The Benefit of Aerobic Methanol Uptake on Denitrification Capacity
The Blue Plains Advanced Wastewater Treatment Plant (AWTP) in Washington, DC has a high rate activated sludge system followed by a suspended growth nitrification-denitrification system. Methanol is used for post-denitrification, because it is the least expensive synthetic compound available that does not leave residual biochemical oxygen demand (BOD) in the process effluent. The process achieves low total nitrogen values in summer, while during winter, the process is more difficult to operate due to increased flows and low temperature. These data suggest kinetic limitations of methylotrophic bacteria. The objective of this project was to evaluate methods for possibly overcoming limitations of methanol addition for full-scale facilities that have limited available cold-weather anoxic solids retention time (SRT). It was hypothesized that methanol addition under aerobic conditions (bleeding a small amount of methanol into the aerobic zone of a BNR process) could alleviate anoxic SRT limitations in a full-scale plant by increasing the active methylotrophic biomass content in the process. There is considerable uncertainty associated with the denitrification potential of aerobically grown methylophic bacteria and its effect on full-scale plant design/operation. Two sequencing batch reactors (SBRs) were operated at the Blue Plains AWTP to address these questions and to assess methylotroph kinetics. This reactor set-up was used to simulate, as closely as possible, the Blue Plains AWTP nitrification/denitrification system, with aerobic and post-anoxic reaction periods (nitrification was not attempted in the SBRs since the focus of this study was denitrification). For both reactors A and B, methanol was added at the beginning of the anoxic period. For reactor B only, methanol was added at the beginning of the aerobic period. SBR data suggested that aerobicallygrown methylotrophs have similar anoxic denitrification potential as strict anoxically-grown methanol utilizing bacteria. These preliminary results indicate that aerobically-grown methylotrophs have the potential to denitrify with kinetics similar to that grown anoxically and that a bleed of methanol into the aerobic zone of a BNR process could alleviate anoxic capacity limitations, albeit with increased methanol demand.