TiO2 heterogeneous photocatalysis (HP) applied to the oxidation of benzalkonium chloride (BKC), a bactericide widely used in pharmaceutical and cosmetic formulations and risky for the biota, is presented. Although BKC is rapidly degraded by HP, mineralization degree is low, indicating the formation of intermediate compounds more resistant to the photocatalytic treatment. Combination of HP with a biological treatment improves the treatment. Both, use of TiO2-photocatalysis as a pretreatment to improve the biodegradability of wastewaters and as a polishing decontamination step after a biological treatment are analyzed. The biotreatment can be used when the effluent contains a relatively low BKC concentration and enough nutrients and carbon to sustain the life of the biomass. The bioreactor reduces both COD and BKC concentration. Once the level of COD is reduced, photocatalysis can be used for the total BKC elimination from the solution. On the other hand, HP can be used to reduce BKC concentration in effluents containing mostly this compound. Once the BKC concentration is low enough to be innocuous to the biomass, the effluent can be combined with a solution containing nutrients and carbon and treated by the biological method. (C) 2011 Elsevier B.V. All rights reserved.