Garbage In, Resource Out

Landfill gas powers a dryer that produces Class AA pellets using biosolids from three treatment plants in Manatee County, Fla.

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Manatee County borders the Gulf of Mexico in southwest Florida. Named for the sea cows that swim in its waters, it’s a laid-back tourist destination with more than 150 miles of beaches. The county hasn’t grown as fast as other parts of Florida, but that doesn’t mean it isn’t progressive, especially in handling solids from three water reclamation facilities (WRFs).

 

“We saw the writing on the wall: Class B solids are a thing of the past,” says Christian (Chris) Collins, who serves as both the biosolids supervisor and the MARS (Manatee Agricultural Reuse System) coordinator for the county.

 

The writing came in the form of legal action. “There’s a push Florida-wide to stop land spreading of Class B solids,” Collins says. After the lawsuits started, land application sites began to disappear, and the county looked at various systems to upgrade the quality of its biosolids. Using criteria such as reputable long-term service, experience in the field, and solid safety records, the county chose the Andritz Separation Inc. drum drying system to produce Class AA biosolids pellets.

 

“The Andritz dryer and pelletizer is basically a drum dryer with a heat source at our end,” says Collins. In this case, the heat source is landfill methane, which fuels the burner that heats the dryer to produce nitrogen-rich pellets.

 

Come together

The county’s biosolids come from the North, Southeast, and Southwest WRFs. Solids are transported via pipeline or truck to the residuals management facility (RMF) at the Southeast WRF, where they are dried and pelletized.

 

The three reclamation facilities use activated sludge systems. At the Southeast WRF, the 11.0-mgd flow is split between three anoxic basins and three aeration basins, followed by four clarifiers. Waste activated sludge from the clarifiers is pumped to a gravity belt thickener and then on to two 1-million-gallon sludge storage tanks, where it is aerated using coarse-bubble diffusers. Stabilized solids are dewatered using two belt filter presses.

 

At the North WRF, the 5.4-mgd flow is treated with two Carrousel oxidation ditch treatment units, each with a 0.6-million-gallon anoxic basin and a 3.1-million-gallon aeration basin, followed by two clarifiers. Waste activated sludge from the clarifiers is pumped to two dissolved-air flotation thickeners, two aerobic digesters, and then on to three belt filter presses.

The Southwest WRF treats its 18 mgd using four primary clarifiers and four aeration basins, two sized at 1 million gallons and two at 0.75 million gallons, followed by four secondary clarifiers. Biosolids are managed first by two gravity thickeners with dissolved-air flotation, then on to six 2-meter belt filter presses.

 

The Ashbrook Simon-Hartley (Klampress-85) belt filter presses used at each WRF produce material at about 16 percent solids. Solids from the North and Southwest facilities are pumped straight from the belt filter presses into 30-cubic-yard trucks. A single contractor hauls material from both plants, each about 10 miles from the RMF.

 

On arrival at the Southeast plant, solids are emptied into bins outside the dryer building. Solids generated at the Southeast WRF are pumped directly to a bin inside the dryer building. Solids from the outside bins are pumped to the bin inside the dryer building, where the pelletizing process begins.

 

Since the inception of pelletizing, operators at the three reclamation facilities have had to pay more attention to biosolids consistency. “It has surely created more awareness,” says Collins. “The dryer works better when processing cake in the 15 to 20 percent solids range. In the past, the plants wanted to make the cake as dry as possible because it saved on hauling costs.”

 

Into the drum

From the bin, material is conveyed to the dryer drum, where air heated in the burner dries the solids. As the material tumbles about the screened drum, pellets form. Separate consecutive screens sort them by size. Pellets that are too small go back to the beginning and mix with incoming material. Pellets that are too large are crushed and go to the beginning.

 

“About 25 percent go back to the beginning and 75 percent go to the silos,” Collins says. “The two storage silos can hold 100 tons of pellets each, and we produce about 20 tons a day.” The three plants produce enough solids to run the dryers four days a week.

 

“We don’t have enough sludge to run seven days a week,” says Collins. “We’re planning to get with local utilities to see if we can take their biosolids.” For now, Collins and his crew of four get the dryers up and running on Monday and operate them through Thursday, enough time to empty the storage bins. Then they shut down from Friday through Sunday. When it comes to routine maintenance or fixing repairs, the downtime is a benefit.

 

“On Friday we have the maintenance guys come over,” says Collins. “Some maintenance items can go unattended while we’re running, like a small meter leak — that’s not enough to keep us down. But larger things, like the oxygen temperature gauges, we have to take care of immediately, and we don’t want to wait until Friday.”

 

For the time being, pellets are taken to the Lena Road Landfill where they are mixed with soil and used as cover material. “It’s a symbiotic relationship,” says Collins. “They give us their methane, and we provide them with cover material.”

 

It’s a gas

From inception, the team planned for the dryer to operate on county-generated methane, but the exact source needed to be decided. The staff considered using digester methane from the Southwest WRF, but only briefly. That plant is in the middle of town, surrounded by houses, and near Sarasota Bay. In addition, the plant does not produce enough methane to run the dryer.

 

A more realistic methane source was the Lena Road Landfill, on 1,200 acres next to the Southeast WRF. With the North and Southwest plants just 10 miles away, landfill gas was clearly the most viable option.

 

The landfill generates about 2,000 cfm of methane, more than the burner needs. Excess methane is flared. “We are tied into their air permit,” says Collins. “We’re going through an air permit upgrade right now. There’s a lot more data to collect.”

 

Air permitting has been one of the more challenging aspects of the project. “We have to collect an astronomical amount of data on a daily and hourly basis,” Collins says. “And we have to have it on record when the Department of Environmental Protection visits the site.”

 

Because the heating value of methane is lower than that of pipeline natural gas, the burners use a blend. Natural gas is used to start the system, and then landfill gas is blended in. The final operating mixture is 80 percent landfill methane and 20 percent natural gas.

 

In the future

The county is pleased with the success of the pelletizing process, which transforms biosolids from Class B to Class AA, suitable for distribution as fertilizer. The county is now looking for a contractor to purchase all the pellets. “Their only responsibility would be to hold a Florida fertilizer license and take all of our product,” Collins says. “County staff will continue to operate and maintain the facility.”

 

When a contract is signed, Collins will continue as MARS coordinator and biosolids supervisor. He doesn’t expect his workload to change — and that’s a good thing. “It’s been non-stop activity,” he says. “We all love it because we’re not just pushing buttons. There is always something going on. Someone’s watching SCADA, someone’s on the floor. We’re taking solids from three plants, so if something happens, we have to fix it. We have to keep it running.”



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