Ciliate populations and metals in an activated-sludge plant

Twenty-nine species of ciliate protozoa were recorded in samples of activated sludge collected monthly from tanks at Davyhulme Sewage Works, Manchester, U.K., over a two-year period (1991–1992). The most frequent species present included Aspidisca cicada, Chilodonella uncinata, Vorticella convallaria and Vorticella microstoma. The total ciliate population showed clear seasonal variation, with highest counts between February and August. Only two species showed significant correlation with ambient temperature. Species diversity had little variation, with 8–13 ciliate species being identified in most samples. Significant positive interspecies correlations were noted for ten species, but were not consistent for both years. Mean concentrations of the metals Fe, Ni, Zn, Cu and Cr in the liquid phase of activated sludge varied considerably over the two annual sampling periods, with no seasonal pattern. Major ciliates present were able to tolerate high concentrations of contaminant metals, including Fe (> 2000 ppb), Zn (> 500 ppb), Cu (> 60 ppb) and Cr (100 ppb). Specific positive and negative ciliate species—metal correlations were observed in each annual sampling period, but were not consistent for both years. The lack of consistent correlations over the two-year sampling period between populations of individual protozoan species and single biological (other species) or physicochemical (temperature, metal concentrations) factors is in accordance with a complex (multifactorial) determination of protozoan populations in activated sludge.