Perchlorate reduction from a highly contaminated groundwater in the presence of sulfate-reducing bacteria in a hydrogen-fed biofilm

We used a hydrogen (H₂)-based biofilm to treat a groundwater contaminated with perchlorate (ClO₄^-) at ∼10mg/L, an unusually high concentration. To enhance ClO₄^- removal, we either increased the H₂ pressure or decreased the electron-acceptor surface loading. The ClO₄^- removal increased from 94% to 98% when the H₂ pressure was increased from 1.3 to 1.7atm when the total acceptor surface loading was 0.49gH₂/m²day. We then decreased the acceptor surface loading stepwise from 0.49 to 0.07gH₂/m²day, and the ClO₄^- removal improved to 99.6%, giving an effluent ClO₄^- concentration of 41μg/L. However, the tradeoff was that sulfate (SO₄^2-) reduction occurred, reaching 85% conversion at the lowest acceptor surface loading (0.07gH₂/m²day). In two steady states with the highest ClO₄^- reduction, we assayed for the presence of perchlorate-reducing bacteria (PRB), denitrifying bacteria (DB), and sulfate-reducing bacteria (SRB) by quantitative polymerase chain reaction (qPCR) targeting characteristic reductases. The qPCR results documented competition between PRB and SRB for space within the biofilm. A simple model analysis for a steady-state biofilm suggests that competition from SRB pushed the PRB to locations having a higher detachment rate, which prevented them from driving the ClO₄^- concentration below 41μg/L.