Assessing activated sludge morphology by laser and image analysis

Abstract

An excess of filamentous bacteria is a common settlement problem affecting the activated sludge process. In this study activated sludge morphology was monitored by image analysis and laser scattering. These were compared with the conventional sludge volume index. Good correlations between the two optical techniques for floc size were found when the small and interfering particles (< 50 μm) were removed from the analysis (activated sludge size 100–500 μm). A simpler image analysis procedure was developed using common software and equipment available in most laboratories. This should enable reproducible microbial morphology measurements to be used more widely routinely to predict the onset of poor settlement. The image analysis was used to determine the standard index of filament-induced poor settlement, namely the total extended filament length. The results in common with some previously noted work did not show a simple link between either floc size or total extended filament length and sludge volume index. A new indicator of excess filaments is suggested, based on the frequency of occurrence of just the long extended filaments (> 100 μm) which was shown to give better prediction of poor settlement (bulking).