The treatment performance and microbial population of two biofilters treating seawater-diluted and freshwater-diluted coke-oven wastewaters, respectively, was compared in this study. The treatment performance did not differ between the two biofilters. The DOC removal efficiency of coke-oven wastewater was as low as 50% at a DOC volumetric loading of 0.7 kg-C m(-3) per day, while the fractional BOD removal was as high as 90%. This fact indicated that some less-biodegradable, i.e. refractory, organic matters remained in the effluent of the biological treatment process without degradation. The microbial population in the biofilter treating seawater-diluted wastewater differed from that in the biofilter treating freshwater-diluted wastewater. The quinone profiles of the microorganisms in the biofilters showed that Pseudomonas putida, (dominant quinone ubiquinone-9). actively contributed to the biological degradation of phenolic compounds contained in coke-oven wastewater. The microbial diversities of the biofilters treating seawater-diluted and freshwater-diluted coke-oven wastewater calculated based on the composition of all quinones were 5.2-9.4 and 5.8-6.2, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.