Estimating Methanol Emissions from the Denitrification Process at the DC Water Blue Plains Facility

The District of Columbia Water and Sewer Authority (DC Water) conducted a study to estimate the annual atmospheric emission rate of methanol from the nitrification/denitrification process at the Blue Plains Advanced Wastewater Treatment Plant. To provide an accurate assessment of the daily emission rates that compose the annual total, which reflect the effects of variable operating conditions, a methanol fate model was constructed, consisting of a methanol biokinetics model of the nitrification/denitrification reactor anoxic zones coupled with a mass transfer model. Methanol data from multiple anoxic zone mixed liquor sampling events was collected for comparison to the model-predicted concentrations for the purpose of validating the model. Through a rigorous statistical analysis, the model was found to over-predict the liquid phase methanol concentrations in the anoxic zones by 6 to 13%.Gas phase methanol data was collected from the surface of a limited number of anoxic zones with a flux chamber apparatus for comparison to the emission rates calculated from the methanol data and the model-predicted concentrations. The emission rate estimated with the flux chamber data was roughly 50% of the modeled values. Despite the differences, the annual emission rate estimates are in agreement that methanol emissions for a typical operating year are on the order of 0.5 ton/year or less. Under a worst case operating year (2003), corresponding to historically high flows, the estimated emission rate is approximately 1.4 tons/year or less, which is well below the regulatory 10 ton/year threshold limit for an individual Hazardous Air Pollutant (HAP).