Dichloromethane biodegradation under nitrate‐reducing conditions

An Acinetobacter sp. was isolated from activated sludge that grows on dichloromethane (DCM) as the sole source of organic carbon and energy under both nitrate‐reducing and aerobic conditions. Inhibitor experiments with chloroacetonitrile indicated that glutathione was involved in dechlorination of DCM under aerobic and anoxic conditions. Yield and endogenous decay coefficients were estimated using volatile suspended solids (VSS) data from five pairs of aerobic and denitrifying reactors, operated in a daily draw‐and‐fill mode at residence times of 2.5, 3, 4, 5, and 6 days. The maximum daily amount of DCM added was 11.8 mM, which is equivalent to a chemical oxygen demand (COD) of 378 mg/L. For aerobic conditions, the yield and endogenous decay coefficients were 0.31 mg VSS/mg DCM COD and 0.14 day−1, respectively, versus 0.23 mg VSS/mg DCM COD and 0.090 day−1 for anoxic conditions. The DCM depletion curves from all 10 reactors were used to estimate the maximum specific growth rate, which was 1.1 and 0.89 day−1 under aerobic and anoxic conditions, respectively. In contrast, there was no significant difference in the maximum specific substrate utilization rate, which was approximately 3.8 mg DCM COD/mg VSS · d when either oxygen or nitrate served as the electron acceptor. The stoichiometry of DCM and nitrate consumption was 0.58 mole NO−3/mole of DCM. At this rate, the cost of supplying nitrate as a terminal electron acceptor is higher than for oxygen, but other factors (such as DCM volatilization during addition of oxygen and lower biomass yield during denitrification) may favor use of nitrate.