Abstract: The invention relates to a method for cleaning water or an aqueous flow electrochemically by flotating impurities contained in water for collecting the impurities from a surface of the water. The method includes conveying the water flow to be cleaned through at least one particle bed which behaves bipolarically under electric voltage, which bed is formed of an anode and a cathode and metal particles arranged between the anode and the cathode; leading a changing direct current to the particle bed to maintain electrochemical reactions on anodic regions and cathodic regions of the particles; and dissolving metal of the particles electrochemically to water to split water to micro bubbled hydrogen gas H2 for the flotation and to hydroxide ions OH— for increasing pH of water. The invention relates also to an apparatus for cleaning water or an aqueous flow electrochemically.