The Impact of Digester Retention Time on Microbial Extracellular Polymer Production and Sludge Dewaterability

Abstract

Anaerobic digestion of wastewater sludge has found widespread use as a method of sludge stabilisation, but the waste sludge remaining can be more difficult to dewater. One factor thought to influence sludge dewaterability is the level of microbial extracellular polymer present in the sludge. Utilising laboratory scale anaerobic digesters, this study aimed to assess the effect that digester retention time has on extracellular polymer production and its associated impact on sludge dewaterability. Six digesters were constructed and operated, as matched pairs, over a range of retention times (10, 15, and 30 days) for 4 full retention periods. The end point extracted extracellular polymer yield was related to the sludge dewaterability, as measured using the capillary suction time test. There was a strong, positive correlation (r2=0.999) between the extracellular polymer yield and sludge dewaterability, but no relationship was found between extracellular polymer yield and digester retention time. Further analysis of the extracted polymer organic and inorganic content demonstrated that the digester retention time had no effect on polymer composition, but polymer composition did appear to have an impact on the sludge dewaterability.