BACKGROUND Biofilters can be used to eliminate different gaseous pollutants including alcohols (e.g. methanol, ethanol) and methane (CH4) individually or in a mixture. In this regard, the biofilter adaptability to feed composition variation is an industrial requisite. Inlet gas composition changes also give a better insight about carbon input and end-points in biofilters. In this study, the gradual conversions of two biofilters from methanol and ethanol to CH4 were investigated. RESULTS Both biofilters reached 100% removal efficiencies for the alcohols with no initial inoculation. Keeping the total inlet load constant (30 +/- 1.3 g m(-3) h(-1)), CH4 was progressively substituted in the feed with corresponding alcohol:CH4 mass ratios of 3:1, 1:1, 1:3 and 0 g(alcohol):g(CH4). Maximum CH4 removal efficiencies of 52% and 29% were obtained (respectively) for biofilters started with methanol and ethanol. By moving from alcohols to CH4 biofilters, the gas phase output carbon increased from 273 to 666 g(carbon) day(-1) and from 377 to 681 g(carbon) day(-1) respectively for the methanol- and ethanol-based biofilters. CONCLUSION This study showed a successful treatment based on inlet pollutant alteration from methanol or ethanol to CH4 in two separate biofilters. However, the methanol-based biofilter displayed a better performance and a shorter acclimation time for CH4 conversion. (c) 2019 Society of Chemical Industry