An evaluation process for optimizing activated-sludge floc-formation.

Abstract

Traditionally, the quality of activated sludge has been evaluated by the relative numbers of protozoa. This investigation has focused on the properties of the activated sludge floc. Specifically floc parameters such as strength, size, sludge density index, and number of filamentous organisms were evaluated. These sludge properties influence the efficiency of the final settling tanks and effluent quality. A high count of filamentous organisms in activated sludge cause bulking and foaming in the final settling tanks, which results in a poor effluent quality and it also affects sludge treatment processes. Floc properties vary over time and lead to different types of sludge. Historical data from the Detroit Wastewater Treatment Plant were reviewed and categorized on the basis of sludge type. The information derived allowed for better operational control of the filamentous bacteria and which lead to the optimization of floc-formation. As a result, the following guidelines are proposed for plants which treat low substrate wastewater. The mean cell residence time (MCRT) should be reduced when; the total ciliate count exceeds 20,000 organisms/ml, the pH is below 7.0 and the respiration rate is above 12.0 mg O$\sb2$/gm mixed liquor volatile suspended solids (MLVSS)/hr. A high ciliate concentration and a low pH are indicative of old sludge. Also, low pH indicates nitrification and an increased respiration rate reflects filamentous bacterial growth. When the respiration rate decreases below 9.5 mg O$\sb2$/gm MLVSS/hr, it signals a severe depletion of substrate (food) and this also stimulates the growth of filamentous bacteria. At this point, a reduction in primary treatment will provide a higher biochemical oxygen demand (BOD$\sb5)$ loading for the activated-sludge process. This manipulation will selectively favor floc-forming bacteria whose exocellular polysaccharide polymers increase the bio-absorptive capacity of the activated-sludge biomass. In conclusion, the results of this research emphasizes the need to continuously monitor the properties of activated sludge for early diagnosis and prevention of undesirable floc-formation and filamentous bacterial growth. As filamentous bacterial problems decrease floc become stronger and the bulk of solid materials are captured by the settling floc-matrix.