Membrane Biofilm Reactor Process for Nitrate and Perchlorate Removal

The objectives of this project were to evaluate the performance of an innovative hydrogen-fed membrane biofilm reactor (MBfR) for nitrate and perchlorate removal, and identify the system operational and design parameters that affect the biological reduction process. The MBfR contained hollow-fiber membranes within a cylindrical module. Hydrogen was fed to the fibers filling the inside and passively diffusing through the membrane avoiding the formation of a hydrogen atmosphere and serving as an electron donor for the biofilm growing on the outside of the hollow fibers. The biofilm within the reactor was developed from the indigenous bacteria present in the groundwater and was not artificially inoculated or amended. Following the MBfR, an aeration process was employed to oxygenate the water in preparation for its introduction into a distribution system as a drinking water source. A subsequent media filter captured any sloughed biomass and provided a support for aerobic organisms to remove any residual dissolved hydrogen.  Process Demonstration The MBfR process was demonstrated at pilot-scale to reduce perchlorate contaminated groundwater (55 ?g/L) to below the current 4 ?g/L California Department of Health Services (CaDHS) perchlorate action limit. The simultaneous removal of influent dissolved oxygen and nitrate to below detection limits was also observed. Their removal has been observed with the MBfR system operated at system flow rates corresponding to theoretical hydraulic residence times between 15 and 60 minutes. In addition, measured hydrogen consumption closely matches theoretical calculations based on stoichiometry and anaerobic biomass development. Microbial Ecology Bench-scale investigations into the microbial ecology of the mixed cultures in MBfRs revealed that perchlorate reducing bacteria (PCRB) were found to be present in a denitrifying system that had not been previously exposed to perchlorate. However, a dominant PCRB species increased from 14 to 21 percent of total bacteria when 100-?g/L perchlorate was added to the influent. Increasing perchlorate reduction led to further increases in the dominant PCBR and the perchlorate-removal capacity of MBfRs. Another important finding is that oxygen alone can serve as a primary acceptor for perchlorate reduction, and that the oxygen reduction appeared to be more favorable for perchlorate reduction than was nitrate reduction. Ultimately, full-scale application MBfR technology could effectively and economically be used to replace costly treatment technologies currently used by municipalities to treat perchlorate-contaminated drinking...