RESUMO
Microbiological analysis of activated sludge is an important tool for monitoring wastewater treatment plants (WWTP). The utilization of the sludge biotic index (SBI) provides helpful information in examining the quality of biological treatment process and has been tested for several different systems. Although its utilization has been increasing, it is still not widespread, especially in Brazil. Also, its applicability has been considered limited for some particular systems. Thus, it becomes important to evaluate the relations among operational and biological parameters of each WWTP in order to characterize the system and its variations. In this work, microscopic analysis were performed once a week for 1 year (n = 54) and the results were compared to the physicochemical, operational parameters and efficiency of the plant along the period. The four seasons were comprised and analyzed, as we cannot neglect the influence of environmental changes in this subtropical region. Not only had we found a strong influence of the evaluated parameters on the structure of the biological community but there is also a good correspondence of SBI with the performance of the WWTP. More importantly, including microscopic analysis in the operational routine made it possible to notice even the slightest changes in the biological community that were not enough to diminish the SBI classification of the sludge, but were satisfactorily informative to show in advance to operators when to take corrective actions about an increase of COD and BOD in the influent and when it was necessary to discard the exceeding sludge.