Mike Wiseman and his operations team knew they would face a total nitrogen limit in their new permit, due in 2016.

So, instead of waiting to see the actual limit, and instead of resigning themselves to a costly plant upgrade, they set out last year to see how low they could push nitrogen output at the Asheboro (N.C.) Wastewater Treatment Plant with a biological nutrient removal process using existing equipment.

They’ve driven it down as low as 5 mg/L with adjustments to the aeration schedule and the addition of a carbon source supplied by a food producer. And they’ve done it while facing other challenges that go with operating a 9 mgd (design) plant that has seen its average flow drop from 6 mgd to 3 mgd after the closing of local industries.

The team’s innovations have earned recognitions that include:

• A 2010 Central Region Operations and Maintenance Excellence Award from the North Carolina American Water Works Association and Water Environment Association
• A 2010 Wilbur E. Long Operator of the Year Award from the same organization for John Stake, assistant plant manager
• A 2013 Wilbur E. Long Operator of the Year Award for Chris Schadt, lead operator

Multistep process

The Asheboro plant was built in 1962 with primary settling tanks and trickling filters. Upgrades followed in 1975, 1986 and 1996. The 1986 upgrade added an activated sludge process for ammonia removal to meet federal Clean Water Act requirements, according to Wiseman, plant manager.

In today’s process, influent passes through an automatic-cleaning Flex Rake bar screen (Duperon Corp.) installed six years ago. “It picks up anything from household toilet paper to tree limbs and tires,” says Wiseman. “Since it has been in service, we haven’t had to do a thing to it.”

The wastewater then passes through grit removal (PISTA grit system from Smith & Loveless), six rectangular primary clarifiers with chain and flight scrapers and three trickling filters (Ovivo), two from the original plant and one added in 1986.

After passing through four secondary clarifiers with the same mechanical system as the primaries, the water is lifted by Fairbanks-Nijhuis pumps to the two aeration basins (Schreiber). Six 40 hp blowers (Aerzen) and three centrifugal blowers (Hoffman & Lamson) — two 300 hp units and one 85 hp — provide airflow. New fine-bubble diffusers (Schreiber) were installed two years ago.

After aeration, the flow passes through three final clarifiers (Envirex/Evoqua Water Technologies) and a DynaSand deep-bed sand filter (Parkson Corp.) before discharge to Hasketts Creek.

Impacts of economy

For years, the plant hummed along, taking a substantial industrial flow along with domestic wastewater from a population of about 25,000. Then the economy took a turn, and textile plants that formed most of the industrial base began moving out.

“If you look back 10 years, our flow was 70 percent industrial and 30 percent domestic,” says Wiseman. “Now it’s flip-flopped — we’re 70 percent domestic and 30 percent industrial. Ten years ago, our average flow was 6 mgd. Now we’re treating 3 mgd.”

That has posed significant challenges. “In losing half our flow, we also lost a lot of alkalinity,” says Wiseman. “Now we have lower-pH influent coming in. The textile makers had been sending us chemicals and solids that were high in pH, and we didn’t have to add alkalinity to sustain our process. We had been averaging pH 7 to 7.5. When the textile mills left town, our pH dropped to 6 to 6.5. We had to find a chemical to supplement that cost-effectively.”

They tried caustic soda until a team member was injured in handling it, then switched to magnesium hydroxide — safer and effective, but costly. “Now we use a lime slurry, which is very cheap and does exactly what we need it to do,” says Wiseman.

Low-flow challenges

The flow reduction alone required process adjustments. “When we lost half our flow, the detention time through the plant increased more than twofold,” Wiseman recalls. “We went from 24 hours to about 52 hours. That created an issue with tanks going septic.” In addition, the trickling filters removed BOD so effectively that too little nutrient was available to feed the microorganisms in the aeration basins.

They experimented with taking tanks out of service to shorten detention times. “But when you take tanks offline, if you don’t exercise the mechanics in them, they can rust up and freeze,” Wiseman says. “In addition, it’s difficult to put tanks back into service, and we have a big problem with I&I. In a high-flow situation we could go from 3 mgd to 20 mgd in about two hours, so the operators were scrambling to put tanks back online so we could handle the flows without losing solids from the aeration basins.”

Another challenge related to the solids process: Biosolids were batch-fed to the belt filter presses for dewatering and the filtrate sent back to the headworks. “At 6 mgd, the flow gave us enough dilution so that the high-ammonia filtrate didn’t cause problems as it went through the plant,” says Wiseman. “But at 3 mgd, it caused huge spikes in air demand, and we ended up with more ammonia leaving the plant than we cared for.”

So in 2010, Stake stepped up with a plan to use a primary clarifier that had been taken out of service as an equalization basin for the press filtrate. “Now we pump our filtrate to that empty tank and use a pump to feed it into the process at a steady rate,” Wiseman says. “So instead of having slug loads of filtrate going through and disrupting the process, we are feeding in a low dose all the time. It has helped tremendously.

“The tank also serves as an equalization basin during high flows. At 275,000 gallons, it doesn’t last long at high flows, but it does enough to take the edge off. Most of our high flows are short-lived — two hours or less.”

The nitrogen challenge

Perhaps the biggest challenge still lies ahead. Plant personnel expect the plant’s 2016 permit to include a total nitrogen limit. Schadt, lead operator, observes, “We had been averaging about 20 mg/L total nitrogen leaving the plant. Facilities similar to ours that have a nitrogen limit are at about 4 to 5 mg/L. We’re looking to reduce ours down to something close to what we think our permit might be.

“We wanted to be proactive in what we could do with the tanks and equipment we had and see if it was possible to get our nitrogen down. If not, we would be looking at some kind of upgrade, probably costing upward of a million dollars.”

The key was to establish an effective nitrification-denitrification process in the circular aeration basins, which include a traveling bridge and perimeter air diffusers. That in turn depended on creating an anoxic zone where denitrification could occur.

“In reading and talking to equipment manufacturers, we learned that by cutting the air off for a specified period, and then aerating for a specified period, we could achieve the necessary conditions,” says Schadt. “We then discovered that we didn’t have enough carbon source to allow the microorganisms to feed and drive the oxygen down.

“Fortunately, we have a cereal company in Asheboro, MOM Brands, that has a sugar water byproduct. We were able to work out a deal where we receive that byproduct in tanker form to use as our carbon source. Using one of our old existing caustic soda tanks and a pump from one of our old plate-and-frame dewatering presses, we put together a feed system for the sugar water. We feed it continuously at 1 to 2 gpm, depending on the strength of the sugar water. By trial and error, we came up with feed rates that work.”

The process is simple: The air is on for two hours (nitrifying), then cut off for two hours (denitrifying). That amounts to eight four-hour on-off cycles over a 32-hour detention time in the basins. At present, while the system is still in an experimental stage, control is manual. “Now is the time to experiment, since we don’t have a permit limit yet,” says Schadt. “Right now, we’re trying to adapt the process to cold weather.”

Satisfying outcome

While dropping effluent total nitrogen from 20 mg/L to 4 mg/L, the biological nutrient removal process has affected phosphorus, as well — that parameter has dropped from 1 mg/L to 0.2 mg/L, measured after sand filtration. “We’re still trying to fine-tune the process,” Schadt says. “If we can cut the nitrogen in half from where we are now, that would be great.”

Wiseman calls the arrangement with MOM Brands a win for the company, the plant and the community: “They had been using a private contractor to dispose of the sugar water. We needed a carbon source to make our biological nutrient removal work. They first gave us samples so we could experiment with it. When we proved it could work, we sat down with them and worked out an arrangement that was beneficial to everybody. It is a very good example of a treatment plant and an industry working together.

“We’ve got about $150,000 invested in our BNR system, versus having to do a major plant upgrade. And it was all done inhouse.” It simply proves once again what can happen with a little operator ingenuity.  

More Information

Aerzen USA - 610/380-0244 - www.aerzenusa.com

Alfa Laval Ashbrook Simon-Hartley - 866/253-2528 - www.alfalaval.us

Duperon Corporation - 800/383-8479 - www.duperon.com

Evoqua Water Technologies, LLC - www.evoqua.com

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

Schreiber LLC - 205/655-7466 - www.schreiberwater.com

Smith & Loveless, Inc. - 800/898-9122 - www.smithandloveless.com

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