| An activated sludge microbial community consists mainly of bacteria, which play the major role in removing organic compounds from wastewater. Because the performance of an activated sludge system depends on its microbial community, whatever affects the structure and activity of that community is a major concern. Therefore, the purpose of this research was to investigate the response of an activated sludge microbial community to toxicants. The response was measured in terms of changes in community structure and changes in the community's susceptibility to other toxicants having similar or dissimilar mechanisms of action.; To be able to distinguish between naturally occurring changes and changes caused by exposure to toxicants, prior to the toxicity study, experiments were performed to monitor natural variations in the microbial community structures of two identically operated lab-scale systems. The results suggested that those structures would diverge in character if the cultures were subjected to a significant perturbation upon being started up. In addition, replicate activated sludge systems were found to not be identical, although they can be very similar if started appropriately.; Next, 2,4-dimethylphenol (2,4-DMP), a polar narcotic toxicant, and 2,4-dinitrophenol (2,4-DNP), an uncoupler, were used for acclimation of the biomass. The susceptibility to other toxicants having similar or dissimilar mechanisms of actions was then measured before and after the acclimation period. Exposure to the selected toxicants resulted in adverse effects that were more evident in system performance than in bacterial community structure. Reductions in biomass yield and changes in the susceptibility of the communities to other toxicants were also observed after addition of the compounds. Most importantly, the results showed that exposure of a microbial community to inhibitory levels of a toxic organic compound acting on the cytoplasmic membrane resulted in a community with less susceptibility to other toxicants that act on the membrane, regardless of their mechanism of action. The degree of resistance resulting from that exposure was proportional to the degree of stress imposed by the toxicant. |
