The operation and performance of a biotrickling filter for methanol (MeOH) and hydrogen sulfide (H2S) removal at acid pH was studied. Excess biomass in the filter bed, causing performance loss and high pressure drop, was controlled by intermittent addition, of ozone (O-3) and hydrogen peroxide (H2O2). The results showed that after adaptation to acid pH, the maximum elimination capacity (EC) reached for MeOH was 565 g m(-3) h (-1) (97 % RE). High MeOH loads resulted in increased biomass concentration within the support, triggering reductions in the removal efficiency (RE) for both compounds close to 50 %, and high pressure drop. At this stage, an inlet load of 150.2 +/- 16.7 g m(-3) h(-1) of O-3 was fed by 38 days favoring biomass detachment, and EC recovery and lower pressure dropped with a maximum elimination capacity of 587 g m(-3) h(-1) (81 % RE) and 15.8 g m(-3) h(-1) (97 % RE) for MeOH and H2S, respectively. After O-3 addition, a rapid increase in biomass content and higher fluctuations in pressure drop were observed reducing the system performance. A second treatment with oxidants was implemented feeding a O-3 load of 4.8 +/- 0.1 g m(-3) h(-1) for 7 days, followed by H2O2 addition for 23 days, registering 607.5 g(biomass) L-1 (packing) before and 367.5 g(biomass) L-1 (packing) after the oxidant addition. PCR-DGGE analysis of different operating stages showed a clear change in the bacterial populations when O-3 was present while the fungal population was less affected.