making the product, a lot of farmers had just finished liming their fields before they started their spring crops. We just barely missed that window.

"Give us a year to get the word out. I'm hoping that by this time next year, once farmers start talking among themselves, we'll see plenty of demand. A lot of farmers are saying that in fall when the crops come off, that's when they're going to start using it."


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