Consistent Performer

The City of Mansfield (Ark.) gets reliable compliance and a thriving discharge creek after replacing an old extended aeration plant with a special activated sludge process

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The City of Mansfield (Ark.) wastewater treatment plant has solved its NPDES permit compliance consistency problems with a recently installed special activated sludge process that replaced an aging packaged extended aeration plant.

 

The new treatment plant, which incorporates anaerobic, anoxic, and aerobic treatment with integral sludge reduction, has also significantly improved system reliability and simplified maintenance. Because the receiving stream no longer sees effluent with borderline loading, aquatic life is thriving, and fishing has improved in downstream pools. In recognition, the city received a 2009 Arkansas Water Works & Water Environment Association Special Systems Recognition Award in the wastewater category.

 

During the plant’s first 19 months, it treated 140 million gallons of influent from homes, schools, and restaurants, with total sludge output of only 86,625 gallons. It was not necessary to begin sludge dewatering, and pressing to cake until the plant had been operating for 14 months.

 

Not a hiccup

“This new plant has never even come close to being out of compliance,” says Arnold F. Elmore, former plant manager and now public works director. “Our Discharge Monitoring Reports (DMRs) have been outstanding. Our TSS has been real, real low from what we were used to. It’s like now there is none. BOD has also been outstanding.

 

“The plant always has very good effluent with very low numbers on everything, no matter what the inflow is. The effluent is ready for decanting regardless how much inflow there is. Before, inflow would often overrun the plant. Our DO is a little low, around 6 ppm, but before, it was barely in compliance, around 4 ppm.”

 

The plant’s monthly DMRs show consistent permit compliance (Table 1). “We had terrible weather when we first started up in February 2008 — wet and cold, lots of I/I — but the plant took everything that came in without even a hiccup,” Elmore recalls.

 

“You were supposed to seed the plant to promote biological activity before startup, but the weather wouldn’t permit it, so it just took it straight out and let nature take its course. When the warmer weather came, we were able to maintain the right pH just by changing the anoxic timing, and we corrected some issues with the sludge by changing return rates. No further adjustments were needed.

 

“You don’t know from appearance that it’s just cleaned-up wastewater. And before, even when we were in compliance, we had borderline loading that would cause the creek to turn. Not enough to stink, but still way too much for optimum life support. All that stuff is gone now.”

 

Growing demand

The previous plant, built in 1974, was rated for 250,000 gpd, and average flow was 100,000 gpd. However, the plant was easy to overload, even after extensive repairs to creek crossings and after wet-weather I/I was reduced from more than 1 million gpd to 700,000 gpd.

 

Meanwhile, during its 33-year life, the customer base had grown from 250 homes and two small schools to 665 homes, three large schools, and six restaurants. Then the neighboring city of Hartford asked to connect to the Mansfield system. To accommodate the increased demand, the new plant has a design capacity of 900,000 gpd, and average daily flow ranges from 200,000 to 700,000 gpd.

 

The new plant project started in 2002, driven primarily by severe corrosion of the packaged extended aeration plant. “We had a mandate from the city to replace that facility with a modern plant with increased capacity and reliability, and made from high-quality materials,” recalls David Hopkins, P.E., president and engineering manager for the city’s consulting engineering firm, Landmark Engineering and Surveying, in Little Rock. “Rehabilitating the existing plant quickly fell out as an option because it was too far gone.

 

“After we considered a variety of other options, finalists were an oxidation ditch with secondary clarifiers, or a sequencing batch reactor (SBR) facility.” The small space available — 1.5 acres of city-owned land — worked in favor of SBR technology, with its small footprint and construction comparable to that of an oxidation ditch.

 

Two-train system

The city and its engineers selected an SBR plant from Fluidyne for its low-maintenance and high-reliability jet aeration headers and a new process that included smaller tanks, improved nutrient and sludge reduction, high-quality materials, and complete detailing of the process and schematics.

 

“They were very helpful in assuring that we presented complete construction documents, so what was put out for bids was a complete process,” Hopkins says. “For example, they showed exactly how the control system wiring and conduits connected to pumps, level indicators, switches, sensors, and so on. If you leave that up to contractors, you don’t know what you’re going to get.”

 

The Integrated Surge Anoxic Mix (ISAM) system from Fluidyne is a single-train system with a constant-level anaerobic selector chamber, followed by a surge/anoxic/mix (SAM) tank, and then one or more SBR basins. It incorporates BOD, TSS, and nitrogen removal with sludge reduction in an integrated process. Mansfield chose a two-train system.

 

The system has consistently demonstrated 0.15 to 0.25 pounds of sludge production per pound of BOD removal, and an average daily conversion of influent wastewater-to-sludge of about 0.1 percent. In operation, all influent flow enters the anaerobic chamber, where solids settle as they would in a primary clarifier. Elimination of primary solids at that stage allows for much smaller SBR basins, at equivalent solids retention times, than with conventional SBRs.

 

Influent then continues to the SAM surge basin, also known as the influent equalization basin, which provides flow and nutrient equalization, allowing for treatment optimization at the full range of flows and loadings. Mixed liquor is maintained in the SAM tank for immediate reaction with flow from the anaerobic chamber, in order to suppress odors and initiate and accelerate carbon and nitrogen reactions. Mixed liquor is recycled from the top of the SBR tank for removal of scum by a proprietary flow and scum control subsystem.

 

Nitrates are recycled to the SAM tank for denitrification. There, reactions are accelerated in the presence of unreacted carbon from the raw sewage entering that tank. Aeration and energy requirements are reduced as nitrates are fully reduced to nitrogen.

 

“With the old plant, it was a constant struggle to keep in compliance,” Elmore says. “That doesn’t happen anymore, not even when we had extremely high flow when there was flooding in town. I want my water with nothing in it, and if fecal is clear, the rest will follow suit. It’s the first thing I look at. When I see that thriving discharge creek, it just tickles me to death. It makes me feel like a success.”

 

About the Authors

Erick Mandt is vice president and Jason Bumgarner is a field service engineer with Fluidyne Corp., a designer and manufacturer of wastewater process equipment and technology based in Cedar Falls, Iowa. They can be reached at erickm@fluidynecorp.com and jasonb@fluidynecorp.com.



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