The paper focuses on the properties of flocs that are required to service the needs of solid–liquid separation in conventional treatment units. Beyond the basic requirements of minimum size and surface nature to enable effective trapping on a collecting surface, field data indicate that floc water content affects treatability (as gauged by the hydraulic loading rate or treatment rate). Floc blanket clarifiers, DAF and filter units are examined in terms of implications of floc size, density and strength, insofar as they interact with the process dynamics. It is shown that these features come into play both individually and collectively and are sensitive to the floc fractal structure. It is argued that floc strength often plays a critical role in separation processes through its impact on floc size. Recognising the benefits of densification, the paper provides specific advice on how densification can be achieved. Among the many options considered, it is noted that the most straightforward means of securing greater density or strength, is by use of smaller flocs, this also benefiting dewaterability.