NOVELTY - The concentration of organic substance in mixed waste liquid obtained by mixing electrolytically-treated organic waste liquid, and organic waste liquid which is not electrolytically treated is determined by specific relations. The minimum ratio of the content of electrolytically-treated organic waste liquid with respect to the content of mixed waste liquid is determined, based on the concentration of organic substance, such that the mixing rate of electrolytically-treated organic waste liquid, and organic waste liquid which is not electrolytically treated is determined.    USE - Processing of organic waste liquid e.g. methane fermentation waste liquid (claimed) discharged by coagulation sedimentation method and activated sludge method.    ADVANTAGE - The organic waste liquid is efficiently processed.    DETAILED DESCRIPTION - The concentration (Y in mg/L) of organic substance in mixed waste liquid obtained by mixing electrolytically-treated organic waste liquid, and organic waste liquid which is not electrolytically treated is determined by the relations: Ax X1+B (I), Cx X2+D (II) and E (III), where X1 is 0-M vol.% and X2 is M-N vol.%, X1 and X2 are ratio of the content of electrolytically-treated organic waste liquid with respect to the content of mixed waste liquid, M is a value of X corresponding to the intersection of straight line of relation (1), and straight line of relation (2), N is a value of X corresponding to the intersection of the straight line of relation (2), and straight line of relation (3), A, B, C and D are constants, and E is concentration (mg/L) of the organic substance contained in the electrolytically-treated organic waste liquid. The minimum ratio of the content of electrolytically-treated organic waste liquid with respect to the content of mixed waste liquid is determined, based on the concentration of organic substance contained in mixed waste liquid, such that the mixing rate of electrolytically-treated organic waste liquid, and organic waste liquid which is not electrolytically treated is determined. An INDEPENDENT CLAIM is included for apparatus for processing of organic waste liquid.