NOVELTY - Alkaline wastewater containing sulfur-based chemical oxygen demand (COD) compound and nitrogen component, is mixed with seawater and flown into a fixed bed type bioreactor (4). The mixed water is contacted a carrier (5) in the bioreactor at a pH of 8.5-10.5 under anoxia sulfur oxidation conditions. The microorganism group in seawater is adhered to the carrier, the sulfur-based COD compound is reduced by oxidation by the microorganism group, and the nitrogen component is removed as nitric oxide, dinitrogen oxide, and/or nitrogen.    USE - Treatment of wastewater e.g. ammoniacal-liquor activated-sludge treated water, or mixed wastewater obtained by mixing blast furnace slag immersion water and ammoniacal-liquor activated sludge treated water (all claimed).    ADVANTAGE - The method enables simultaneous removal of sulfur-based COD compounds and nitrogen components, stably reducing sulfur-type COD. Since wastewater can be processed in alkaline state, acidifying reagent cost is reduced. Generation of hydrogen sulfide gas, which is dangerous with respect to human body, is suppressed. Since autotrophic-type bacteria with slow proliferation rate are used, the amount of surplus sludge is decreased.    DETAILED DESCRIPTION - Alkaline wastewater containing sulfide, bisulfide, sulfite, thiosulfate and/or thiocyanate ion as sulfur-based chemical oxygen demand (COD) compound, and nitrite and nitrate ion as nitrogen component, is mixed with seawater and flown into a fixed bed type bioreactor. The mixed water is contacted a carrier in the bioreactor at a pH of 8.5-10.5 under anoxia sulfur oxidation conditions. The microorganism group in seawater is adhered to the carrier, the sulfur-based COD compound is reduced by oxidation by the microorganism group, and the nitrogen component is removed as nitric oxide, dinitrogen oxide, and/or nitrogen.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the wastewater treatment process.    Wastewater tank (1)    Seawater tank (2)    Mixing tank (3)    Fixed bed type bioreactor (4)    Carrier (5)