The role of flagellated and ciliated Protozoa in lagoon and grass filter sewage treatment systems

The abundances and grazing impact on bacteria of ciliated protozoa and heterotrophic nanoflagellates in a lagoon series and grass filter sewage treatment system, at the Melbourne Water Treatment Plant at Werribee in Australia, were investigated between April and July 1995. Bacterial concentrations decreased along the lagoon series and the grass filtration systems, but were twice as high in the latter. Heterotrophic nanoflagellates (HNAN) followed the same trend in distribution as their bacterial food supply, but were 10 times higher in the grass filter. Ciliates, however, showed a much more variable distribution in both systems and similar densities. Calculated grazing rates indicated that HNAN were responsible for removing the highest proportion of bacterial biomass in both systems. In the grass filter ciliate grazing was usually below 11% that of the HNAN and in the lagoons ciliates never exceeded 11% of the HNAN grazing levels. HNAN were important in these two treatment systems as the major consumers of bacteria and contributed to the production of good quality effluents. This contrasts with other types of sewage treatment where ciliates have been shown to be the major grazers of bacteria.