Faced with a future mandate to limit phosphorus and nitrogen discharged to Jordan Lake, the City of Mebane (N.C.) expected to upgrade its activated sludge plant to a biological nutrient removal system.

Interestingly though, effluent ammonia levels were below 1.0 mg/L even though the aeration basin dissolved oxygen (DO) was below the recommended 1.0-2.0 mg/L for nitrification. Because of the low DO, the plant was also removing some nitrates.

Encouraged by the nitrification and denitrification at low DO, the plant staff looked into simultaneous nitrification-denitrification. “We installed timers and started cycling two of the six floating aerators on and off to maintain DOs averaging less than 1.0 mg/L on a given day,” says Dennis Hodge, wastewater director. “The results were looking good — ammonia averaged below 1.0 mg/L and nitrates less than 5.0 mg/L.”

Plant staff members felt they could do even better with improved controls. They added InsiteIG dissolved oxygen probes and oxygen reduction potential probes and controllers to the aeration basin, and a SCADA system (Trihedral Engineering) to monitor and adjust the aerators. They also pilot-tested a s::can analyzer with ammonia, pH, temperature and nitrate probes (s::can Measuring Systems), and eventually installed an s::can system. Since then, effluent ammonia has been within permit limits, and nitrates have averaged below 2.0 mg/L.

The plant’s innovative practices earned a 2013 Operation and Maintenance Excellence Award from the NC Professional Wastewater Operators Committee central region.

Fast growth

Built in 1981, the 2.5 mgd (design) Mebane treatment plant sits in one of the state’s fastest growing areas. The city is growing at 4 percent per year, mostly because of its equal distance between Raleigh and Greensboro. A 1992 upgrade doubled capacity and added a 200 gpm rotary drum thickener (Parkson Corp.), a 70-foot-diameter secondary clarifier (Walker Process Equipment), a coagulant feed system for phosphorus removal and an Aqua Guard influent screen (Parkson).

A 2013 upgrade added a MiniDisk effluent filter (Aqua-Aerobic Systems) to replace old traveling bridge filters. Other plant equipment includes:

• Jet surface aerators (Aqua-Aerobic Systems)
• Return activated sludge and waste activated sludge pumps (Pentair - Fairbanks Nijhuis)
• Two secondary clarifiers (Ovivo)
• Chemical feed pumps (Blue-White Industries)
• Effluent blower (Hoffman & Lamson)

Influent passes through the bar screen, then to the grit chamber, aeration basin, final clarifiers, disk filter, chlorine contact basin and sulfur dioxide basin. From there, it flows to a reaeration basin, which adds dissolved oxygen to meet the required 6.0 mg/L minimum before discharge. Waste activated sludge is treated with polymer, and the water is removed in the rotary drum thickener. Thickened material is discharged to two aerobic digesters for further treatment.

The plant treats an average of 1.1 mgd from 4,500 residential connections. Most of the growth in Mebane has been residential and commercial, but the plant has an industrial pretreatment program, required in North Carolina for plants with flows greater than 1.0 mgd. Says Hodge, “During the 1980s and 1990s, the industrial contribution was about 50 to 60 percent of the daily flow, but the textile and furniture companies have gone elsewhere. The industries that are here now are fairly low process water users.”

Biosolids are dewatered on site by Synagro and transported off site. “We land-applied in the past, but now we have a vendor with a mobile belt press who takes the biosolids to a compost facility,” says Hodge. “For now, it’s cheaper than buying equipment and having our employees operate it.”

Growing the team

A team of eight keeps the Mebane plant operating smoothly. Says Hodge, “None of our operators were hired with more than a Grade I license. That gave us a unique opportunity to grow our own operations team.”

Hodge holds Grade IV biological wastewater operator, land application/residuals operator, and American Water Works Association advanced utility leadership certifications. He has been with the plant for six of his 36 years in the field.

The other team members are:

• Tony Bowes, lead operator (Grade IV, 10 years)
• Rickey Connally, operator/mechanic (Grade IV, six years)
• David Douglas, senior operator (Grade II, six years)
• Amanda Hill, laboratory technician (Grade III biological wastewater operator, Class I wastewater laboratory analyst, three years)
• Amy Varinoski, compliance manager (Grade IV biological wastewater operator, Class III wastewater laboratory analyst, Grade III pretreatment specialist, five years)
• Lee Parker, part-time operator assistant (three years)
• Carol Walker, administrative support (Grade II wastewater operator in training, three years)

The plant is staffed Monday through Friday from 7 a.m. to 5 p.m. and checked on weekends by the on-call operator. Operators handle routine daily checks and process adjustments, including chemical feed rates and flow rates, and field testing for dissolved oxygen, chlorine residual and ammonia. Says Bowes, “We do all our own equipment maintenance, except for electrical, and we also do all the mowing and painting.”

Meeting the limits

Effluent is discharged to Moadams Creek, which flows into the Haw River and eventually into Jordan Lake. “Because our effluent is the majority of the flow in the creek, our limits are some of the most stringent in the state,” says Hodge.

The TSS limit is 30 mg/L; BOD is 5.0 mg/L in summer and 10.0 mg/L in winter. The plant is meeting the nitrogen removal mandate to protect Jordan Lake. “The mandate was supposed to go into effect in 2016 but has been pushed to 2019,” says Hodge. “We will probably be able to meet the limit until 2021, when we will be looking at our options to meet our future wastewater needs.”

In North Carolina, the limit is based on pounds, not concentration. “Discharge into the lake is based on our flows,” explains Hodge. “The nitrogen limit is 40,000 pounds per year and the phosphorus is 5,000 pounds per year.”

Operators have met the nitrogen and phosphorus limits by using the SCADA to monitor nitrate, ammonia, pH and dissolved oxygen. “We use this data to manipulate the aeration based on the ammonia and nitrate levels,” says Hodge. “We continually monitor the data and try to keep the ammonia below 2.2 mg/L as a peak during the day, and attempt to keep the nitrates below 2.0 mg/L while maintaining a DO range of 0.2 to 0.8 mg/L. We will soon be using the ammonia and nitrate readings to automatically adjust the DO as needed.”

Like family

The operators’ biggest challenge is the weather. Rain events can increase influent flow, and winter’s colder temperatures can decrease nitrification system efficiency, making it harder to remove ammonia. Last winter, the plant partially lost nitrification when 4 inches of rain fell in one week and temperatures dropped rapidly.

“We were in noncompliance, so we did some testing and found that it was temperature-related,” says Hodge. “During that week, the temperature dropped from 19 to 8 degrees C. We started partially diverting the incoming water to a currently unused aeration basin, and it gave us breathing room to try and save the microorganisms. When something like this happens, we might as well bring a cot to work and sleep here.”

But the staff members don’t complain. “We’re a great team — very family-like,” says Hodge. “Although everyone has a specialty or area of expertise, no one has an attitude of ‘It’s not my job.’”

Bowes agrees: “Everyone is interested in getting the work done and helping each other out.” For example, the staff worked as a team several years ago to switch from gas to liquid chlorination. They did all the work in-house, saving an estimated $20,000.

“They decided to take on the project, constructing a new building, installing the tanks and chemical feed pumps, and running the piping,” says Hodge. “We ended up with a very nice structure, as well as pride in knowing we accomplished it ourselves.” Working around the usual duties, the staff completed the project in about a month. Most had been involved in construction before, either at another job site or at home.

The greatest future challenge will be to stay one step ahead of regulations. Says Hodge, “The rules will continue to get more stringent, which is why we have to be careful how far out we go in the planning stage. It’s always an unknown for us.”

Hodge enjoys the challenge, though: “My philosophy is: Don’t get by, get better. If you’re just getting by, then you must be doing the same thing you did yesterday without improvement.”  

More Information

Aqua-Aerobic Systems, Inc. - 877/271-9694 - www.aqua-aerobic.com

Blue-White Industries - 714/893-8529 - www.blue-white.com

Hoffman & Lamson, Gardner Denver Products - 866/238-6393 - www.hoffmanandlamson.com

InsiteIG - 985/639-0006 - www.insiteig.com

Ovivo USA, LLC - 512/834-6000 - www.ovivowater.com

Parkson Corporation - 888/727-5766 - www.parkson.com

Pentair - Fairbanks Nijhuis - 913/371-5000 - www.fairbanksnijhuis.com

s::can Measuring Systems, LLC - 888/694-3230 - www.s-can.us

Synagro Technologies, Inc. - 800/825-5698 - www.synagro.com

Trihedral Engineering Limited - 800/463-2783 - www.trihedral.com

Walker Process Equipment, A Div. of McNish Corp. - 800/992-5537 - www.walker-process.com