At the end of first phase, more than 99.5% removal of all four compounds was achieved at the steady state condition. These results were consistent with the reported acclimation periods from several weeks to several months. (Rene et al., 2005) reported that the removal was more efficient in the lower part rather than in the upper part of a compost based biofilter treating toluene for a similar up-flow configuration and an inlet load (IL) of 3–12 g m3 h1 (Rene et al.,2005). (Elmrini et al., 2004) observed that most of the o-xylene was eliminated in the first lower/inlet section of a peat-based biofilter for relatively small IL (12–34 g m3 h1) (Elmrini et al., 2004).