Perchlorate as a secondary substrate in a denitrifying, hollow-fiber membrane biofilm reactor

In recent years, several oxyanions have emerged as drinking water micropollutants, including arsenate, selenate, bromate, and, most recently, perchlorate (ClO₄^-). Conventional water treatment processes typically are ineffective in removing these compounds, and advanced treatment processes are expensive. Biological reduction may provide a suitable treatment alternative, since these compounds can serve as electron acceptors. Other acceptors, such as nitrate (NO₃^-), must act as a primary electron acceptor. We tested our denitrifying, autotrophic, hydrogen-oxidizing hollow-fiber membrane for ClO₄^- reduction. The reactor is highly suited to drinking water treatment, as hydrogen (H₂) is inexpensive, non-toxic, and does not leave residuals that can cause regrowth. When 1 to 2 mg/L ClO₄^- was supplied to reactor, which was at steady-state with 5 mgN/L NO₃^- but unacclimated to ClO₄^-, ClO₄^- removals increased from 40 to 99% over three weeks. Removals to 4 μg/L were also achieved in a natural groundwater having 6 to 100 μg/L ClO₄^-. Tests with variable NO₃- and H₂ showed that ClO₄^- reduction requires less than 30 μgN/L NO₃- and at least 300-μg/L effluent H₂. Therefore, partial denitrification is probably not consistent with excellent ClO₄^- removal. Mineral medium produced a gradual loss of ClO₄^--reducing bacteria, but they were re-enriched when tap water replaced minimal medium.