Substrate effects on the enhanced biotransformation of polychlorinated hydrocarbons under anaerobic condition

An investigation involving the effects of different substrates on the biotransformation of heavily chlorinated hydrocarbons was performed under anaerobic condition to assess the availability of the substrate on the in situ bioremediation of contaminated groundwaters. The substrate-fed batches were anaerobically incubated with either acetate, glucose, methanol, or humic acid (HA) as supplemental substrate with concentrations ranging from 10 to 30 mg/L. Experimental results indicated that the rates and extents of the disappearance of the chlorinated compounds differed depending on type and concentration of the supplemental carbon sources employed. Better removal efficiencies were observed for glucose-, methanol-, and acetate-fed batches, and least for HA-fed batches. The sequence of the efficiency of enhancement is consistent with the sequence of the reducing potentials of the dechlorination reactions. When the substrate concentrations in the batches were increased from 10 to 30 mg/L, highest removal efficiencies of carbon tetrachloride (CT), tetrachloroethylene (PCE), and 1,1,1-trichloroethane (TCA) increased from 96%, 49%, and 61% to 99.9%, 61%, and 69%, respectively. The viable counts of microorganisms determined by the direct epifluorescence counting technique revealed that the batches with higher concentrations of the supplemental substrate produced higher bacterial cell numbers. Moreover, from microscopic observations, different compositions of bacterial population were found. Small bacteria with spheric shape were observed when culture bottles were amended with either acetate or HA. Whereas, large bacteria with rod-shape were predominant in bottles amended with glucose. Also, higher DNA contents were demonstrated for glucose-fed batches.