Divide and conquer is a phrase usually associated with political or military strategy.

It could also describe nutrient removal improvements at the Cookeville (Tennessee) Wastewater Treatment Plant, where operational changes to the oxidation ditches have enabled the plant to meet more stringent permit requirements.

The changes involved dividing each of four ditches into separate vertical zones and turning them on and off to create anoxic conditions to facilitate nitrification-denitrification. Tom Graham, plant superintendent, credits the idea to his assistant, John Buford.

“The state had actually hired a consultant who came in and tried different methods here but couldn’t find a solution with the extended aeration type of plant we have,” Graham recalls. “But John’s idea has worked out great.”

Serving two cities

Cookeville (population 35,000) lies 80 miles due east of Nashville. The city’s sewer system consists of more than 300 miles of pipe and 20 lift stations.

The treatment plant is an oxidation ditch facility built in 1983 (Lakeside Equipment Corporation) and expanded to twice its original size in 1995. Design flow is 14 mgd with a maximum capacity of 30 mgd; average flow is just under 8.0 mgd. The plant serves Cookeville and the neighboring city of Algood and is managed by the Cookeville Department of Water Quality Control.

Wastewater enters the plant through a headworks facility that includes two 1/4-inch multirake bar screens (Vulcan Industries), and a pair of aerated grit chambers. The wastewater is lifted 50 feet to the oxidation ditches by three 60-inch screw pumps (Lakeside Equipment Corporation) and then flows by gravity through the treatment system.

The plant is really two pairs of Lakeside oxidation ditches (2.36 mgd each) that mirror each other. Each has six surface-mounted brush rotors and a pair of clarifiers (Walker Process Equipment, A Div. of McNish Corp.).

The system includes four small-channel sludge selectors that reduce filamentous growth in the return activated sludge.

After biological treatment, the wastewater settles in four circular clarifiers, two 90 feet and two 100 feet in diameter. The newer portion of the plant is equipped with tertiary clarifiers that double as storm-mode treatment clarifiers during extreme wet-weather events.

A Duralon UV system (WEDECO - a Xylem Brand) with 96 lamps replaced an older disinfection system in 2016.

Effluent discharges to Pigeon Roost Creek, which flows to the Falling Water River. Effluent quality is consistently high; BOD averages 1.9 mg/L and TSS 4.29 mg/L. The plant reduces nitrogen to 3.0 mg/L and phosphorus to about 1.4 mg/L. For its excellent performance, the plant has won Clean Water Professionals of Kentucky-Tennessee Outstanding Performance Awards every year except one since 2007.

All beneficial

Cookeville’s biosolids management program earned the 2019 Beneficial Reuse of Biosolids Award from the Clean Water Professionals organization. “One hundred percent of our biosolids are recycled to farmland,” Graham says. “Our program is completely beneficial. There are not many utilities in Kentucky or Tennessee doing that.”

Waste solids are pumped to two aerated holding tanks and then dewatered to about 18% solids on a Komline-Sanderson belt press. The staff is working on adding a thickener to increase solids content, but in the meantime, it uses an innovative approach to accomplish that.

“We’ve adopted a different wasting scheme,” Graham says. “We turn off our return activated sludge pumps for 12 to 24 hours, then begin wasting the solids from the plant. This helps increase solids from about 0.75% to greater than 2% before being sent to the sludge holding tanks. That increases the final solids percentage off the belts.”

A conveyor moves the cake to a lime stabilization process (RDP Technologies) where the solids are pasteurized to create a Class A Exceptional material. The solids are heated to at least 158 degrees F, then mixed with quicklime and held in the lime system for at least 30 minutes.

The end product, about 800 dry tons a year excluding lime, is spread on hay and corn fields by owners of a private 5,500-acre farm about 30 miles from the treatment plant. It’s a win-win, as the farm owner gets a free beneficial product and the city avoids hauling costs.

Experienced team

Graham is proud of his small, but experienced and competent seven-member team. He and Buford are Grade 4 operators. Mance Brown is the biosolids operator; Bryan Martin and Glen Kleisley are operator trainees; and Ronnie Umbarger and Tim Henry are maintenance and repair technicians.

Graham praises their well-rounded approach: “Everybody knows the plant operations. They know what they need to do. We work eight-hour shifts each day. Team members alternate on-call duty during off hours. Any one of us can do just about everything. It’s a great example of teamwork.” Most staff training is done on the fly, including orientation for newcomers.

The team’s experience is enviable. Graham has logged almost 37 years at the facility, and Buford has 11. Umbarger and Henry have a total of nearly 50 years’ experience. “They’re indispensable,” Graham says.

“Everything runs smoothly until something breaks. Then we count on the maintenance crew to repair the equipment so that normal operations can resume as soon as possible. We do most of our maintenance and repairs in-house, which saves Cookeville a tremendous amount of money.”

The staff members’ knowledge will also help as Cookeville faces future challenges, which include higher flow rates and the addition of new plant equipment and the associated operational challenges.

Removing nutrients

The challenge of nitrogen removal has been pretty much solved, thanks to Buford’s ingenuity. “John’s an excellent scientist,” Graham says. “He came up with the solution by himself.”

The state’s consultant tried a few approaches to increase nutrient removal, but nothing worked. “When he left, John started playing with the rotors,” says Graham. “That was in 2016.” At first, Buford experimented with turning the rotors on and then off, by hand since no automation was available. That didn’t work, either.

Then he had the idea to divide each oxidation ditch into three vertical zones, each served by a pair of rotors. “It was like rotors A and B, then C and D, then E and F,” Buford says. The plant purchased 24 Tork timers for the ditch rotors and two timers for the sludge holding tank blowers, so that each pair could be turned on and off in a timed sequence.

With the help of the maintenance crew, the timers were installed and the new experiments began. Under the current protocol, which could be the ultimate solution, the first two rotors run for 3 1/2 hours and then shut down for 30 minutes. The solids settle to the bottom, creating an anoxic zone and letting the bugs take the dissolved oxygen content down to zero or near zero (an ORP near -220).

Then, the next two rotors turn on and the next zone is aerated; the ORP rises to about 175 before that zone is shut down. Then the third pair of rotors are turned on and off. The entire sequence takes 12 hours. “We’ve reduced the total nitrogen approximately 85% from what it used to be,” Buford says.

Graham adds, “We have plant operators, engineers and college students here on tours to observe the operations. Most have never seen anything like it. It’s working really, really well.”

There’s more. While the plant’s nitrogen discharge has been reduced, the power bill is lower, too. “We’re seeing a savings of about $250,000 a year by alternating the rotor operation,” Buford says.

From biosolids to training, from nitrogen removal to power consumption, Cookeville has used staff ingenuity to truly conquer its challenges.