OPTIMIZATION OF STRATEGIES FOR SEPARATE CENTRATE TREATMENT VIA PARTIAL NITRIFICATION AND DENITRIFICATION IN NEW YORK CITY WATER POLLUTION CONTROL PLANTS

The New York City Department of Environmental Protection (NYCDEP) has been researching methods of treating a high-strength ammonia recycle stream (centrate) to reduce the total nitrogen (TN) discharge to Long Island Sound and Jamaica Bay. One technology under investigation is a two-stage reactor system consisting of biological oxidation of ammonia to nitrite ("nitritification") and subsequent denitrification using methanol as the carbon source and electron donor. Restricting the oxidation of ammonia to nitrite instead of completely oxidizing it to nitrate is economically advantageous since the oxygen requirements are lower and the less methanol is required for denitrification.The impact of SRT, pH, temperature, alkalinity addition, reactor configuration, variable influent characteristics and the methanol feed rate on nitrite production and denitrification efficiency was evaluated. Over a SRT range of 2 to 6 days, a pH range of 6.7–7.7 and an average temperature of 21°C, 35 to 80% ammonia oxidation was achieved, producing NOx composed of 60 to 95% nitrite. At the average temperature, ammonia oxidation was not impacted significantly by the presence of excessive bicarbonate alkalinity to support autotrophic biomass growth. Increasing the SRT to values over 4 days under these conditions led to increased nitrification, but the percentage of nitrite in the total NOx decreased significantly. Specific nitrification rates of 1.0-1.5 mgN/mgVSS/day were obtained from in-situ batch tests.The denitrification reactor experienced poor settling of the mixed liquor solids in the clarifier, causing continuous washout of the biomass and the need to reseed often with mixed liquor solids from a step-feed BNR reactor. However, up to 80% denitrification was realized when the centrate contained a high solids concentration over an extended period of time. In-situ batch tests were used to determine the methanol requirements for denitrification using nitrite and nitrate as the electron acceptors. As anticipated, less methanol was required for nitrite reduction than for nitrate. Overall TN removal across the two-stage reactor system ranged from 14% to 42%.