The wastewater from coke-ovens for steel industries contains high concentrations of toxic compounds such as cyanide and phenol as well as ammonia. If the treated wastewater is to be discharged to enclosed water bodies, nitrogen removal is needed in addition to the removal of toxic compounds. Yet, as nitrifiers are in general known to be sensitive to toxic compounds, the application of biological methods such as nitrification/denitrification has been thought to be difficult. Yet, there are cases where biological nitrification has been applied successfully. The authors examined the applicability of the nitrification/denitrification process for the treatment of coak-oven wastewater diluted with sea water. In the preliminary experiment, the authors observed the accumulation of nitrite instead of nitrate in the nitrification step. In an attempt to clarify the cause of partial nitriifcation, the authors operated a test plant using simulated coak-oven wastewater prepared with chemical reagents for a period of about one year. The nitrifiers population was monitored by molecular methods including fluorescence in situ hybridization (FISH), PCR-DGGE, and real time PCR.
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