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

Comparative studies were carried out on both pilot-scale and laboratory sys tems (microcosms) in order to determine the Escherichia coli removal rates and to investigate the E. coli removal kinetics in various designs of const ructed 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 austral is, and the type of planting medium (pea gravel or soil). The average E. co li 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 expl ained by a combination of lower retention time and possibly less efficient microbial communities. Removal kinetics studies show that most E. coli remo val occurs in the first third of the system, however there was a clear diff erence in the removal kinetics between the planted gravel bed and the unpla nted soil bed with the population of E. coli decreasing faster in the grave l bed. A comparison of two hybrid microcosms revealed that the system compr ising a planted soil followed by a planted gravel gave better E. coli remov al than the system comprising a planted gravel followed by a planted soil. (C) 2000 Elsevier Science B.V. All rights reserved.