Managing the interactions between sulfate- and perchlorate-reducing bacteria when using hydrogen-fed biofilms to treat a groundwater with a high perchlorate concentration

- Groundwater with high ClO4- and SO42− was treated successfully.
- A two-stage system controlled the H2 delivery and the microbial ecology.
- Suppressing SO42− reducers allowed complete perchlorate remediation.
- The onset of SO42− reduction changed the community structure.

A groundwater containing an unusually high concentration (∼4000 μg/L) of perchlorate (ClO4-) and significant (∼60 mg/L) sulfate (SO42−) 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 SO42− 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. SO42− reduction ranged from 10 to 60%, and lower SO42− reduction corresponded to lower H2 availability (i.e., lower H2 pressure or membranes with lower H2-delivery capacity). Minimizing SO42− reduction improved ClO4- removal by increasing the fraction of PRB in the biofilm. High SO42− flux correlated with enrichment of Desulfovibrionales, autotrophic SRB that can compete strongly with denitrifying bacteria (DB) and PRB. Increased SO42− 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.

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