Managing the interactions between sulfate- and perchlorate-reducing bacteria when using hydrogen-fed biofilms to...

A groundwater containing an unusually high concentration (∼4000 μg/L) of perchlorate (ClO4(-)) and significant (∼60 mg/L) sulfate (SO4(2-)) was treated with hydrogen (H2)-fed biofilms. The objective was to manage the interactions between sulfate-reducing bacteria (SRB) and perchlorate-reducing bacteria (PRB) by controlling the H2-delivery capacity to achieve ClO4(-) reduction to below the detection limit (4 μg/L). Complete ClO4(-) reduction with minimized SO4(2-) reduction was achieved by using two membrane biofilm reactors (MBfRs) in series. The lead MBfR removed >96% ClO4(-), and the lag MBfR further reduced ClO4(-) to below the detection limit. SO4(2-) reduction ranged from 10 to 60%, and lower SO4(2-) reduction corresponded to lower H2 availability (i.e., lower H2 pressure or membranes with lower H2-delivery capacity). Minimizing SO4(2-) reduction improved ClO4(-) removal by increasing the fraction of PRB in the biofilm. High SO4(2-) flux correlated with enrichment of Desulfovibrionales, autotrophic SRB that can compete strongly with denitrifying bacteria (DB) and PRB. Increased SO4(2-) reduction also led to enrichment of: 1) Ignavibacteriales and Thiobacteriales, sulfide-oxidizing bacteria that allow sulfur cycling in the biofilm; 2) Bacteroidales, heterotrophic microorganisms likely using organic sources of carbon (e.g., acetate); and 3) Spirochaetales, which potentially utilize soluble microbial products (SMPs) from autotrophic SRB to produce acetate.