Wastewaters containing biocides constitute an increasing environmental pollution concern. In this work, the feasibility of implementing a pre-treatment based on the Fenton process to minimize the negative impact of biocide compounds, such as benzalkonium chlorides (BAC), on the activated sludge system was studied. The behavior of the microbial community in the presence of a BAC solution untreated (BAC100) or pre-treated by homogeneous Fenton oxidation (BAC100/HF) was analyzed. The addition of

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... C100 to the activated sludge system caused a deleterious effect. The cellular lysis process caused a 30% decrease in biomass concentration. The inhibition of respiratory activity was 73%. In contrast, BAC100/HF caused no significant decrease in the biomass concentration or detectable inhibition of the respiratory activity. Besides, the consumption of easily oxidizable products was observed. Results indicated that the homogeneous Fenton process as a pre-treatment of wastewaters containing BAC could become a feasible alternative to protect active biomass in biological treatment systems.