On-line optical monitoring of activated sludge floc morphology

An on-line device for the optical monitoring of activated sludge flocs was set up and tested in situ at a municipal wastewater treatment plant over eight months. A charge-coupled device camera was used to image the flocs and filaments, and an automated image analysis programme was used to analyse the morphological parameters of the flocs. Results were compared to those obtained from offline laboratory analysis for water quality indicators, such as suspended solids. The results showed that the activated sludge process has produced different levels of settling quality. Large and round flocs enabled good settling, whereas small and irregularly shaped flocs, as well as the large amount of filaments, led to poor settling. The image analysis results obtained from the on-line device indicated that the poor settling was most likely caused by filamentous bulking. Such bulking was most likely due to the compositional variations in incoming wastewater; the poorly settled sludge was characterised by much lower solids content, which could have been responsible for the growth of filamentous bacteria. The image analysis results also revealed that the changes in floc morphology occurred slowly, indicating the potential for optical monitoring to control and optimise the activated sludge process.