A Sludge Biotic Index (sbi) for the Evaluation of the Biological Performance of Activated Sludge Plants Based on the Microfauna Analysis

-An objective index, the Sludge Biotic Index (SBI), based on the microfauna associated with activated-sludge mixed liquor, has been devised to monitor activated-sludge plant performance. The proposed method makes it possible to define the biological quality of the sludge using numerical values (biotic index) of 0-10, and to group SBI values into four quality classes. The method is based on two principles. First, the dominance of the microfauna keygroups changes in relation to the environmental and operational conditions of the plant. In particular, some protozoa such as small flagellates and the ciliates Vorticella microstoma and Opercularia spp have a negative influence on the index value. Secondly, the number of morphological species is reduced as the performance of the plant gets worse. The advantage of this method over other schemes is that it provides numerical values which enable the operator to monitor the prevalent plant operating conditions on a daily basis. Key words--protozoa, activated sludge, Sludge Biotic Index, indicator species, plant performance