Treatment of perchlorate- and nitrate-contaminated groundwater in an autotrophic, gas phase, packed-bed bioreactor

The biological degradation of perchlorate was examined using a laboratory-scale, autotrophic, packed-bed biofilm reactor. The reactor was operated in unsaturated-flow mode and continuously fed water containing perchlorate (ClO₄^-) (as an electron acceptor), and a gas mixture of hydrogen (5%) and carbon dioxide at a retention time of 1.5min. In the absence of nitrate, perchlorate removal rate (r_P, ppb/min) in the reactor was found to be first order with respect to perchlorate concentration (c, ppb) according to r_P=0.16±0.06c^0.97±0.12 (n=11, R²=0.97, p<10^-5). Perchlorate removal rates in the hydrogen feed were found to be comparable to rates found by others for fixed film bioreactors using either hydrogen gas or organic electron donors such as acetate, although the rate coefficient was reduced to slightly less than unity (r_P=0.22±0.08c^0.91±0.08; n=19, R²=0.89, p<10^-5). When nitrate was present in the water, similar perchlorate removals were achieved despite nitrate concentrations three orders of magnitude higher than perchlorate concentrations. Perchlorate was removed by an average of 25±5% from a perchlorate-contaminated groundwater containing 73±2ppb of perchlorate and 21±2ppm of nitrate. This removal was slightly higher than the removal of 17±3% measured for a synthetic groundwater containing 79±3ppb of perchlorate and 22±2ppm of nitrate. In both cases, there was an average of 10% nitrate removal.