Survival of indicator microorganisms and enteric pathogens in wetlands

Wetlands containing aquatic plants have been found to economically provide a mechanism for improving the microbial and other quality of wastewater. The purpose of this study was to elucidate the effect of vegetation and sedimentation on the survival of enteric microorganisms in constructed wetlands. The first part of this study was designed to determine the effect of vegetation on the survival of Escherichia coli, Salmonella typhimurium , bacteriophage MS-2 and poliovirus in wetlands. The organisms were added to the water from six wetland systems, containing different aquatic plants. The wetland systems received either fresh water or secondary sewage. The presence of aquatic plants significantly increased the die-off of E. coli, S. typhimurium, bacteriophage MS-2, and poliovirus in fresh water and secondary sewage. Additional work on the survival of E. coli in non-sterile, filter sterilized, and autoclaved wetland water indicated that one of the plausible mechanisms of bacterial die-off in constructed wetlands is through increased microbial competition or predation. The next phase of this study investigated the survival of indicator microorganisms in wetlands similar to field conditions. E. coli, bacteriophage MS-2, and PRD-1 were added to tanks which were unvegetated or contained different aquatic plants. E. coli die-off in unvegetated tanks was greater than the vegetated tanks. Temperature was found to be significantly correlated with the die-off of E. coli. Inactivation of bacteriophage MS-2 in unvegetated tanks was also higher than the vegetated tanks. In contrast, PRD-1 inactivation was greater in the vegetated tanks compared to the unvegetated tanks. Thus, a combination of unvegetated and vegetated wetland would probably result in the greatest reduction of microorganisms by the wetlands. The last phase of this study was to examine the relative occurrence and survival of indicator microorganisms and pathogens in the water column and sediments of two constructed surface flow wetlands. On a volume/wet weight basis the concentration of fecal coliforms and coliphage in the water column and sediment was similar. Giardia and Cryptosporidium concentration in the sediment were one to three logs higher in the sediment compared to the water column. The die-off rates of all the organisms were greater in the water, except for Giardia muris. Giardia die-off in the sediment was greater than in the water column. These results suggest that sedimentation may be the primary removal mechanism for the larger organisms in surface flow wetlands. Overall, these studies suggest that vegetation, microbial competition or predation, temperature/sunlight, and sedimentation play important roles in microbial reduction in constructed wetlands. The effect of vegetation on microbial survival appeared to be indirect, through increasing microbial competition. However, vegetation in constructed wetlands may offset the effect of temperature and sunlight, resulting in a longer survival of microorganisms. Thus a combination of non-vegetated and vegetated wetland would probably result in the greatest reduction of microorganisms from wetlands. Future experiments are needed to examine such combined wetlands