Abstract: This invention relates to a process for the controlled flocculating or precipitating of foreign substances from a liquid by the addition of a flocculating agent, at least a partial quantity of the liquid being treated with a dosed amount of the flocculating agent, the flocculation or precipitation caused thereby in said partial quantity being measured by a turbidity measurement and the addition of the flocculating agent to the liquid to be treated being determined on basis of the turbidity measurement. An apparatus for the carrying out of such a process also constitutes the object of this invention.