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The New York City Department of Environmental Protection (NYCDEP) is implementing step-feed biological nitrogen removal (BNR) with supplemental carbon addition in existing NYC water pollution control plants (WPCP) to reduce the total nitrogen discharge to Western Long Island Sound and Jamaica Bay. During the winter periods, the switch zones in the step-feed BNR tanks may be aerated to sustain nitrification, resulting in a reduced total anoxic volume and the need for higher specific denitrification rates (SDNR), thereby allowing the plants to continue to comply with their total nitrogen discharge permit limits. To achieve the plant performance requirements at the lowest cost a seasonal supplemental carbon addition strategy has been proposed.

The NYCDEP has evaluated the use of methanol and ethanol as carbon sources to enhance denitrification at its Applied Research Facility located at the 26th Ward WPCP. The facility houses several pilot-scale step-feed biological nitrogen removal (BNR) systems with each pilot operating continuously on primary settling tank effluent and a selected source of supplemental organic carbon. Pilot 3 uses methanol while pilot 2 uses ethanol as the carbon source. The two sources of carbon were selected on the basis of the low unit cost of methanol and the expected higher specific denitrification rate (SDNR) of ethanol.

The objective of this study was to measure the specific denitrification rate of VSS acclimated to methanol and ethanol for two sources of carbon, methanol and ethanol, individually and in different blends. In addition to these tests, the SDNR provided by a sludge that has not been exposed to either methanol or ethanol was measured. All measurements were conducted using bench scale completely mixed batch reactors operated at room temperature and at 13°C to simulate winter wastewater temperatures. The most salient findings of this study include;

The SDNR, measured in reactors where the source of VSS was pilot 3 (methanol acclimated), and methanol the supplemental carbon, can be expressed as (SDNR)T = 0.0738 (1.11)(T-20) where SDNR is in mg NOx-N per mg VSS per day and T is temperature in degrees centigrade. The SDNR values had a similar trend with temperature but of slightly higher values when methanol was fed to reactors with VSS from pilot 2 (ethanol acclimated).

The SDNR, measured in reactors where the source of VSS was pilot 2 (ethanol acclimated), and ethanol was the supplemental carbon, can be expressed as (SDNR)T = 0.161 (1.13)(T-20) where SDNR is in mg NOx-N per mg VSS per day and T is temperature in degrees centigrade. The SDNR values for ethanol were higher than for methanol, namely 2.18 times higher at 22°C and 1.89 at 12°C. However, even when the source of seed was pilot 3, the SDNR values were 1.6 times higher than for methanol.

A gradation of methanol/ethanol blends was fed to a reactor with VSS from pilot 3. The presence of ethanol resulted in maximum increases in the SDNR of approximately 1.5 at 22°C and 1.9 at 13°C.

In a series of batch experiments using a combination of ethanol and methanol and VSS from pilot 2, pilot 3, and an aeration tank from 26th Ward WPCP, (VSS not acclimated to either ethanol or methanol), the data suggested that acclimation is needed for both ethanol and methanol to achieve maximum benefit. Ethanol facilitated a higher value of SDNR for all three sources of VSS, namely: for the un-acclimated seed −2.1 times, for the methanol-acclimated seed −1.6 times, for the ethanol-acclimated seed −2.3 times.

Ethanol may be the preferred source of carbon for start-up of the denitrification process. Once denitrification is achieved, ethanol can be readily replaced by methanol for a more cost effective operation from the Spring through Fall operating periods.

Document Type: Research Article


Publication date: 2007-01-01

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