Microscopic Image Analysis versus Sludge Volume Index to Monitor Activated Sludge Bioflocculation: A Case Study

The influent ratio of Mono- over Polyvalent cations (M/P ratio) influences activated sludge bioflocculation. A more thorough insight through means of conventional macroscopic parameters, Sludge Volume Index (SVI), and Effluent Suspended Solids (ESS), and microscopic image analysis was pursued in this work. Therefore, a long term experiment was conducted during which activated sludge was exposed twice to a transition in influent M/P ratio from 1.5 to 15. Two distinct observations were made. The first transition indicates the existence of a loosely attached layer evidenced by the temporary release of small sludge particles as observed by image analysis. This release was followed by a decrease in the amount of small sludge particles and increase in floc size indicating good bioflocculated sludge. The second transition results in a massive release of small particles leading to complete sludge deflocculation. (De)flocculation was monitored by both SVI and microscopic image analysis, whereas during the first transition the change in bioflocculation condition could only cleary be monitored by means of microscopic image analysis. Based on this study, microscopic image analysis proves to be a powerful monitoring tool to assess the bioflocculation condition of the activated sludge, whereas SVI is unable to accurately monitor its bioflocculation condition.