The role of ciliated protozoa in the root zone method of wastewater treatment was assessed by analyzing ciliate community structure in four experimental subsurface flow wetlands (planted or unplanted in either soil or gravel) and estimating Escherichia coli removal due to ciliate predation. A total of 22 different ciliate taxa were isolated from the four reed beds. The first third of each bed contained a higher abundance and diversity than the final third of the bed. There was a qualitative correlation between physicochemical conditions and ciliate community structure: microaerophilic species dominated the organic-rich and oxygen-poor environment of the unplanted soil bed; aerobic and facultative bacterivorous species dominated the better oxygenated gravel beds; a combination of these two communities was found in the planted soil bed. The average grazing rates of ciliates was around 5 times higher in the planted gravel bed (49 bacteria/ciliate/hour) than in the unplanted soil bed (9.5 bacteria/ciliate/hour). Taking into account the retention time and ciliate abundance, it was calculated that ciliates, by their predatory activities, are capable of removing up to 2.35 × 105 and 0.45 × 105E. coli in the first third of the planted gravel bed and unplanted soil bed, respectively. These results are discussed in relation to variation in E. coli removal kinetics. The potential for using ciliate communities as indicators of conditions within constructed wetlands is also assessed.

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