Dispersion effects analysis and constructed wetlands treatment of oyster shell contacted beds water purification systems by tracer test

Abstract

Being the advanced study of utilizing wasted oyster shells as contacted bed media, the purpose of this study is to discuss the influence of fluid mixing on constructed wetlands treatment. We choose Erchong Floodway in Taipei County as our experimental site, and use NaCl as the tracer to perform pulse tracer tests to discuss residence time distribution of the oyster shell constructed wetland and dispersion effects of solute transport by using longitudinal dispersion theory. This study provide treatment parameters of the oyster shell wetlands and discuss the applicability of advanced water quality models. The results reveal that hydraulic retention time will be underestimated by using nominal retention time. Mean hydraulic retention time is about 2.68~2.75 nominal retention time in an 8 meter length wetland. This underestimation will cause errors of the efficiency of water purification. By tracer tests, the mean dispersion coefficient of oyster shell wetlands is 0.015 m2/min, between the schoenoplectus and soil subsurface treatment wetlands. The study also points out that the dispersion coefficient can be seen as a constant in such low velocity surroundings. Eventually, we compare 1-D plug flow model (PF) and plug flow model modified by dispersion (PFD), and find out that it will overestimate the efficiency of water purification without considering fluid dispersion effect.