A Novel Testing Approach for BNR Optimization in NYC
- Sarah Galst PE, Robert Sharp PE, PhD, Michael Lynch PE, Paul Pitt PhD - Hazen and Sawyer
- Keith Mahoney PE - New York City Department of Environmental Protection
Background to the New York City BNR program
The New York City Department of Environmental Protection (NYCDEP) is currently in the final stage of implementation of its over 20-year upgrade of four Wastewater Treatment plants (WWTPs) to provide Biological Nitrogen Removal (BNR) to reduce Total Nitrogen (TN) discharges to the East River. BNR upgrades are needed to reduce effluent TN discharges from approximately 18-20 mgN/L to 7-8 mgN/L. The Hunts Point (HP) WWTP (200 mgd) is one of the BNR plants, and has recently completed installation of a supplemental carbon system to optimize overall nitrogen removal. This paper will describe the unique approach to optimization of carbon addition to high-rate step-feed BNR process, and the development of a simple, operational tool for plant staff to maintain required levels of treatment.
This unique full-scale comparison directly demonstrated the impact of carbon dose and location, as well as the influence of operational variables such as flow splits, treatment of nitrogen rich return streams, and in-tank dissolved oxygen levels. Full-scale sampling result, including the comparisons between “test” and “control” ATs, were combined with results from dynamic simulations of a calibrated BioWin™ model equipped with a glycerol nitrite-lock add-on to develop a simple operation tool to allow operators to effectively meet required TN criteria year-round.
BNR Upgrades at HP
New four-pass (A through D) step-feed BNR facilities were completed in 2010 at the HP WWTP which included the following:
- Baffles and mixers to create anoxic and anoxic-oxic switch zones
- Process air upgrades (blowers, air filters, diffusers)
- Froth Control (Froth Hoods, RAS Chlorination, Surface Skimming, Polymer Addition)
- Increased RAS and WAS capacity
- Improved metering and control for RAS/WAS
- Chemical facilities for alkalinity, polymer, and sodium hypochlorite
- Separate centrate treatment in an existing aeration tank
Following the implementation of the initial BNR upgrades, new carbon (glycerol) addition facilities were constructed at the HP WWTP. Glycerol was chosen as the supplemental carbon source because it is safer to handle and store compared to methanol and ethanol, and was shown to be an effective carbon source.
In order to effectively optimize BNR treatment with glycerol addition at the HP WWTP, a novel approach was taken which included using one aeration tank (AT) as a ‘control’ reactor, with no carbon addition, while two other ATs were selected as ‘Test’ reactors, each receiving carbon at different doses and dosing locations. One of the Test reactors was evaluated with multiple-point glycerol addition, while the other was tested with single-point addition and a modified flow distribution to minimize operational complexity.
Intensive sampling and monitoring were conducted over three months in the summer/fall of 2015, including diurnal nitrogen profiles of the control and test aeration tanks, specific denitrification rate batch tests, and operational monitoring. The results of these sampling events were discussed with plant staff during weekly meetings, and changes to improve effluent quality were implemented and tested the following week.
Results demonstrated that the addition of glycerol resulted in a reduction in effluent TN concentration by approximately 2 mg/L. Optimization of the multiple-point addition ‘test’ tank showed that dosing Passes B and C provided the best overall nitrogen removal, while avoiding the higher effluent Nitrite concentrations that can be observed from the ‘nitrite-lock’ phenomenon common in glycerol-addition facilities (R. Sharp, Kinetics of Glycerol Acclimated Biomass: Implications on Plant Operations and Performance, WEF/IWA Nutrient Removal and Recovery, Vancouver, Canada, July 2013). Additionally, a one-point carbon addition strategy was deemed successful during the warm-weather testing, with equivalent performance observed in terms of overall effluent TN removed. However, the question of optimization during colder weather months was not addressed in this warm weather sampling program; as such, a modeling exercise was conducted to provide guidance for year-round optimization of plant operations.
A BioWin process modeling effort was conducted, with a model calibration and verification to the performance observed during the summer/fall 2015 testing program. A Glycerol Accumulating Biomass (GAB) add-on, previously created by Envirosim (R. Sharp, D. Houweling, Modeling Glycerol Acclimated Biomass: Using BioWin Add-On to Model the Nitrite Lock, WEFTEC, New Orleans, LA, September2014), was used to simulate the nitrite-lock, and more accurately predict performance with glycerol addition.
The calibrated model was used to determine four seasonal operating targets (summer, spring, fall, and winter) for solids inventory, dissolved oxygen, flow distribution, operation of anoxic/oxic switch zones, carbon/alkalinity dosing locations, and carbon/alkalinity flows.
Generation of SOPs
This unique approach to carbon optimization allowed for a combination of filed monitoring and modeling results to be used in the development of an easy-to-use visual poster which was provided to plant staff. This poster outlined the seasonal operational targets for solids inventory, dissolved oxygen, flow distribution, operation of anoxic/oxic switch zones, carbon/alkalinity dosing locations, and carbon/alkalinity flows. Operational targets were provided for two conditions; all ATs online and one AT out of service for routine maintenance/repair. Additionally, the poster provided operational procedure to follow in the event that effluent quality began to deteriorate, with modifications to various parameters titrated against effluent ammonia concentrations.
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