Abstract: A method for treating a liquid wherein the liquid to be treated is caused to flow through a tank equipped with at least one electrode assembly in the base thereof for the generation of gas bubbles by electrolytic action, whereby suspended materials, usually but not necessarily solids, in the liquid are carried to the surface of the tank by said bubbles to form a layer of thickened material, characterised by the step of dissolving air under pressure into the liquid before it is caused to flow through said tank, whereby the amount of gas available for flotation purposes exceeds that available from the electrolytic action alone.

Abstract: A method of filtering a liquid comprising the steps of providing a bed of particulate material, withdrawing material from the base of the bed, supplying replacement particulate material to the bed in such a manner that the bed has a free inclined upper surface, and causing the liquid to be filtered to flow through the bed by entering the same through said inclined upper suface, and apparatus for carrying out such method. Preferably the particulate material is comprised by particles of various sizes whereby a classifying action takes place to give a structured filter bed.