Capability of ciliated protozoa as indicators of effluent quality in activated sludge plants

The aim of this study is to determine the relationship between the ciliate populations density and effluent quality in activated sludge plants. A total of 231 samples taken at three activated sludge plants were analyzed over a three year period. Seven physico-chemical parameters were examined and protozoa (in particular ciliate protozoa) and small metazoa were counted by means of optical microscopy. Effluent quality was determined from BOD5 and suspended solids concentration. For data analysis the species were classified in ranges in terms of abundance. Mean and standard deviation of effluents BOD5 and SS were calculated for each range. It was found that as the abundance of each species population increased, both BOD5 and suspended solids from effluents tended to a particular range of values (the optimal range) whereas the standard deviation diminished. Thus the higher the ciliate species population density, the better the capability of the species as an indicator, which is something reflected in the standard deviation. This capability of ciliated protozoa to act as indicators of effluent quality will also be limited by the other factors influencing the presence of species. It was observed that the correlation coefficients between ciliates and effluent quality depend on the range of physico-chemical values studied, i.e. whether they are superior or inferior to the “optimal range”. The values of each speciesʹ BOD5 optimal range varied from 4 mg/l to 18 ppm, suggesting that ciliates are good indicators between 4 and 18 ppm in activated sludge plants. In all cases observed, less common species such as Acineta tuberosa, Euplotes sp. and Zoothamnium sp. were indicators of high effluent quality. By contrast, the species that reach the highest densities and are the most common, such as Uronema nigricans, Vorticella microstoma and Opercularia coarctata, indicated lower effluent quality.