MICROBIAL POPULATION SEGREGATION DURING TREATMENT OF POLLUTED GROUNDWATRE IN AN ION EXCHNGE MEMBRANE BIO-REACTOR

Ion Exchange Membrane Bioreactor (IEMB) used to treat groundwater polluted with high perchlorate (423 mg L^-1), nitrate (204 mg L^-1) and chlorate (268 mg L^-1). The contaminating anions are transported across an anion exchange membrane from a water-compartment into a bio-compartment by Donnan dialysis where they microbiologically reduced. Glycerol was used as a carbon source and electron donor in this study. An enrichment culture that was cultivated on perchlorate used to seed the reactor. Biofilm developed in the bio-compartment side of the membrane controlled anions total flux across the membrane. The total abundance of functional genes coding for perchlorate and nitrate reduction was below 10% of the total population. On the membrane biofilm, structural changes in perchlorate reductase gene (pcrA) abundance were obtained with 7×10⁵ copies /cm² at the membrane inlet rising to ~10⁷ copies /cm² at the outlet. The microbial populations in the reactor were markedly different from the membrane biofilm. At the end of the experiment, the suspended biomass contained 33% γ-proteobacteria, 6% α-proteobacteria, 2% Firmicutes and 41% bacteroidetes. The biofilm consisted of 34.6% γ-proteobacteria, 33.8 % α-proteobacteria, and 8.6% bacteroidetes. It is proposed that in the reactor, high fermentable carbon and low electron acceptors selected for γ-proteobacteria with Klebsiella as a dominating genus. The fermentation products, ethanol, 1,3-prpandiol and acetate, used by α-proteobacteria like Azospirillum in the biofilm to reduce the incoming electron acceptor. Still, the role of bacteroidetes in the reactor is not known. These results suggest that the niche exclusion of microbial populations in between the reactor and membrane is controlled by the fluxes of the electron donors and acceptors.