- The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.