Characterization of microbial populations in landfill leachate and bulk samples during aerobic bioreduction

Aerobic microbial populations in landfill leachate and bulk material were characterized during an engineered aerobic bioreduction process in a test cell of a municipal landfill in Atlanta, Georgia, USA. Assessment of the microbial ecology (bacterial numbers, species, and substrate utilization patterns) of this engineered system was undertaken to determine its biological status during the progression of the remediation process. Counts of aerobes in leachate increased by two orders of magnitude during the first 5 months of air injection. Bacterial counts in solid samples collected from various depths in the cell varied more than three orders of magnitude during the fifth month of treatment, exceeding counts in leachate by as much as three log units. In the ninth month of treatment, bacterial counts in bulk material were non-detectable in some cases, suggesting stability of the degraded waste material. Although bacterial species in leachate and bulk samples varied with sample collection date, eight species were identified in samples from multiple sampling dates. Only two Gram positive and six Gram negative species were isolated from both leachate and bulk material, and none of the yeast (Candida sp. or Cryptococcus sp.) isolated from solid samples was found in leachate. Analysis of the substrate utilization patterns of individual bacteria isolated from leachate collected on sequential sampling dates indicated a decrease in the percentage of Gram negative bacteria able to metabolize selected sugars with a concomitant increase in the percentage of Gram positive bacteria able to metabolize them. The amino acids tested were not readily utilized by Gram positive or Gram negative bacteria from either sample type. The observed decrease in percentage of bacteria able to metabolize specific substrates may have resulted from a decrease in substrate availability as waste stabilization, which was the goal of the project, began. (C) 2001 Elsevier Science Ltd. All rights reserved.