Method for removing selenium and apparatus for removing selenium

1. A method for removing selenium from a wastewater from a facility that gasifies a coal fuel containing selenium in a reducing atmosphere, the method comprising:

a valence change step of adding an oxidant to the wastewater containing selenium, and oxidizing selenium having a valence lower than +4 to change the valence of the selenium to +4, and

a solid-liquid separation step of adding a flocculant to the wastewater, forming a floc containing the selenium for which the valence has been changed in the valence change step, and subjecting the floc to solid-liquid separation,

wherein the valence change step includes a step of adjusting a pH of the wastewater to 4 before adding the oxidant to the wastewater,

the solid-liquid separation step includes a step of adjusting a pH of the wastewater to 7 using an alkali agent before adding the flocculant to the wastewater,

wherein hydrogen peroxide is used as the oxidant in the valence change step and an oxidation-reduction potential of the wastewater is controlled to a value of at least 393 mV but not more than 433 mV so as to change the valence of the selenium to +4, or sodium hypochlorite is used as the oxidant in the valence change step and an oxidation-reduction potential of the wastewater is controlled to a value of at least 630 mV but not more than 900 mV so as to change the valence of the selenium to +4,

the flocculant comprises an inorganic flocculant and a polymer flocculant, and

the polymer flocculant is an anionic polymer flocculant or a nonionic polymer flocculant.

2. The method for removing selenium according to claim 1 , wherein an iron salt is added to the wastewater in the valence change step.