In this paper,the flocculation-photocatalysis treatment of the mixed wastewater from secondary-fiber paper mill was carried out,with TiO₂ colloid prepared by the hydrolysis of titanyl organic compound as the flocculant and the photocatalyzer. Then,the flocculation mechanism of TiO₂ colloid and the optimal photocatalysis conditions were investigated. The results indicate that ( 1) TiO₂ colloid particles with the average diameter of 13. 5 nm is of
high photocatalytic activity; ( 2) when the flocculation performs at a TiO₂ dosage of 0. 025% ( mass fraction) with a pH value of 4,and when the photocatalysis performs at a TiO₂ dosage of 0. 050% with a pH value of 3,the COD of wastewater sample decreases from 909. 7mg /L to 103.0mg /L,and the total COD removal reaches 88. 7%; ( 3) the flocculation of TiO₂ colloid is mainly due to the bridging between TiO₂ colloid and the fine fibers as well as the dissolved colloidal substances( DCS) in the wastewater; ( 4) the COD removal of TiO₂ colloid is high in the earlier stage of photocatalysis,and then slowly changes after 6 hours; and ( 5) TiO₂ colloid is a kind of reusable photocatalyzer and a small dosage of it helps to reduce the energy consumption and the cost of wastewater treatment.