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

A groundwater containing an unusually high concentration (~4000μg/L) of perchlorate (ClO₄^-) and significant (~60mg/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 (4μg/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.