Hybrid membrane bioreactors have emerged to alleviate membrane fouling, the drawback of conventional membrane. In this study immobilized acclimatized activated sludge on either polypropylene or polyurethane carriers were exploited to run two types of hybrid bioreactors and compare their performances with one conventional MBR in treatment of a synthesized phenolic wastewater at 1000 mg/L and a hydraulic retention time of 13 h. Immobilization on polyurethane foam resulted in phenol removal of 99% as compared to 72.5% for polypropylene and 70.6% for the conventional MBR. Complete membrane fouling occurring at trans-membrane pressure of 0.6 bar was observed after 5,9 and 21 days for conventional MBR, polypropylene and polyurethane hybrid bioreactors, respectively. Additionally, analyses of soluble microbial products showed that proteins positively affect membrane fouling. The performance of hybrid bioreactor containing polyurethane foams under phenol loading shocks was also examined by sudden variations from 1000 to 1250 and 1500 mg/L in influent phenol concentration where it was found that although persistent shocks had adverse effect on phenol removal, complete retrieval of phenol removal could be achieved after elimination of shocks. Among bioreactors examined, a hybrid bioreactor using polyurethane foam is therefore proposed as a superior bioreactor considering both performance efficiency and membrane fouling for wastewater treatment. (C) 2013 Elsevier B.V. All rights reserved.