Sequencing Batch Reactors Help Drive Down Nutrients

An upgrade to sequencing batch reactors, overseen by an involved and committed staff, helped a Kentucky plant get and stay in step with tightening nutrient limits.
Sequencing Batch Reactors Help Drive Down Nutrients
Kevin Kirby, assistant chief operator, checks the sludge level in the sequencing batch reactor.

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How can the biggest be a perfect fit for the smallest? The sequencing batch reactor system at the newly upgraded Bowling Green (Kentucky) Municipal Utilities Wastewater Treatment Plant solves the riddle.

It’s one of the largest SBRs in the world, able to treat average daily flows of 12 mgd with four basins, yet it was the best choice for improved treatment, including nitrogen removal, on the city’s small landlocked site.

“Because our facility is situated between public park areas, businesses and the river flood plain, we knew we needed to go with a process that had a small footprint but would still provide good treatment at a reasonable cost,” says Doug Kimbler, water and wastewater superintendent. “SBRs were the solution to our dilemma.”

Kimbler, who was hired by BGMU in 1991 and was around for the renovation and expansion of the former treatment works, says the old plant was reaching the end of its useful life at the same time the new plant permit was about to include seasonal limits for ammonia nitrogen.

“The old plant simply couldn’t meet those limits,” says Kimbler. “With that in mind, we began to look at treatment methods that would allow us to meet any new permits for the foreseeable future.”

The operations team was heavily involved in the process selection and had input that is now paying off in operational efficiencies. Financed through low-interest Kentucky infrastructure loans, the project cost about $55 million.

Efficient aeration

The SBR anchors the new plant, with headworks on the front end and UV disinfection following. Influent flows through a pair of 3 mm Bandscreen Monster screens (JWC Environmental). Material removed is processed in a Screenings Washer Monster (JWC Environmental). PISTA grit units (Smith & Loveless) remove grit before submersible pumps (Flygt - a Xylem Brand) move the flow to the SBRs.

The SBR process consists of four 3-million-gallon OMNIFLOW basins (Evoqua Water Technologies), each measuring 100 by 68 by 26 feet deep, for a total footprint of 28,600 square feet. Each basin has Goulds motive pumps and ABS mag-lev turbo blowers (Sulzer Pumps Solutions). Kimbler praises the blowers’ energy efficiency and quiet operation: “They work great. It’s so quiet compared to our old positive displacement blowers.”

Each SBR equalizes, aerates, settles and decants the flow in a timed sequence, all in a single batch operation. Operators can achieve nitrification, denitrification and biological phosphorus removal by varying the conditions from aerobic to anaerobic to anoxic. The four basins are staged. “We use all four,” Kimbler says. “We may have two operating on the same cycle while one is filling, and the fourth is ready for decant.”

As a result, while the plant is rated at 12 mgd, it operates more like a 16 mgd plant. “We’re very efficient,” Kimbler says. “We can handle high flows in extreme wet weather conditions.”

After biological treatment, the wastewater is disinfected in a pair of UV light channels (TrojanUV), each with two banks of lamps. Space remains for a third UV channel if required in the future. While the replacement of the old chlorination system with UV has increased power consumption, “we felt it was a good tradeoff,” Kimbler says. 

The flow then moves on to an effluent holding tank. Some is used around the plant, while the rest is re-aerated by two Roots blowers (GE Energy) to achieve the required 7.0 mg/L dissolved oxygen (DO) before discharge to the Barren River.

Quality products

To process the solids decanted from the SBRs, the facility uses three repurposed digesters from the previous plant for holding and thickening. “These had lain empty for over 25 years,” Kimbler says. “We refurbished them and added ABS blowers and fine-bubble diffusers to aerate the contents and keep a DO of about 1 mg/L.”

Solids are then fed to a pair of 26-inch centrifuges (Centrisys), which produce cake at 18 to 20 percent solids. Moyno cake pumps and a SPIRAC auger and cake holding system transport the cake to an IC10000 indirect heat dryer (Therma-Flite).

The dryer was designed to operate with feed material at 18 to 20 percent solids. The centrifuges can consistently achieve 25 percent solids; material is dried to that level if for any reason some solids need to be hauled to landfill.

The dryer produces a Class A biosolids granular mixture of up to 1/4-inch particles. That makes the product easy to spread on fields with a common fertilizer buggy. The indirect heat process uses hot oil to heat the conveying screws and the dryer jacket to about 560 degrees F. “We operate somewhere in the 500s,” says Kimbler. “That ensures adequate heat transfer to the biosolids.” Dried biosolids are stored in silos with a capacity of 100 dry tons each.

For now, BGMU supplies all of the dried product, branded BGREEN, to one farmer. “We’ve had some interest from other farmers and the general public, but we’ve chosen not to get into sieving or sizing it or bagging it,” Kimbler says. Working with just one end user also keeps distribution simple.

Watching costs

A cost analysis has shown that the dryer costs less than lime stabilization or hauling to a landfill 60 miles away. Landfill tipping fees add even more cost, and in any case, the utility prefers beneficial use. “It’s our intent to get to the point where 100 percent of our solids are going to the dryer,” Kimbler says.

BGMU employees are disciples of life cycle costing, a budget and management tool used extensively in the collections system. “Sometimes cheapest is not the best,” Kimbler says. “There’s always a mid-point between capital and operating costs. People often make a mistake by not considering life cycle cost over 20 years.”

The biosolids dryer is a case in point. “We did a life cycle analysis,” Kimbler says. “We used lime stabilization and hauling as the baseline and then evaluated the capital, operating and maintenance costs of three different biosolids drying systems. The Therma-Flite dryer came out best.”

The plant’s liquid and solids trains are monitored and controlled by a SCADA system, which Kimbler calls “an extra set of eyes.” Operators can log in and run the plant from any number of locations. Data is gathered and displayed in real time.

Role of operators

Bowling Green places substantial responsibility in operators’ hands. In fact, the operations team was instrumental in selecting process equipment as the plant was upgraded. The team includes Heather Stringfield, chief operator; Kevin Kirby, assistant chief operator; and Chad Oliver, Tony Elrod, Andrew White, Trevor Riddle and Casey Brindley, plant operators.

Other team members include Benzie Timberlake, chief chemist; Mason Hamilton, industrial pretreatment coordinator; Todd Baize and Allen Hunter, industrial pretreatment assistant coordinators; Mitchell Blair, maintenance specialist; Scotty Alexander, instrumentation and control specialist; and Scott Neighbors, project manager.

“From the moment we decided to renovate, then go with a new design, we strived to include plant personnel as much as possible,” Kimbler says. “From initial decisions, to facility visits, to sitting in progress meetings with the construction company and engineering staff, we wanted to make the best possible use of our best resources: our plant staff.

“We didn’t make any decision until the operators and maintenance personnel had a chance to look at the equipment. They knew the SBRs, the pumps and the other pieces of process equipment, and there was consensus on the selection among the operators. They were excited. By being involved from the very start, our people were vested in the new plant.”

The involvement in equipment selection also boosted the utility’s team concept. “I’m fortunate,” Kimbler says. “Our crew works well together. It’s a very tight group. My job is to clear a little path in front of them and get out of the way.”

That attitude has as much to do with the success at Bowling Green as the new treatment processes: “Our project involved building a new plant while running a dying plant, at the same time. We never had a hiccup, never had a violation. It could not have gone better.”   


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