The role of ciliated protozoa in subsurface flow wetlands and their potential as bioindicators

The role of ciliated protozoa in the root zone method of wastewater treatme nt was assessed by analyzing ciliate community structure in four experiment al 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 fir st third of each bed contained a higher abundance and diversity than the fi nal third of the bed. There was a qualitative correlation between physicoch emical conditions and ciliate community structure: microaerophilic species dominated the organic-rich and oxygen-poor environment of the unplanted soi l bed; aerobic and facultative bacterivorous species dominated the better o xygenated gravel beds; a combination of these two communities was found in the planted soil bed. The average grazing rates of ciliates was around 5 ti mes 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 re tention time and ciliate abundance, it was calculated that ciliates, by the ir predatory activities, are capable of removing up to 2.35 x 10(5) and 0.4 5 x 10(5) E. roll in the first third of the planted gravel bed and unplante d soil bed, respectively. These results are discussed in relation to variat ions in E. coli removal kinetics. The potential for using ciliate communiti es as indicators of conditions within constructed wetlands is also assessed . (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.