The operation of a pilot scale membrane bioreactor (MBR) provided feedback for the world's largest centralized MBR plant treating petrochemical wastewater, located in the industrial area of Porto-Marghera, Venice. The main objective was to study the robustness of MBR technology under variable operating conditions of the petrochemical industry. We aimed to reduce the idle volumes of biological reactors and to enhance biomass activity. Five runs were conducted, initially aiming to represent the operating conditions of the full scale MBR and then alternations were introduced, including the addition of more external carbon source, the reduction of the anoxic compartment volume, changes in configuration and an increase of influent load. Ammonification was not effective in the pre-denitrification configurations, since the average organic nitrogen removal ranged from 29% to 60%. Nitrification was very satisfactory since ammonium concentration was usually lower than 0.5 mg NH4-N L-1. Increased acetic acid addition was effective, as it enhanced oxidation activity and denitrification rate. The reduction of the anoxic reactor volume and the abolition of internal recycling resulted in a decrease of denitrification rate. Petrochemical wastewater composition affected the biological processes of ammonification and denitrification. The low denitrification during the nitrification, post-denitrification configuration was attributed to the low organic carbon to total nitrogen ratio of influent wastewater. The doubling of inflow rate did not significantly compromise permeate quality. The MBR demonstrated to safeguard the effluent quality even under sudden and drastic transient conditions. Caustic soda caused inhibition of nitrifiers by 56% and refinery wastewater up to 60%. (C) 2012 Elsevier B.V. All rights reserved.