In order to observe the impact of different water compositions on sludge dewaterability, assessments of floc sizes using a particle size analyzer and of sludge dewaterability based on the capillary suction time (CST) test were carried out. Synthetic raw water had small floc sizes, and synthetic domestic wastewater had both larger median floc sizes and a better correlation between sludge dewaterability and median floc sizes. The floc size distribution results showed that synthetic raw water is associated with a narrow particle size distribution. In comparison, synthetic domestic wastewater produced a wider distribution. However, the CST values were similar for both waters. Compared to synthetic wastewater, natural wastewater had the largest distribution with generally larger particle sizes.