Client: Decatur, Arkansas
Solution: ISAM™ - Integrated Surge Anoxic Mix
Background:
The public works manager for the City of Decatur, AR reports that replacement of their aerated pond system wastewater treatment plant (WWTP) with a special activated sludge process WWTP has provided for compliance with a more stringent National Pollutant Discharge Elimination System (NPDES) permit, while coping with a regular 90-95% weekend decrease in wastewater input, and major seasonal variations in ambient temperature.
The design 2.2 million gal./day (MGD), 1.6-1.8 MGD average usage plant was started up in 2009, following a 10-month installation, and a five-year effort to find a suitable replacement for the previous 1.375 MGD pond system, which had been installed in 1989.
While the previous system had performed adequately for biological oxygen demand (BOD) and ammonia and organic nitrogen (TKN), the replacement effort was launched in the late 1990’s when new limits for nitrates were introduced. Staged reductions for phosphorous limits began in 2004.
The new plant has consistently provided 96-98% biological phosphorous reduction to help meet the new limits, and nitrates reduction to meet the drinking water standard mandated by discharge to a “losing stream”.
In addition, it was not necessary to begin sludge dewatering, and pressing to cake, until the new plant was operating for six months. At least 50% reduction of sludge is being realized during the special activated sludge process.
“The regulators did not wait on us,” recalled James Boston, the City’s public works manager, who came on board in 2003. “The new instantaneous 10 mg/L nitrate limit had spelled the beginning of the end for the pond system, which was especially prone to inadequacy when water temperatures fell below 10 degrees C in the winter. It couldn’t reliably meet the new, very unforgiving instantaneous limit, where 11 mg/L on one day would require us to write a non-compliance report for a whole month of data. It wasn’t averaged like everything else. And our discharge goes to losing stream, which means possible entry to aquifer, so we went to drinking water standard for nitrates.”
“Meanwhile, reduced phosphorous was being staged in--- from reportable, to 3 mg/L, to 2 mg/L, to 1 mg/L, and upgrading the pond system for that had been deemed cost-prohibitive,” he continued. “And whatever we were going to do had to also be able to cope with the routine weekend shutdown of a local poultry processing plant that contributes 1.4-1.6 MGD of pre-treated flow on weekdays, and also handle the seasonal temperature variations. When it got cold, I didn’t want to have to just wait for warmer weather.”
“We’ve been amazed to get the numbers we got with this new plant,” he concluded, “especially with the poultry plant as such a major contributor to our input. It’s like having cheeseburgers during the week, going to salad for the weekend, and then back to cheeseburgers the following week. And it hasn’t mattered if they had any bumps or hiccups in their pretreatment operation.”
“We’ve been consistently getting 96-98% biological reduction of phosphorous. We’ve also been seeing at least 50% reduction of sludge during the process, before it goes to the holding tank and sludge press. It was great to see a much greater reduction there than we had calculated.”
Boston reports NH3 now at < 0.1 mg/L, vs. permit level of 5-7 mg/L seasonal; CBOD at < 2 mg/L vs. 10 mg/L; TSS at < 2 mg/L vs. 10 mg/L; nitrates at 6 mg/L vs. 10 mg/L instantaneous; and phosphorous at 0.2 mg/L vs. 1.0 mg/L.
The city’s 500 sewer connections include the homes of its 1600 residents; the poultry plant; and a pressure washer manufacturing plant that contributes 800,000 gal./mo. of process water and wash water.
The washer plant’s input is considered basic industrial; i.e., not including any heavy metals or other special contaminants. The poultry plant is a kill, process, marinate, and pack operation, though is not a renderer, or “protein plant”. It has dissolved air flotation (DAF) installed as pretreatment for its wastewater before discharge to the city.
Under new management since 2009, the poultry plant’s TKN output is now about 50-65 mg/L and its phosphorous 6-12 mg/L. The domestic input from the city for TKN is 35-45 mg/L, and phosphorous 3-6 mg/L.
The poultry plant’s weekend steep volume reduction originally challenged the city from Saturday, 4:00-5:00 am to just after midnight Sunday, and is now from Friday, 9:00-10:00 pm to Sunday, 8:00 pm.
‘We had a major challenge to be able to deal with that change in the time for the volume interruption, as well as changes in nature of that plant’s output to us while we were getting our new plant installed,” recalled Boston. “TKN had been 60-65 mg/L, and was now 50-65 mg/L. Phosphorous was down from 10-15 mg/L to 6-12 mg/L. The hardest was BOD --- we had designed the new plant for 300-350 mg/L, expecting 275 mg/L --- and we got 175 mg/L.”
The low BOD-to-TKN ratio impacts denitrification and nitrates levels. However, the plant is able to stay well below the nitrate limit without the use of supplemental carbon.
Boston started his new job in 2003 with a list of new plant options from the city’s engineers, McClelland Consulting Engineers of Fayetteville, AR. The selection included alternative oxidation ditch systems, as well as alternative sequencing batch reactors (SBR’s), as finalists that were deemed capable of handling the BOD, phosphorous, and nitrates requirements.
“OEM costs were the major consideration, and we also didn’t like any surface aeration option, with the cooling of our water during the winter months very likely causing a problem for nitrates compliance,” he recalled. “The nitrogen-fixing bacteria stop working when it’s cold. Another issue was dissolved oxygen. If oxygen is over 0.5 mg/L, the bacteria won’t work to pull it off the nitrates; they’ll take the oxygen that’s handy in the water. With the Fluidyne plant option that we selected, they have a stage of the process that handles all that.”
Boston noted that oxidation ditches were regarded as too capital-intensive, with major requirements for concrete and large motors, and not offering as much control of wastewater as SBR options did. He said the Fluidyne SBR-type system selected had the smallest footprint, and also one of the lowest electricity costs --- by an estimated 50-75%. It was also appealing that all pumps in that plant were the same brand.
“In addition to having selected a very sophisticated product that was not too hard to operate, we were lucky to have very good engineers and a very good contractor,” he said. “McClelland’s Johnny Quinn and Todd Beaver, and Crossland Heavy Contractors’ (Columbus, KS) Todd Zimmerman and Jason Lundry were tireless on the issues, day in and day out, especially the change orders. I had heard of other jobs where it was a nightmare to put through change orders.”
The change orders mainly dealt with groundwater issues, and having to raise the plant.
“Then we had Fluidyne’s Jason Bumgarner on site for a week during plant startup, training us in how it works and what to be looking for, and then their Targie Mandt for another week to reinforce all that and help us make some programming adjustments. They trained us on everything from the headworks through the end of their plant. We feel we have a real good understanding of how the process works, which sure does help us in our everyday operations.”
Fluidyne Corporation 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. The system at Decatur consists of three trains, each capable of operating independently, and providing redundancy.
Fluidyne’s system has consistently demonstrated 0.15-0.25 lbs of sludge production per lb. of BOD removal, compared to 0.5-1.0 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 soluble carbon produced in the anaerobic 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 Decatur, AR 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.