Polycyclic aromatic hydrocarbons (PAH) are regarded as environmental pollutants. A promising approach to reduce PAH pollution is based on the implementation of the natural potential of some microorganisms to utilize hydrocarbons. In this study Proteiniphilum acetatigenes was used for bioaugmentation of sewage sludge to improve the PAH removal. Bioaugmentation experiments were performed in parallel semi-continuously fed reactors started up with digested primary and secondary sludge. Three bioaugmentation approaches were investigated: A1, addition of bacteria once during starting up; A2, addition of bacteria at the beginning and then every 2nd day and A3, addition of encapsulated bacteria once during starting up. Removal of PAH was found to be both biotic and abiotic. All three approaches had a positive effect of the biological removal of PAH. Highest biological removal of individual PAH (up to 80%) was observed using continuous addition (approach A2) of the bacteria to the reactors. In general, the effect of bioaugmentation was higher in the reactors fed with primary sludge compared to the reactors fed with mixed sludge. Bioaugmentation resulted in biological removal of low molecular weight PAH in the reactors fed with primary sludge using all three approaches while clear biological removal of the medium- and high molecular weight PAH only was observed if the bacteria were added continuously (approach A2). (C) 2008 Elsevier B.V. All rights reserved.