A method for removal of dissolved materials and/or particles from a liquid stream comprising flocculation by vigorously mixing polysaccharides or hydrocolloids into the liquid stream and subsequent gelation of the polysaccharides or hydrocolloids with cations capable of gelling same. Large, strong gelled agglomerates of pollutant and polysaccharide or hydrocolloid are formed with the help of suitable cations. The gelled agglomerates are well suited to separation from a liquid stream by flotation, in a sieve, in a centrifuge, in a cyclone or via other conventional separation processes. Also described is a method for mixing of un-dissolved polysaccharide or hydrocolloid with a polluted aqueous stream by shearing the polysaccharide or hydrocolloid into the aqueous stream to improve logistics and provide economic savings in treatment of large volumes of water. Also described is the use of dilute solutions of polysaccharides for treatment of a variety of effluent streams. An application of the invention is provided for removal of hydrocarbons within the oil industry, but also for other kinds of oil-containing water and also water with organic or particulate pollution.