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 batch reactors (SBRs), fed a glucose‐based synthetic wastewater and operated at different solids retention times (SRTs), and from five full‐scale, activated‐sludge systems treating different wastewater was determined using the dilatometric technique. Storing sludge at 4 °C before analysis resulted in a lower measured BW value. The SRT and surface charge were not correlated to the BW content of sludge from the bench‐scale SBRs (SRT range of 4 to 20 days) or full‐scale systems (SRT range of 4 to 25 days). A greater BW content was strongly associated with a smaller median floc size in bench‐ and full‐scale studies, indicating the importance of floc size in relation to the BW content. In bench‐scale studies, greater levels of total extracellular polymeric substances (EPS) correlated with an observed increase in the BW content of sludge. Individual EPS components either weakly correlated to or were not associated with changes in BW. Sludge samples from 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 no relationship between EPS and BW content of the sludge.