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Studies on the processes of bacteria elimination in constructed wetlands
Citation key AlexandrinoFernandes2006
Author Alexandrino Fernandes, Maria de Fátima Azevedo
Year 2006
School Technische Universität Berlin, FG Umweltmikrobiologie
Abstract The bacteriologic purification efficiency of three multistage subsurface flow constructed wetlands with vertical and horizontal reed beds was analyzed by culture independent methods. All wetlands are located in Germany and treat domestic wastewater of 100-1,000 population equivalents. Microscopic enumerations of the wastewater bacteria at different purification stages revealed that bacteria removal efficiency of the vertical beds was high (between 93% and 99.5%), whereas the contribution of the horizontal reed beds to the bacteriological purification of wastewater was low and erratic (average of 46%). However, when assessed with standard cultivation procedures, the purification efficiency of the horizontal reed beds was higher than that of the vertical reed beds (98% vs. 95%). Comparison of the data obtained by microscopic counts with standard cultivation revealed that the fraction of culturable bacteria was especially low in the effluent of the horizontal reed beds. Consequently, the purification capacity of the horizontal beds was significantly overestimated by the standard cultivation procedures. A PCR-based approach for rapid, sensitive and reliable detection of four waterborne, VBNC forming enteropathogenic bacteria (Campylobacter jejuni, Escherichia coli O157:H7, Helicobacter pylori and Yersinia enterocolitica) was tested in the wastewater samples. The highest sensitivity was obtained with the protocol for Y. enterocolitica with 5 cells per 100 ml of treated wastewater and 200 cells per 100 ml of settled wastewater. The lowest sensitivity was obtained for E. coli O157:H7 with 250 target cells per 100 ml of treated wastewater and 10,000 target cells per 100 ml of settled wastewater. All PCR methods could be performed within 12 hours and were specific and reproducible in a background of 1011 non-target cells per 100 ml of wastewater. None of the investigated pathogenic bacteria were detected in the investigated constructed wetlands. Therefore, in order to study defined aspects of the processes of pathogen elimination in vertical beds, the fate of a pathogenic Y. enterocolitica strain was investigated in a lab-scale fixed-bed reactor. The reactor was constructed and operated in order to simulate a reed bed for wastewater treatment. The reactor was percolated with primary settled wastewater amended with the pathogenic strain of Y. enterocolitica. The TaqMan assay was selected and optimized for tracing Y. enterocolitica in the fixed-bed reactor. The assay displayed low detection limits (200 target cells ml-1 of wastewater and 400 target cells g-1 of filter medium). After three weeks of operation biofilm formation in the reactor bed was low: only 0.16% of organic matter accumulated in the filter and bacterial densities varied between 106-107 cells g-1 of dry weight filter medium. Despite of low biofilm formation in the filter bed, wastewater bacteria were retained at a nearly constant rate (97%). The retention of Y. enterocolitica by percolation amounted to 99.8%. A significant elution of bacteria from the reactor filter bed was not detected. Effluent bacterial populations were constituted by a sub-population of the not retained influent bacteria. A similar retention mechanism seemed to have taken place in the full-scale reed beds. These assumptions are supported by the following data: • Cultivation and in situ hybridization indicated that the physiologic activity of the filter medium bacteria was lower than the physiologic activity of both the influent and effluent bacteria. • The morphology of the effluent bacteria was rather similar to a sub-population of the influent bacteria and not to the morphology of filter medium bacteria • The amounts of influent and effluent bacteria were nearly proportional After 8 days of percolation with Y. enterocolitica, the pathogen accumulated in detectable amounts in the filter bed. Y. enterocolitica concentration was highest in 3 cm depth (104 cells g-1 dw) and decreased to 102 cells g-1 dw in 30 cm depth. However approximately 99% of the total retained Y enterocolitica cells were eliminated in the reactor filter bed. The autochthonous wastewater bacteria were eliminated by approximately 96%. Abiotic factors like low moisture and organic matter content, and relatively high temperatures (25ºC) probably contributed strongly to bacteria elimination in the reactor column. In the full-scale plants, the physical factors were more favorable for bacteria survival in the filter beds, since moisture and organic matter content were higher and temperatures in general lower. On the contrary, antagonistic relationships between bacterial species and predation probably played a more significant role in bacteria elimination in the full-scale filters than in the lab-scale filter. In both the constructed wetlands and the fixed-bed reactor protozoa were almost only detected in the aqueous phase and in spots of the filter beds with high bacterial density. The results derived from the analysis of the full-scale and lab-scale systems suggest that, for long-term operation, the vertical flow operation modus combined with low biofilm formation can constitute a feasible strategy for reliable bacteriologic wastewater purification in subsurface flow constructed wetlands.
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