Abstract: The invention relates to a method and system for treating an aqueous stream having a first flow rate and containing solid matter exhibiting first settling properties, the method comprising adding a modifying agent to the aqueous stream at an addition rate sufficient to change the first settling properties of the aqueous stream to obtain a modified aqueous stream having solid matter exhibiting second settling properties different from the first settling properties; taking batchwise samples of the modified aqueous stream to a settling vessel having a volume; determining a settling property of the solid matter of the samples in the settling vessel; and conducting the modified aqueous stream to a separation unit in which solid matter is separated from the modified aqueous stream. The invention provides a convenient way of monitoring and/or controlling aqueous streams conducted for example to purification processes.