modifying agents are added to the stream samples can be taken after each of respective addition points so that the effect of these modifying agents can be assessed separately according to one embodiment both a coagulant and a flocculant are provided to the aqueous stream the coagulant preferably as a first modifying agent at a first addition point and the flocculant as a second modifying agent at a second addition point downstream from the first addition point the aqueous stream is conducted to a separation unit comprising e g a sedimentation basin or a flotation unit having a volume the ratio of the volume of the settling vessel to the volume of a separation unit is typically 1 100 to 1 10 000 000 in particular about 1 1 000 to 1 1 000 000 in other words the settling vessel in which the turbidity and or any other settling property is determined in considerably much smaller in volume than the actual separation unit whereby considerable reduction of monitoring and or control delay is achieved in a preferred embodiment the settling property determined is the turbidity of the aqueous sidestream in the settling vessel after a certain period of time for example 60 1200 seconds measured from the interruption of the sidestream it is also possible to measure as the settling property the change of turbidity or change of amount of solid matter locally present in the sample i e the settling velocity of the solid matter over a predetermined period of time the period can be generally about 1 to 1200 seconds long or.