PURPOSE: To make the operation of a water-treating plant easier by carrying out a back-wash of a filter installed on the top of a sedimenting tank by opening an opening connecting with a back-wash tank by a float, and by positioning a sludge valve at a height at which the total volume of filtered substances remains in a sedimenting section.

CONSTITUTION: Water to be treated is transported by a pump (6) through a flow-in pipe (5), where a measured amount of additives necessary for treating the water are added, to enter a sedimenting tank (4) along its tangential line. Agglomerated substances are held at a filter (1) positioned on the top of the sedimenting tank (4). The water passed through the filter (1) pushes up a float (15) to close an opening (16), flows into a back-wash tank (14) through a by-pass (17) and then is controlled and discharged by a flow-out valve (20). Back-wash starts in cases, a treating volume is too small, etc. By opening sludge valve (11), the float (15) opens the opening (16), generating a flow rate to the water in the back-washing tank (14) enough to agitate and clean the filter (1). The sludge valve (11) is installed at a height with which a volume of a sedimenting section (4) corresponding to the total volume of the substances on the filter (1) is left.