It　is　worthy　to　discuss　the　mechanism　of　iron　oxidation　in　the　biological　filter　with　DO　since　the　iron　oxidation　happens　easily.　The　limit　concentration　of　Fe2+　is　22-23　mg/L　that　can　be　removed　by　air　contact　-oxidation　under　weak　aeration.　The　research　on　need　for　the　Fe2+　and　Mn2+　nutrient　for　the　iron　and　manganese　oxidizing　bacteria　showed　that　most　of　them　belonged　to　the　poor　nutrient　bacteria　in　the　filter.　The　quantity　of　Fe2+　that　need　is　very　little　in　culture　and　mature　periods.　The　quantity　of　Mn2+　that　need　is　much.　These　proved　that　most　of　the　Fe2+　was　removed　by　chemical　oxidation　and　only　a　small　quantity　of　it　was　removed　by　biological　oxidation　in　the　biological　filter.