The present invention is directed to methods for purifying wastewater, including water which may or may not contain fatty acids or oils, by producing in situ a hydrophobic flocculant including a water-insoluble metal hydroxide. In the preferred methods, metallic cations are produced by the gradual dissolution of consumable, similar metal sources, preferably, a fluidized bed of metallic particles. The motion of the particles of the fluidized bed maintains the surfaces thereof clean of oxides and other contaminants in order to facilitate the gradual dissolution thereof. Oxidation of the dissolved cations to form the water-insoluble metal hydroxide is achieved by flowing an oxidizing agent, e.g., air or pure oxygen, through the system. Contaminants are entrapped and removed within the insoluble flocculants, including the water-insoluble metal hydroxide and, optionally, metallic soaps which are formed in the presence of fatty acids. Thus, clean, contaminant-free water may be separated and removed from the insoluble flocculants and entrapped contaminants.