Patent ID: 7799232

Claim:
A method for treating wastewater to remove contaminants from wastewater by adding a reducing iron compound to wastewater containing contaminants to precipitate the contaminants, followed by separating the precipitate by a solid-liquid separation to remove the contaminants from the wastewater, the method comprising: a reducing iron compound addition step of adding a reducing iron compound to wastewater; a precipitation step of leading the wastewater to which the reducing iron compound is added to a reaction tank, and forming a precipitate; a solid-liquid separation step of separating the formed precipitate by a solid-liquid separation to obtain a sludge; and a sludge return step of alkalinizing all or a portion of the separated sludge to form an alkaline sludge, followed by returning to the reaction tank, wherein in the precipitation step, the wastewater to which the reducing iron compound is added and the alkaline sludge are mixed and are allowed to react in a non-oxidizing atmosphere under alkaline condition to form a reducing iron compound precipitate as the precipitate, thereby incorporating contaminants in the precipitate to remove the contaminants from the wastewater, wherein the reducing iron compound precipitate formed in the reaction tank is a mixture of green rust and iron ferrite, the reducing iron compound precipitate is formed so that a ratio of bivalent iron ions to total iron ions in the reducing iron compound precipitate (Fe2+/Fe(T)) is 0.4 to 0.8, and wherein a pH of the alkaline sludge returned to the reaction tank is adjusted to 11 to 13, a pH in the reaction tank in which this alkaline sludge is mixed is adjusted to 8.5 to 11, and the reducing iron compound precipitate is formed in a non-oxidizing atmosphere.