To prevent the flowing out and extinction of sulfur oxidizing bacteria even when the content of nitrate nitrogen in a raw water is low and also to prevent the production of hydrogen sulfide.
A sulfur oxidizing bacteria holding material comprising particles or lumps of a carbonaceous sedimentary rock is allowed to be present in a denitrification system. The atomic ratios of the carbonaceous sedimentary rock are preferably O/C: 0.05 to 0.65, H/C: 0.25 to 1.5 and N/C: 0.001 to 0.1, and a denitrification material comprising sulfur and calcium carbonate and the sulfur oxidizing bacteria holding material are preferably allowed to be present in water as separate particles or lumps. Further, the sulfur oxidizing bacteria holding material is selected from at least one component of the group consisting of shale, black shale, oil shale, coal, brown coal, lignite, peat, cannel shale, cannel coal, asphaltic soil, asphalt, silicified wood and fossil wood.
HIRATO YASUHIRO
Tomohiro Nakamura
Eiichi Sano