Bound water content of activated sludge and its relationship to solids retention time, floc structure, and surface properties

The bound water (BW) content of sludge from four bench-scale sequencing bat ch reactors (SBRs), fed a glucose-based synthetic wastewater and operated a t different solids retention times (SRTs), and from five full-scale, activa ted-sludge systems treating different wastewater was determined using the d ilatometric technique. Storing sludge at 4 degreesC before analysis resulte d in a lower measured BW value. The SRT and surface charge were not correla ted to the BW content of sludge from the bench-scale SBRs (SRT range of 4 t o 20 days) or full-scale systems (SRT range of 4 to 25 days). A greater BW content was strongly associated with a smaller median floe siz e in bench- and full-scale studies, indicating the importance of floe size in relation to the BW content. In bench-scale studies, greater levels of to tal extracellular polymeric substances (EPS) correlated with an observed in crease in the BW content of sludge. Individual EPS components either weakly correlated to or were not associated with changes in BW. Sludge samples fr om the bench-scale SBRs with greater BW content were found to have greater sludge volume index values, For the full-scale systems studied, there was n o relationship between EPS and DW content of the sludge.