Schwing Bioset Process Helps a Community Maximize Beneficial Use of Resources

A lime stabilization process helps an Arkansas city meet its goal of producing Class A biosolids to enable the resumption of beneficial use.

Schwing Bioset Process Helps a Community Maximize Beneficial Use of Resources

Biosolids are dewatered to 18 percent solids in a gravity thickener and this 1.5-meter three-belt press (MSD Environmental Services) before being mixed in with quicklime and sulfamic acid.

Innovative solutions in wastewater are not limited to treatment plants in big cities. Small or midsized operations can be progressive as well.

The Russellville (Arkansas) Pollution Control Works Facility was faced with a challenge when its Class B biosolids no longer could be land-applied. Rather than commit long-term to landfilling the material, plant officials explored alternatives and decided to upgrade to a Class A product.

They chose lime stabilization using the Schwing Bioset process. Since 2016, the facility now processes an average 81,000 pounds of Class A material per month for distribution to farmers at no charge. 

History of upgrades

Russellville, between Little Rock and Fort Smith, is home to several major manufacturers and the state’s only nuclear power plant. Operated under contract by City Corp., Pollution Control Works Facility serves a population of 30,000 including all of Russellville and the nearby town of Dover. The wastewater collections system consists of 170 miles of gravity sewer, 14.1 miles of force main and 18 lift stations.

The plant has undergone periodic updates in its 55-year history, but it made a major shift in its approach a couple years back. “Up to that point, and still to a large part today, we were a fairly typical operation,” says Randy Bradley, wastewater operations manager.

The plant is designed for 7.3 mgd. The wastewater goes through Duperon bar screens, grit removal and primary clarifiers, and then to three aeration basins before final clarification. A final stop is chlorine contact followed by dechlorination with sulfur dioxide.

Change in plans

In the past, the facility’s primary and waste activated sludges were pumped into a digester and through a belt press for dewatering. The material was then trucked to permitted properties for land application.

“However, in 2014, land ownership changed on one of those parcels, and the new owners no longer wanted biosolids on their land,” Bradley says. “That was a substantial loss in available area, and we’re restricted solely to a parcel we owned. At that time, we were generating about 2,400 pounds per day of Class B biosolids. We might have been able to make that work, but we had just added another aeration basin and clarifier to the front end of the process, so we were going to be generating more solids. Something had to be done.”

The facility team conducted an intensive study of options, which included increasing the digester volumes or improving the existing ones, composting, and biosolids drying. “Drying the material was a significant initial investment,” Bradley says. “And as I talked to people at other facilities, I discovered there was a fairly high maintenance cost on that equipment.

“When we went to a northwest Arkansas composting facility, we found that solution to be very labor-intensive, and it would demand much more acreage than we had available. In addition, we weren’t sure of a reliable availability of the organic material needed for the process. We needed another viable option.”

A good tip

Meanwhile, the Russellville facility had issues that prevented keeping the biosolids in the digester for the retention time required to meet Class B specifications. That meant taking the biosolids to a landfill.

“We absolutely hate to send anything to landfill — and not just because of the costs involved in doing so, though those costs are substantial,” Bradley says. “Landfills have space issues of their own, and biosolids can ultimately serve a benefit. And yet, we had no choice, so we contracted with Denali Water Solutions to haul off our material.

“It just so happened that they’d been working on some potential projects with Tom Welch, a regional manager from Schwing Bioset. They told us about the Schwing Bioset process for creating Class A biosolids.”

Bradley contacted Schwing Bioset and was intrigued by what he learned. He took Lance Bartlett, utility engineer, and Chesley Jackson, senior operator, on a trip to Florida to see the process in operation. The rest is history.

At Russellville, the Schwing Bioset process takes biosolids dewatered to about 18 percent solids in a gravity thickener and a DBP 1.5-meter Model 3DP three-belt press (MSD Environmental Services). The material drops into a hopper with a twin-screw mixer in which quicklime and sulfamic acid are added and blended.

The mixing alleviates issues such as unreacted lime in the final product and the associated costs. A Schwing Bioset KSP-25 piston pump then sends the blended material into a 35-foot-long reactor where the acid/quicklime mixture generates temperatures in the range of 140 degrees F, raises the pH, stabilizes the biosolids, and yields the Class A product.

Built for expansion

Getting the process fully online was anything but business-as-usual. In a deal that involved the city, Schwing Bioset and Denali Water Solutions, a mobile system was brought to Russellville for a pilot program to prove out the process.

“The final agreement involved Bioset leasing a mobile system to Denali, who operated it for us, charging us a monthly fee to cover the labor, operation, and lease of the equipment and placing of the material at an on-site dirt yard,” Bradley says. “However, after we purchased our permanent system and the installation was nearing completion, we decided we had the comfort level to operate it and manage the disposition of the product ourselves. And bringing that process in-house would save us a substantial operating cost.

“Largely due to the help Schwing Bioset provided — technicians spent the first two weeks with us — the install was very smooth,” Bradley says. “During the design phase, we gave Hawkins-Weir Engineering a projection for where we might be in 10 to 20 years. They designed the facility for future area development. As a result, the reactor is sized to handle two belt presses. If growth warrants it, we can just move another belt press in without skipping a beat.”

Great for the soil

Once the material exits the Schwing Bioset process, it is trucked to an area next to the plant, spread out using a front-end loader, and allowed to dry. Once dry, it is moved into piles and, after periodic testing for Salmonella, given away to area farmers.

“We have several farmers competing for it at times, which is great for us,” Bradley says. “While the material has nutrient value, its ability to boost the soil pH is its real selling point. Almost all the soil in Arkansas is pretty acidic, so farmers welcome that pH boost.

“Next spring we are looking at possibly doing some type of bid for it. Simply recouping some of our costs, even enough to pay for fuel for the loader, would be a nice bonus. Right now, just having someone haul it off for us and make good use of it is a huge improvement. This entire project could not have gone better nor had better results.”


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