The overall aim of this study was to investigate the activated sludge by using sludge activity (TTC) screening and microscopic method in determining the differences in the activated sludge of the three different wastewater treatment plants located in Hungary.Activated sludge system is the most extensively used biological wastewater treatment process. The process does not function without oxygen content (it is aerobic), it involves the incessant flow biological method for removal of soluble organic materials which are engrossed through the cell walls of the microorganisms and into the cells and conversion of these materials into more microorganisms, carbon dioxide, water, and energy.