BACKGROUND: A membrane-aerated biofilm reactor (MABR) has previously been used to provide both anaerobic and aerobic conditions for mineralisation of perchloroethylene (PCE). However, very little is known about the stability of this reactor under hydraulic and PCE shock loads. An MABR was therefore subjected to sudden hydraulic and PCE shock loads in order to investigate its stability under such conditions. RESULTS: After each shock, the reactor responded with an increase of chemical oxygen demand (COD) and volatile fatty acids (VFA)s, a breakthrough of PCE and its biodegradation intermediates in the effluent, and a decrease in methane production. Although some PCE biodegradation intermediates were found in the effluent during each shock loading,the MABR performance recovered without the accumulation of any particular PCE biodegradation intermediates during PCE shock loads. During the hydraulic shock loads, the MABR was unstable at hydraulic retention times (HRTs) of 6 h with PCE and its biodegradation intermediates detected in the effluent. However, these intermediates were degraded when the HRT was reset to 48 h. CONCLUSIONS: This study suggests that MABRs can withstand fluctuations in influent strength and flows which occur in wastewater treatment works. (C) 2009 Society of Chemical Industry