PURPOSE: To perform effectively oxidation of ferrous ion by treating aq. soln. contg. chlorine and iron with iron bacteria to oxidize to ferric ion and returning the sludge contg. iron bacteria precipitated by this treatment to an oxidation tank.

CONSTITUTION: Aq. soln. contg. at least chlorine ion and Fe(II) ion is introduced into an oxidation tank, and iron bacteria cultivated separately in a liquid culture medium contg. Fe(II) ion and Cl ion are introduced into the same tank to oxidize Fe(II) ion in the introduced liquid to Fe(III) ion. In this case, diatomaceous earth as a carrier agent for the iron bacteria and nutrient are added to the oxidation tank. When Fe(II) ion is oxidized to Fe(III) ion in the oxidation tank, iron bacteria-contg. sludge consisting essentially of iron hydroxide is formed; solid matters in the treated liquid is separated from liquid in a settling tank, and separated iron bacteria-contg. sludge is reused repeatedly in the oxidation tank to perform operation continuously.