Plant Achieves Enhanced Biological Nutrient Removal Through High-Efficiency, Small Footprint Processes
Compact biological aerated filtration and high-rate sand-ballasted settling allows the Metro Syracuse, NY, wastewater treatment plant to consistently meet stringent effluent ammonia and phosphorous limits.
Facing a 12-month rolling average effluent phosphorus limit of 0.12 mg/L, and summer and winter effluent ammonia limits of 2.0 mg/L and 4.0 mg/L respectively, plus scarce available space at its treatment plant site, Onondaga County realized it needed innovative treatment technologies. The subsequent addition of high-efficiency, small footprint processes for enhanced biological nutrient removal has allowed the plant to consistently meet these very low limits, and has played a key role in the markedly improved water quality conditions in Onondaga Lake.
Onondaga County’s Metropolitan Syracuse Wastewater Treatment Plant (Metro) provides high quality wastewater treatment for approximately 270,000 residents of the City of Syracuse and surrounding communities. The plant is designed to treat an average monthly flow of 84 million gallons per day, with peak flows of 126 mgd and a hydraulic capacity of 240 mgd. The plant discharges treated effluent to Onondaga Lake.
In 1998, Onondaga County signed an amended consent judgment (ACJ) with the State of New York to significantly increase the level of treatment at its Metro plant. The ACJ is designed to improve the water quality of Onondaga Lake and achieve full compliance with state and federal water quality regulations by December 1, 2012. In addition to Metro plant improvements, many pollution abatement and cleanup efforts are being focused on this lake to enhance its role as an important aesthetic and recreation resource for Central New York.
When the ACJ was drafted, the Metro treatment plant was considered to be a major source (>50 percent) of phosphorus to Onondaga Lake. Furthermore, Metro plant effluent was the largest source of ammonia to the lake, averaging about 90 percent of the total annual input.
Demonstration Project
To develop design criteria and determine the effectiveness for year-round removal of ammonia, Onondaga County Department of Water Environment Protection (WEP) initiated a demonstration project to remove ammonia from wastewater at the Metro plant using biological aerated filtration, testing two different systems at various temperatures and flow rates. The county also implemented a pilot project to evaluate six different phosphorus treatment technologies. In addition to nutrient removal performance, physical size was a major consideration when evaluating these technologies. The Metro plant is located next to Onondaga Lake, and the length of pilings (275 feet) required for construction dictated that the new facilities following secondary treatment be as small in size as possible.
The Metro plant’s ACTIFLO® system consists of four treatment trains, each rated for 31.5 mgd, and uses microsand as a ballast to greatly increase the settling velocity of flocculated material. The process employs typical coagulation chemistry along with a high molecular weight polymer to flocculate material and adhere it to the microsand.
Based on the results of the demonstration project, pilot study and evaluated bid, WEP selected the BIOSTYR® biological aerated filtration technology for ammonia removal and the ACTIFLO® high-rate flocculated settling for advanced phosphorus removal. The selected technologies had the smallest footprints of any commercially available alternative and, through the extensive competitive trials, were shown to provide the lowest operational costs possible for the high level of treatment required.
This significant investment in state-of-the-art wastewater treatment technology has achieved far lower discharges of ammonia and phosphorus.
Ammonia Removal
The BIOSTYR technology combines biological treatment with upflow filtration in a single, multi-cell structure. By combining fixed film biological treatment with filtration, reactor volume is minimized and the need for final clarifiers is eliminated. The process is a biological aerated filter (BAF) with a submerged media bed. Wastewater flows upward through the media bed, and air is injected through an air grid located below the bed at the bottom of the cell and rises upward concurrently with the wastewater.
Each of the plant’s 18 BIOSTYR cells has 11.8 feet of polystyrene beads held in place by a combination of concrete slabs with removable plastic nozzles. The beads are buoyant and provide a highly effective surface area for nitrifying bacteria to attach and convert the ammonia to nitrate. A pre-cast concrete nozzle deck, located above the media, is outfitted with nozzle-type strainers that retain the media while allowing water and air to pass through the cell.
The biological aerated filtration system, in operation since May 2004, provides high quality effluent, according to John Saraceni, Process Control Director for the Metro plant.
“We were mandated a 2006 effluent ammonia limit of less than 2.0 mg/L in the summer and 4.0 mg/L in the winter. We were also mandated a 2012 effluent ammonia limit of 1.2 mg/L summer and 2.4 mg/L winter,” Saraceni said. “We began meeting our 2012 limit soon after we started up our BIOSTYR system, and our current permit now mandates this more stringent limit.”
Saraceni reported that system operation has been easy.
“We make sure all the equipment serving it is running properly, but otherwise it runs itself. We’ve taken it off-line a few times for equipment maintenance and the process has come back very quickly each time.”
Saraceni also said the ammonia removal process is not affected by cold temperatures.
“We can have cold winters here, with our water temperature getting down to 48°F, and sometimes a little lower. This hasn’t seemed to affect the process.”
Enhanced Phosphorus Removal
The Metro plant’s ACTIFLO enhanced ballasted flocculation/clarification system, which came on-line in 2005, is achieving effluent phosphorus limits of 0.12 mg/L on a 12-month rolling average. The system uses coagulation, flocculation (assisted by polymers), and sedimentation (assisted by microsand) to convert the soluble phosphorus to a particulate form that is readily removed.
The system consists of four treatment trains, each rated for 31.5 mgd, and uses microsand as a ballast to greatly increase the settling velocity of flocculated material. The process employs typical coagulation chemistry along with a high molecular weight polymer to flocculate material and adhere it to the microsand. The system provides short mixing times and high clarifier rise rates.
In the sand-ballasted flocculation process, raw water is first mixed with a coagulant in a high-shear environment where it is retained for two minutes. In the next tank, the water is injected with a polymer along with microsand and mixed aggressively for approximately another two minutes. When the water enters the “maturation zone,” gentle shear is applied for an additional six minutes. The microsand-ballasted flocs increase in size, trapping smaller flocs before the water enters the sedimentation tank, where the large flocs immediately begin to settle.
At this stage, the clarified water counterflows upward through settling tubes to collection troughs. The microsand and other solids in the ballasted flocs that settled in the bottom of the tank are then pumped to a hydrocyclone centrifuge. The microsand is cleaned and re-injected for reuse and the waste solids are removed.
“We are meeting our effluent phosphorus limit,” Saraceni, said. “Our ACTIFLO system is designed to take influent phosphorus of 0.75 mg/L per day and bring it down to 0.12 mg/L per day, which it does well under these parameters. For our plant, to get effluent phosphorus levels this low, there’s virtually no wiggle room.”
The plant experiences incredibly high storm flows during the rainy season. What most plants experience three or four times a year, the Metro plant can see three or four times a month. During these periods, the system’s effluent phosphorus levels remain consistent.
Successful Nutrient Removal
Improvements to the treatment system at the Metro plant are primarily responsible for the improved water quality conditions in Onondaga Lake. Phosphorus and ammonia concentrations in the lake have declined significantly as loading reductions have been achieved at the wastewater treatment plant.
The biological aerated filter system has resulted in year-round nitrification. Prior to the installation and various other improvements to the treatment system, plant effluent was the largest source of ammonia to the lake, averaging about 90 percent of the total annual input. Today, it contributes significantly less.
Slightly more than half of the phosphorus entering Onondaga Lake had come from the Metro plant before the installation of high-rate sand-ballasted settling. In 2005, the year the process was added, the wastewater plant contributed only about 29 percent of the total external phosphorus load to the lake.
Wastewater treatment plants are today facing more demanding requirements to improve nutrient removal, especially in areas threatened with eutrophication. Many of these areas, like Onondaga County, have aggressive plans aimed at reducing nutrient loading, thereby placing stepped-up pressures on treatment plants to perform. Onondaga County found that advanced wastewater treatment technologies providing high-efficiency, small footprint processes provide it the best solution for meeting its tough nutrient removal challenges.
About the author: Tom Perry is Regional Product Manager for Kruger Inc., a Veolia Water Solutions & Technologies company.
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