Client: Mansfield, Arkansas
Solution: ISAM™ - Integrated Surge Anoxic Mix
Background:
Mansfield, Arkansas - Plant Management for the city’s recently installed special activated sludge process, which replaced an aging packaged extended aeration plant, reports the move has solved compliance consistency problems with its National Pollutant Discharge Elimination System (NPDES) permit. The new plant, which incorporates anaerobic, anoxic, and aerobic treatment with integral sludge reduction, has also provided significantly improved system reliability, and much easier ongoing maintenance. Meanwhile, the plant’s discharge creek is no longer receiving input with borderline loading, allowing aquatic life to thrive, and improving fishing in downstream pools.
In recognition of its excellence of operation, and service to the public during 2008-2009, the City of Mansfield received the Arkansas Water Works & Water Environment Association Special Systems Recognition Award for the wastewater category.
During the plant’s first 19 months of operation, it treated 140,308,266 gal. of influent from residences, schools, and restaurants, with total sludge output of only 86,625 gal. It was not necessary to begin sludge dewatering, and pressing to cake, until the plant was operating for 14 months.
“This new plant has never even come close to being out of compliance,” declared Arnold F. Elmore, the plant manager, who has been promoted to public works director. “Our Discharge Monitoring Reports (DMR’s) have been outstanding.”
“Our TSS has been real, real low from what we were used to with the old plant; it’s like now there is none. There’s nothing visible in the water coming out. BOD has also been outstanding. The plant always has real good effluent with real low numbers on everything, no matter what the inflow is.
“The effluent is ready for decanting regardless of how much inflow there is,” he noted. “Before, inflow would often overrun the plant. Our DO is a little low, around 6, but before, it was just barely in compliance, around 4.”
The plant’s monthly DMR’s include sampling for Dissolved Oxygen (DO), pH, Total Suspended Solids (TSS), Ammonia Nitrogen (NH3-N), and Biochemical Oxygen Demand (BOD). Monthly average concentrations, as mg/L, have ranged from 5.20-6.70 for DO; <1.00-6.59 for TSS; 0.05-0.23 for NH3-N; and <2.00-4.12 for BOD. In addition, fecal coliform has ranged from 1-6 colonies, while pH has always been within permit requirement.
“We had terrible weather when we first started up in February, 2008 wet and cold, lots of I/I (inflow and infiltratin), but the plant took everything that came in without even a hiccup,” Elmore recalled. “You were supposed to seed the plant to promote biological activity prior to 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 some issues with the sludge were corrected by changing return rates,” he noted. “No further adjustments were needed.”
“You don’t know from appearance that it’s just cleaned-up waste water,” he added. “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.”
Elmore said the previous plant, constructed in 1974, was rated for 250,000 GPD. Average flow was 100,000 GPD. However, the plant was easy to overload, even after extensive repair work was performed on creek crossings, and after system I/I was reduced from over 1 million GPD down to 700,000 GPD. Meanwhile, during its 33-year service life, the customer base had grown from 250 residences and two small schools to 665 residences and three large schools, plus six restaurants. Then recently, the neighboring city of Hartford, which wasn’t able to build its own wastewater treatment plant, wanted to connect to the Mansfield system. To accommodate the increased demand, the new plant has a design capacity of 900,000 GPD. Average daily flow ranges from about 200,000 to 700,000 GPD.
“With the old plant, it was a constant struggle to keep in compliance,” Elmore recalled. “That hasn’t happened anymore, even when we had extremely high flow when there was flooding in town. For example, ammonia had often run out of control before, and now ammonia nitrogen is typically less than 0.2 mg/L.”
“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. Makes me feel like a success. I’m not a tree hugger, but I do have a soft spot for my water.”
He added that Charlie Little, his Class I Operator, loves the plant, with no more sludge drying beds to deal with, and trouble-free operation.
The city’s consulting engineer said the new-plant project for Mansfield started in 2002, driven primarily by severe corrosion of the packaged extended aeration plant, which was no longer being manufactured, and therefore difficult to find parts for.
“We had a mandate from the client to replace that facility with a modern plant with increased capacity and reliability, and made from high-quality materials,” recalled David Hopkins, P.E., president and engineering manager for Jewell Engineers Inc., now known as Landmark Engineering and Surveying, in Little Rock, AR. “For example, the containment tank was corroded, and there was concern about failure. Treatment was single-train, so if containment was lost, there would be no treatment.”
“Rehabilitating the existing plant quickly fell out as an option because it was too far gone,” he continued. “After we considered a variety of other options, finalists were an oxidation ditch with secondary clarifiers,or an SBR-type facility.”
“The land available was 1.5 acres purchased by the city, which is a small piece of property as sites go. That moved us in favor of SBR because of its small footprint, efficient use of the space, and comparable construction cost. Within that, we wanted materials to be concrete, fiberglass, and stainless steel instead of the carbon steel the old plant had.”
“We looked at the city’s plans and their levels of I/I, and came up with an average need of 350,000 GPD. We then added projected growth in demand, plus the desire of the adjacent town to connect, and increased it to 450,000 GPD, with hydraulic peak of 900,000 GPD.”
Hopkins said there were important differences among SBR technology vendors. “Some vendors had fine bubble diffusion nozzles at the bottom of their aeration tanks, and we were concerned about diffusion baffles tearing and nozzles clogging,” he said. “Fluidyne had jet aeration headers instead, which were preferred for maintenance and reliability.”
“That vendor also was promoting a new and improved process that featured smaller tanks, and improved nutrient reduction and sludge reduction versus competing SBR’s. And they offered high quality of materials, and complete detailing of their process-complete schematics of the system, instead of pieces and parts that had to be put together.”
Hopkins also noted special help from the vendor after the decision was made to go with their process.
“They were very helpful in assuring that we presented complete construction documents, so what was put out for bids was a complete process,” he explained. “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 proof is in the performance we have a happy client.”
Fluidyne describes its special Integrated Surge Anoxic Mix (ISAM™) system as a single-train type, with a constant-level anaerobic selector chamber followed by a surge/anoxic/mix (SAM™) tank, and then one or more SBR basins. It is designed to incorporate BOD, TSS, and nitrogen removal with sludge reduction, in an integrated process. At Mansfield, a two-train system was installed.
The system has consistently demonstrated 0.15-0.25 lbs of sludge production per lb. of BOD removal, compared to 0.5-0.6 for other SBR systems, and an average daily conversion of influent wastewater-to-sludge of about 0.1%, compared to a typical conversion rate for other biological processes of about 2%.
In operation, all influent flow enters the anaerobic chamber, where solids settle in the manner of a primary clarifier. Elimination of primary solids at that stage is said to allow for much smaller SBR basins, at equivalent SRT, than with conventional SBR’s.
Influent then continues to the SAM™ surge basin, also known as the influent equalization basin. This part of the system is said to provide flow and nutrient equalization that allows for optimization of treatment 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 also initiate and accelerate carbon and nitrogen reactions. In addition, mixed liquor is recycled from the top of the SBR tank, for removal of scum by a proprietary flow and scum control sub-system.
Nitrates are recycled to the SAM™ tank for denitrification, with reactions said to be accelerated in the presence of unreacted carbon from the raw sewage entering that tank. Aeration and energy requirements are said to be reduced, as nitrates are fully reduced to nitrogen gas there.
In addition to WWTP applications like the Mansfield installation, Fluidyne says its ISAM™ has also been used as a stand-alone system to thicken and destroy organic sludge from other biological treatment plants, as a means for significantly reducing sludge disposal volume.