Ask Ben Riles why the Moberly (Mo.) Wastewater Treatment Plant won a state award for its biosolids program and he gives part of the credit to city leaders, who decided nearly two decades ago to adopt an emerging technology when it was time to replace an aging facility.

Riles, named chief operator in 2007 after eight years as an operator, has been “kind of spoiled” while working at the dual-cell sequential batch reactor (SBR) facility, designed and built by Aqua-Aerobic Systems. “I’ve never worked at an older plant,” he says.

Although some people questioned the city’s decision to adopt the SBR technology in 1995, Riles observes, “It works quite well, especially under adverse conditions. It adapts and it overcomes.”

A side benefit of the new plant was the decision to locate it next to a 150-acre parcel the city bought so it could apply biosolids on its own property.
The convenience of the biosolids program was a factor last year when Moberly (population 14,000) received the Biosolids Management Award in the small facility category at the annual conference of the Missouri Water Environment Association.

No More Hauling

The city started land-applying biosolids in 1987, trucking the material to local farms. That changed in 1997 with the switch to the city-owned Wastewater Farm. “Most plants just don’t have their own ground, but we have 150 acres, 84 of which are actually in our land application program,” Riles says. A little more than half the permitted land is cultivated, and the city makes money by leasing it to a farmer who rotates corn, soybeans and other crops. The other half of the permitted land is forested and offers area for biosolids application during harvest periods.

Besides generating income from the tenant farmer, having land next to the treatment plant saves the costs of hauling and soil testing at private farms.

No Violations

In the award application, Riles reported that the city had no violations in the past five years of quarterly tests on the Class B biosolids from two aerobic digesters. In 2012, the city met U.S. EPA standards for the Specific Oxygen Uptake Rate (SOUR) test and Volatile Solids Reduction methods. In previous years, the city has regularly passed at least one of the tests each quarter and usually met both standards.

The city consistently meets the soil-quality standards on its land application site, as well. Regulations call for testing the soil every five years for pH, but Riles tests annually.

The next-door land application site means Moberly no longer needs trucks to haul biosolids. Instead, the liquid material is pumped from two holding basins to the Wastewater Farm, where it is applied by a reel/pivot irrigation system. “I can put down easily 187,000 to 200,000 gallons of biosolids per day when the conditions are right,” Riles says.

Simple Process

The Moberly treatment plant has an average flow of 2.5 mgd and a permitted maximum capacity of 5 mgd. During rain events when flow from the city’s combined sewer system increases sharply, influent can be diverted to lagoons still in place at the city’s former treatment plant sites, allowing the system to handle up to 7 mgd on a short-term basis. With a heavy rain, the flow can jump from 2 mgd to 5 mgd within hours, but Riles says, “The quality of biosolids and effluent never really varies.”

Most of the dry-weather flow comes from residential and commercial customers. The city’s few industrial customers pose no challenges to the wastewater system. The treatment process starts with a grit removal screen before the flow is directed to one of the two SBRs. From the reactors, wasted sludge is directed to a pair of aerobic digesters and the liquid effluent passes through a UV disinfection system before discharge. A 2007 plant improvement project added a second biosolids holding basin and upgraded the digesters, “so we could do a better job of controlling biosolids quality,” says Riles.

The city installed Tornado mixers (RWL Water Group – Aeromix) in both digesters. In the new above-ground concrete holding basin, aerators (TideFlex Technologies) mix the biosolids with compressed air. In the existing in-ground holding basin, mixers (Aqua-Aerobic Systems) maintain the consistency of biosolids awaiting land application.

Process Control

Riles operates the biosolids program with a team that includes Doug Farrow, Richard Swank, Donnie Gregory, and Garrett Foote.

To make sure the application system works properly, the operators keep a close eye on solids content in the storage basins. “We like to maintain between 2 and 3 percent solids,” Riles says. “If it gets up to 5 percent, we will actually add effluent to get it back down. If you put it on too thick, you can coat the ground, and what you want is for the ground to be able to absorb it.”

The irrigation system (Kifco Ag-Rain) includes a slurry pump, helping operators apply biosolids at an agronomic rate. Operators monitor soil conditions: If the ground is frozen, or if it is saturated from rain or snow, they do not apply biosolids.

The two holding basins (1.2 million gallons each) provide 180 days of storage capacity. The basins have never reached full capacity, but the city was nearing that point before building the second basin. If the basins ever reach capacity, “We can decant water off the top and send it back to the influent pump station,” says Riles. The basins are not covered, but Riles has seen no significant problems from rail or evaporation.

The storage capacity paid off in 2012, when wet weather and a long winter made land application challenging. In that year the plant generated 489 dry tons of biosolids and the team land-applied just 283 dry tons.

To finance its biosolids operation and recent plant improvements, the city has used State Revolving Fund loans and federal stimulus funding. The upgrades have proven to be a sound investment in protecting water quality and putting biosolids to beneficial use at a highly affordable cost.  

More Information

AEROMIX Systems, Inc. - 800/879-3677 - www.aeromix.com

Aqua-Aerobic Systems, Inc. - 800/940-5008 - www.aqua-aerobic.com

Kifco, Inc. - 800/452-7017 - www.kifco.com

Red Valve Co. / Tideflex Technologies - 412/279-0044 - www.redvalve.com