Effects of salinity on simultaneous reduction of perchlorate and nitrate in a methane-based membrane biofilm reactor

This study builds upon prior work showing that methane (CH4) could be utilized as the sole electron donor and carbon source in a membrane biofilm reactor (MBfR) for complete perchlorate (ClO4 (-)) and nitrate (NO3 (-)) removal. Here, we further investigated the effects of salinity on the simultaneous removal of the two contaminants in the reactor. By testing ClO4 (-) and NO3 (-) at different salinities, we found that the reactor performance was very sensitive to salinity. While 0.2 % salinity did not significantly affect the hydrogen-based MBfR for ClO4 (-) and NO3 (-) removals, 1 % salinity completely inhibited ClO4 (-) reduction and significantly lowered NO3 (-) reduction in the CH4-based MBfR. In salinity-free conditions, NO3 (-) and ClO4 (-) removal fluxes were 0.171 g N/m(2)-day and 0.091 g/m(2)-day, respectively, but NO3 (-) removal fluxes dropped to 0.0085 g N/m(2)-day and ClO4 (-) reduction was completely inhibited when the medium changed to 1 % salinity. Scanning electron microscopy (SEM) showed that the salinity dramatically changed the microbial morphology, which led to the development of wire-like cell structures. Quantitative real-time PCR (qPCR) indicated that the total number of microorganisms and abundances of functional genes significantly declined in the presence of NaCl. The relative abundances of Methylomonas (methanogens) decreased from 31.3 to 5.9 % and Denitratisoma (denitrifiers) decreased from 10.6 to 4.4 % when 1 % salinity was introduced.