Estimating Air Emissions Resulting from Implementation of Supplemental Carbon to Enhance Denitrification in Water Pollution Control Plants

The New York City Department of Environmental Protection (NYC DEP) plans to use supplemental carbon (methanol and/or ethanol) in the activated sludge process to enhance biological nitrogen removal (BNR) at 2 of the City's 14 water pollution control plants (WPCPs). As part of the environmental analysis of this process, the volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions resulting from supplemental carbon addition needed to be quantified. Methods were developed to estimate air emissions resulting from BNR operation with supplemental carbon addition using the TOXCHEM+ model. Specifically, anoxic zones were simulated by using the model's aeration tank unit process with adjustments to more accurately predict typical anoxic kinetics. All unit processes were updated with expected typical BNR operation parameters.Process-specific biodegradation rates and half-saturation constants for methanol and ethanol were developed for the TOXCHEM+ air emissions model. This was done to predict volatilization rates of supplemental carbon sources to aid denitrification in NYC DEP's step-feed BNR biological reactors. Process-specific biodegradation rates for methanol and ethanol were determined based on 40 CFR Part 63, Appendix C, guidance for both anoxic and aerobic conditions. These process-specific biodegradation rates are more representative of anticipated kinetic values for the full-scale facilities than the current default values in the model.Testing was conducted in an aerated reactor batch test (BOX test) at the NYC DEP PO-55A Applied Research Facility, where biomass in the pilot reactors—specifically designed and operated to perform biological nutrient removal using NYC's step-feed BNR configuration—is acclimated to either methanol or ethanol. This testing helped determine the aerobic and anoxic biodegradation rates specific to the step-feed BNR process for methanol and ethanol that reflect the kinetics from an acclimated biomass.Zero-order kinetics were observed down to the detection limit (2 mg/L) of the gas chromatograph Therefore, the half-saturation constant, Ks, could not be directly determined from the data. Extrapolation of data using defining equations determined that the “best-fit” half saturation constant is 0.1 mg/L. Using a half saturation constant of 0.1 mg/L, the experimentally derived Monod biodegradation rate coefficient, kb, is 0.2400 L/mg/hr for methanol in aerobic conditions and 0.0475 L/mg/hr for methanol in anoxic conditions. Likewise, the experimentally derived kb is 0.3430 L/mg/hr for ethanol in aerobic conditions and 0.1433 L/mg/hr for ethanol in anoxic conditions when the half-saturation constant is 0.1 mg/L.However, to provide conservative emission estimates, NYC DEP recommends using a half saturation constant of 0.5 mg/L. Corresponding biodegradation rates are 0.0480 L/mg/hr and 0.0095 L/mg/hr for methanol in aerobic and anoxic conditions, respectively, and 0.0685 L/mg/hr and 0.0287 L/mg/hr for ethanol in aerobic and anoxic conditions, respectively.