Determination of Methanol and Ethanol Biodegradation Rates for the New York City Step-Feed Biological Nutrient Removal Water Pollution Control Plants

Process-specific biodegradation rates and half-saturation constants for methanol and ethanol were developed for use in the TOXCHEM+ air emissions model for predicting volatilization rates of supplemental carbon sources to be used to aid denitrification in the New York City Department of Environmental Protection (NYC DEP) 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 more representative of the anticipated kinetic values for the full-scale facilities than the current default values in the model.Testing was conducted from May–July 2006 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 stepfeed BNR configuration, is acclimated to either methanol or ethanol. Testing was conducted to 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, in the interest of being conservative in estimating emissions, using a half saturation constant of 0.5 mg/L is currently recommended by the NYC DEP. 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.