The ratio of flocs to filaments in activated sludge waste water treatment plants is of extreme importance for the overall performance of the plant. In order to control this ratio the individual concentrations of flocs and filaments need to be measurable. However, no sensors which can measure these concentrations are currently available. It is proposed that by means of image analysis techniques the ratio of flocs to filaments can be determined. Combination of this ratio with the total biomass concentration results in the individual floc and filament concentration. This contribution focuses on the last step of the image analysis procedure, i.e., the classification of objects as either floc or filament. Five different shape parameters, i.e., aspect ratio, roundness, form factor, fractal dimension and reduced radius of gyration, are evaluated and compared. The results indicate that the form factor is the least suitable and the reduced radius of gyration the most suitable shape parameter to accurately classify flocs and filaments in activated sludge images.