Investigation of Escherichia coli removal in various designs of subsurface flow wetlands used for wastewater treatment

Comparative studies were carried out on both pilot-scale and laboratory systems (microcosms) in order to determine the Escherichia coli removal rates and to investigate the E. coli removal kinetics in various designs of constructed wetlands employing horizontal subsurface flow through the root zone of emergent macrophytes (RZM). Parameters investigated include the presence or absence of hydrophytes, in this case the common reed Phragmites australis, and the type of planting medium (pea gravel or soil). The average E. coli removal rates were 41-72% for microcosms, and 96.6-98.9% for pilot-scale systems. The poorer removal rates achieved by the microcosms might be explained by a combination of lower retention time and possibly less efficient microbial communities. Removal kinetics studies show that most E. coli removal occurs in the first third of the system, however there was a clear difference in the removal kinetics between the planted gravel bed and the unplanted soil bed with the population of E. coli decreasing faster in the gravel bed. A comparison of two hybrid microcosms revealed that the system comprising a planted soil followed by a planted gravel gave better E. coli removal than the system comprising a planted gravel followed by a planted soil. (C) 2000 Elsevier Science B.V. All rights reserved.