Teamwork Helps a Staff Adapt and Master a Big Treatment Process Change

Operators at North Carolina’s Contentnea MSD quickly learn to get the most performance from a new biological nutrient removal facility.
Teamwork Helps a Staff Adapt and Master a Big Treatment Process Change
Plant operators including Stephen Berry (left) and James Woodard received training to ensure proper functioning of the facility’s BNR oxidation ditch (Ovivo).

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At the Contentnea Metropolitan Sewerage District, tailgating doesn’t mean getting together for beer and burgers before a football game. It refers to the informal morning gatherings of the eight-member operations staff before they head out for their daily tasks. Ideas are broached. Solutions discussed. Experiences shared.

“We’re a team,” says Chuck Smithwick, manager of the district, based in Grifton, North Carolina. “Everybody works together. We don’t let any good ideas go to waste.”

Adds Renee Smith, operator in responsible charge, “We’re small. Each day we might be doing something different. You put on whatever hat you need to wear that day.”

The daily exchange of ideas and observations helps because the district is operating a brand-new wastewater treatment plant, the culmination of more than $33 million in improvements and upgrades over the last 10 years, driven by new requirements from state and federal environmental agencies. The result? The district meets all discharge parameters, including nutrient reductions.

Step up to BNR

The Contentnea district serves the communities of Winterville (population 10,000), Ayden (5,000) and Grifton (2,800). Average daily flow to the 4 mgd (design) treatment plant is 2.14 mgd. Effluent flows into Contentnea Creek, a part of the Neuse River watershed.

In the old days before the improvement project, the district operated a 2 mgd multistage aeration-clarification facility designed to reduce BOD and TSS and convert ammonia to nitrite in a partial nitrogen removal process. “Our operators did a fantastic job of making the old plant work, but the flow had gone beyond what the old plant could handle, and it couldn’t meet the new nutrient limits,” Smith says. “It wasn’t designed to do what we needed it to do.”

To meet the tighter nutrient requirements, the new treatment works is anchored by a five-stage biological nutrient removal (BNR) Bardenpho oxidation ditch process (Ovivo). The new headworks includes a screen and cyclone grit removal system (Parkson Corp.). The flow then passes through a Parshall flume where flow is monitored.

The headworks is designed for a maximum wet-weather flow of 8 mgd. Smithwick says infiltration and inflow is a problem in the collections system. Each community is responsible for the integrity of its own system. “We’re working with our communities on reducing I&I,” Smithwick says.

Out with N

In the Bardenpho system, the first phase of the oxidation ditch operates in the anaerobic mode for luxury uptake of phosphorus. It is followed by an anoxic zone for nitrogen removal, an aerobic aeration stage for nitrification, a second anoxic stage for further denitrification, and a post-aeration stage to promote settling in the clarifier. Return activated sludge is directed back to both anoxic zones.

Two 90-foot-diameter, 16-foot-deep clarifiers settle solids and produce a clear overflow that passes to a tandem of deep bed sand denitrification filters. Methanol is the carbon source. The filters are “bumped” periodically to release nitrogen in its gaseous form.

The filter also polishes the wastewater.

Filter backwash water is returned to the head of the plant. TSS is not an issue, as the plant easily meets its requirement of less than 30 mg/L. Total nitrogen isn’t an issue anymore: In 2014, the plant effluent averaged just 1.69 mg/L total nitrogen. Final effluent is UV disinfected (WEDECO) before discharge.

Biosolids are aerobically digested, gravity thickened to 2 percent solids and dewatered on two new 800 Series incline screw presses (Huber Technology) rated at 90 gpm. Cake averaging 18 to 20 percent solids is hauled to a compost facility about 90 miles away. The district owns 70 acres surrounding the treatment plant where liquid biosolids can be applied.

“We started operating the screw presses in March 2014, with the goal of reducing land application of biosolids to our property by 50 percent,” says Smithwick. “We typically apply about 2.2 million gallons to our fields annually.” In 2014, the property received 800,000 gallons, fertilizing Bermuda grass and coastal hay for livestock feed. Material is applied from mid-March through October. About 1.5 million gallons was dewatered in the presses in 2014.

Keeping control

The control system (Ovivo) ramps the aerators up and down to control dissolved oxygen in the BNR process and optimize chemical and energy use. PLCs on the individual pieces of equipment provide further process control, and the plant uses a telemetry system to oversee all processes and alert operators to an issue if necessary. “The ditch is very operator-friendly,” Smith says. “We essentially turn it on and let it run.”

The new plant capped a 10-year outlay in capital improvements to the treatment system and the wastewater collections system, including interceptor and pump station work. Much of the funding came from outside sources and zero- or low-interest loans. About 30 percent came as grants from the U.S. Department of Agriculture and Rural Development and the North Carolina Rural Center. Other money came from the state’s Clean Water Management Trust Fund. “Without their help, none of this would have been possible,” Smithwick says.

The district bills the communities it serves, and they in turn bill customers. Smithwick says individual sewer bills have increased slightly, and the communities are implementing flow-based billing over the next two years.

Using the old plant

While the former treatment plant is gone, it is not forgotten. In fact, it serves a purpose. The district uses space in the old plant for solids processing and storage. “Four of the old circular clarifiers have been converted to aerobic digesters for processing solids,” he says. These small tanks (150,000-gallon capacity) have been retrofitted with diffused air blowers. They are used in rotation, week by week.

“We pump biosolids into one of the digesters for a week, then switch to a second digester and pump into that one for a week,” Smithwick says. It’s a batch operation: Each digester holds solids for about a month to condition the material before it is transferred to the dewatering facility or spread on land.

“By using the old plant, we’ve been able to increase our storage from 1 million gallons to 2.4 million gallons,” Smithwick says. “From an operations standpoint, we can’t have enough storage space.” The increased capacity enables the plant to remove, stabilize and properly recycle its biosolids.

Operating innovations

The Contentnea team’s ingenuity has helped the plant perform effectively. Besides Smithwick and Smith, the team includes James Woodard, Ricky Barrow and Stephen Berry, operators; Jimmy Edwards, mechanic; Windy Sammond, lab assistant; and Harriett Pridgen, administrative officer.             

As is typical with new treatment processes, the staff received training from the equipment manufacturers, but Smith and her staff have made their own adjustments to meet their specific needs. “Each facility is different,” Smith says. “We learned the new processes together and we brainstormed with all our operators on how to optimize operations.”

One of the most important lessons learned was that the BNR oxidation ditch system can be most effectively controlled by monitoring dissolved oxygen entering the second anoxic zone. “The manufacturer gave us the idea, and we’ve learned how to operate with the DO a bit lower and get better denitrification in the ditch,” says Smith. “That means less methanol needed in the denitrification filters and less cost. We’ve been able to optimize the efficiency of the denitrification process.”

Plant operators have made another adjustment that improved phosphorus removal. “Weather and mixed liquor suspended solids play a big part in phosphorus removal,” Smith says. During summer, the staff maintains the MLSS at 3,000 mg/L or less and increases the DO content of the basin. In winter, the MLSS increases to about 4,000 mg/L.

“We’ve found the lower MLSS in summer results in better phosphorus removal,” Smith says. In hot weather, increased DO reduces the tendency of phosphorus to release back into the water. The plant also adds alum during summer to increase phosphorus removal.

Peak performance

The tweaks make a difference. The district produces outstanding effluent, averaging well under permitted levels for BOD, TSS, phosphorus and total nitrogen. Effluent phosphorus averaged 0.66 mg/L for all of 2014; total nitrogen averaged 1.69 mg/L.

The plant’s nitrogen permit limit is stated in pounds released per year. “Our permit limits us to no more than 37,100 pounds of nitrogen on an annual basis,” Smithwick says. “In 2014, our actual was 10,492 pounds.”

The district has gone from near the bottom of the list to first in nitrogen removal among the members of the Neuse River Compliance Association, a regional nutrient trading group formed to reduce nitrogen into the sensitive Pamlico Sound estuary.

Before installation of the new treatment processes, the district discharged more than 6 mg/L of total nitrogen at the midyear points of 2008 and 2009, and more than 8 mg/L in 2010. In 2014, the midyear number was 1.52 mg/L, lowest in the compliance association. “It’s much easier now,” says Smithwick. “It’s better for our operators and for the environment.”  



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