Successful operation of the activated sludge process relies heavily on the ability to effectively separate mixed liquor into return activated sludge (RAS) and a high quality final effluent.
Solids separation efficiency is dictated in large part by the relative population of floc-forming and filamentous microorganisms present in the activated sludge. During normal operation, mixed liquor will be composed primarily of floc-formers with relatively low levels of filamentous organisms.
Some population of filamentous organisms is necessary to promote the formation of larger flocs and prevent development of a “pin-floc” condition, which often results in high effluent turbidities (Jenkins, 2004). However, when conditions favoring the growth of filaments over floc-formers occur, a condition commonly referred to as filamentous “bulking” may occur. Filamentous bulking may develop in response to specific wastewater characteristics or secondary process operating conditions. Some facilities may consistently experience bulking as a result of process design and configuration issues that favor the growth of filamentous organisms.
Filamentous bulking can severely interfere with proper settling and compaction characteristics in the secondary clarifier, potentially causing high blanket levels, low RAS concentrations, and loss of effluent solids. During bulking episodes, high sludge volume indices (SVIs) may be measured at levels well beyond 150 mL/g, which is a typical measure of a “good settling” sludge. Poor settling sludge requires significantly more aeration basin and secondary clarifier capacity (Parker et al., 2003), in addition to requiring higher RAS rates, higher waste activated sludge (WAS) flow rates and chemical conditioning dose rates at downstream thickening processes, and higher hydraulic flows to downstream digestion processes.
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