The Moore County (Tennessee) Utility Department needed a new wastewater treatment plant. Its continuous-flow activated sludge package plant was showing its age; when it needed repairs, parts were hard to find.

In 2013, Rick Garland, utility manager, began planning a replacement. Construction started in early 2016, and a new sequencing batch reactor (SBR) plant went online on Feb. 1, 2017, with a design flow of 0.4 mgd and an average flow of 0.3 mgd.

The department chose an EcoCycle SBR from Parkson Corp. with a five-step, fill-and-draw activated sludge process. All treatment steps occur within the system’s two tanks. Garland and operator Amy Smith have found the new process easy to operate, energy efficient, and effective in meeting permit requirements for BOD, TSS, and ammonia for discharge to Mulberry Creek, a tributary of the Elk River.

Efficient treatment

The Moore County Wastewater Treatment Plant in Lynchburg, Tennessee, serves a rural county with a population just over 6,000 but with only 311 sewer connections; most of the homes are on septic systems. “We are so spread out in a big county that we can’t reach all of them,” observes Garland, holder of a Grade 3 wastewater operator license along with water distribution and backflow prevention licenses.

The flow to the treatment plant is residential; the community is home to the Jack Daniel’s distillery, which has its own SBR treatment systems and would only send flow to the Moore County plant in an emergency.

Influent to the plant passes through a bar screen (Vulcan Industries) and then enters the SBR process for a six-hour cycle. The first step is anoxic fill, during which the aeration systems are off, creating anaerobic and anoxic conditions that discourage filamentous bacteria and encourage facultative bacteria that settle efficiently. Residual nitrate is removed in this step, creating anaerobic conditions that promote the growth of volatile fatty acids and phosphorus-removing bacteria. Late in this stage, aeration is activated so the bacteria can begin metabolizing the organic matter they absorbed. Simultaneous nitrification and denitrification occurs in this period.

Once the SBR fill step is complete, the flow is diverted to another basin and the SBR enters the react step, where aeration and mixing occur until biodegradation of organics is complete. Aeration is provided through fine-bubble diffusers that are mounted on retrievable grids so they can be accessed for maintenance without dewatering the tanks. Two 15 hp positive displacement blowers (Universal Blower Pac) supply the process air. Mixing is provided by floating downdraft mixers (Aerator Solutions) in the center of each SBR. Dissolved oxygen (DO) is monitored to determine when residual DO starts to form, indicating that oxygen demand for the batch has been satisfied and treatment is complete. Luxury uptake of phosphorous occurs during this step.

In the settle step, liquid-solids separation occurs. Because no flow enters the reactor in this phase, a perfect, quiescent condition is created. Operators can adjust the settle period, which usually lasts 45 minutes. In the decant step, effluent is withdrawn from roughly the upper four feet of the 19-foot-deep, 50-foot-diameter reactor tank. The floating decanter removes effluent from below the water surface to exclude foam, scum and floatables.

Finally, in the idle step, waste activated sludge is removed to maintain the correct biomass population in the reactor. Aeration and mixing are turned off as the reactor waits for the next cycle to begin.

The SBR-treated effluent proceeds to a post-equalization tank for final aeration and then to a contact chamber for chlorination with bleach, followed by dechlorination with sodium bisulfite and discharge to the creek. Waste activated sludge is delivered to aerobic digesters; the resulting biosolids are land-applied.

Consistent process

Smith observes that the SBR runs with minimal operator attention; a feedback loop based on the DO level determines when to run the aeration systems. Effluent BOD and TSS are about the same as with the old plant, but nitrogen removal improved from 30 to 50 percent up to 75 to 80 percent.

“With the old system, ammonia would spike up and down, and we had to really stay on top of that,” Smith says. “There were times when ammonia was as high as 5 mg/L. With the SBR system, ammonia has been under 1 mg/L all the time.”

As of last May, Garland reported electric bills with the SBR at $1,300 to $1,800 per month, versus an average of about $2,300 per month and as high as $2,700 per month with the old plant. (He noted at the time that the old process was still treating a portion of the wastewater as the SBR was being phased in.)

Smith expressed satisfaction with the training provided by the manufacturer. A Parkson Corp. representative was on site for two weeks for the initial filling of the system and to ensure that all was operating as designed. When the time came to begin wasting sludge, the representative talked Smith through the process over the phone. In a later phone call, he helped Smith reset the process after a two-hour power outage.

“The SBR has been really easy to learn and is a more user-friendly process,” Smith says. “Also, during rain events, we’re not here until 11 o’clock anymore trying to run the water through the system. The SBR can adjust its cycle time to the flow coming in. Right now, we’re on a six-hour cycle, but if we have an increase in flow, it shortens the cycle to four hours. That’s very nice.”
Garland concludes, “I’ve been very pleased with the project and with the assistance we received from Parkson. We couldn’t ask for a better setup.”