Abstract This paper deals with the influence of the global velocity gradient G (in the range of 28–307 s −1 ) on properties of suspension (size and structure) formed during water treatment. Furthermore, it describes the influence of these properties on separation using depth filtration (filtration velocities of 3 and 6 m h −1 ). The methods of image and fractal analysis were used to determine the aggregate size and structure, respectively. The experiments were conducted in a pilot plant (a mixing tank and a rapid gravity sand filter) with ferric sulphate used as coagulant. The experiments confirmed that with an increasing velocity gradient the aggregate size decreased; the aggregate size distribution narrowed (suspension was more homogenous); the fractal dimension D 2 increased (aggregates were more compact) and the fractal dimension D pf diminished (aggregates were more regular). The smallest and most compact aggregates ( d ∼ 60 μm and D 2 = 1.9) formed at G > 200 s −1 displayed the best filterability with the time of filtration up to 78 h. On the contrary, the filtration of large and porous aggregates formed at G ∼ 30 s −1 lasted only about 30 h.