Fixed-bed bioreactor treating perchlorate-contaminated waters

Perchlorate (ClO₄^-) contamination of surface and groundwaters is widespread in the United States, particularly in the arid southwestern states. Perchlorate can be completely reduced to chloride by acclimated bacteria via cell respiration in fixed-bed bioreactors, but design factors, such as reactor packing and loading rate, need further investigation to optimize these systems for drinking water treatment. The use of granular-activated carbon (GAC) as a biofilm support medium was compared to that of sand for perchlorate bioremediation in acetate-fed fixed-bed bioreactors. Perchlorate was reduced in sand column bioreactors from 20 mg/L to below the detection limit (<4 μg/L) at empty-bed contact times (EBCTs) as low as 43 min (detention time of 18 min). GAC-packed columns performed similarly for the first 10 days, but, after the column was backwashed to redistribute biofilm-coated particles in the reactor, reactor efficiency was substantially reduced due to perchlorate desorption. Increasing the concentration of acetate in the feed partially restored reactor performance. However, effluent perchlorate levels fluctuated between 1.1 mg/L and the detection limit due to continued slow perchlorate desorption from the carbon. It was concluded that fixed-film reactors for perchlorate-contaminated waters should contain nonperchlorate-adsorbing packing in the reactor.