City Meets New Strict Discharge Limits While Coping with Weekend Influent Drop

City of Decatur, AR, reports that Replacement of the city of Decatur, Arkansas, aerated pond wastewater treatment system with a special activated sludge process has provided for compliance with a more stringent NPDES permit, while coping with a regular 90-95% weekend decrease in wastewater input, and major seasonal variations in ambient temperature.

The public works manager for the City of Decatur, AR, reports that replacement of the city's aerated pond wastewater treatment system with a special activated sludge process 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 new 2.2 mgd 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.

Especially clear, treated effluent discharging from the ISAM™ process. Typical effluent for 2011 has averaged less than 5 mg/L BOD and TSS; less than 6 mg/L TN; and less than 0.3 mg/L TP.

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 1990s 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 until the new plant was operating for six months, thanks to a 50% reduction of sludge from the new system.

"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," said James Boston, the city's public works manager.

The old system couldn't reliably meet the new, unforgiving instantaneous limit, where 11 mg/L on one day would require the city to write a non-compliance report for a whole month of data.

"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," Boston said.

When considering a new plant, the city wanted a system that could 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.

Chief Operator Mike Liley at soft-touch control panel, which allows operators to check on anything that's going on in the plant, such as when pumps are running, when aeration is blowing, the dissolved oxygen (DO) level, and when sludge is being returned.

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.

New Treatment System

Boston started his new job as public works manager 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 (SBRs), 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 said.

The city eventually selected a an SBR-type system from Fluidyne Corp.

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 selected SBR-type system 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.

SBR basins in settle, aeration, and decanting modes. Elimination of primary solids in anaerobic chamber allows for much smaller SBR basins, at equivalent SRT, than with conventional SBRs.

Fluidyne describes its 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.

The Fluidyne 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 SBRs.

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 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.

The new system has lived up to expectations, Boston said.

"We've been amazed to get the numbers we got with this new plant," Boston said, "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."