A pilot-scale Hybrid Membrane Bioreactor (HMBR) containing both suspended biomass and biofilm was tested for the treatment of a low strength municipal wastewater. The wastewater fed was characterized by a high variability throughout the day, low BOD5/TN ratio and low alkalinity (302 +/- 52 mgCaCO(3)/L). For limiting membrane fouling, an innovative abrasive granular material has been proven in the Microdyn-Nadir membrane. Permeability ranged from 126 to 291 L/(h m(2) bar) during the operational period, achieving a maximum flux of 24 L/(m(2) h). A low BOD5/TN ratio of the raw wastewater, led to insufficient denitrification, with an average nitrogen removal of 49%. This fact, in turn, caused a decrease in the pH due to the lack of alkalinity. This study underlined that wastewaters characterized by high variability throughout the day, low BOD5/TN ratio and/or low alkalinity content require carefully design of the MBR systems. It was shown that a low pH in the HMBR led to a strong membrane fouling increasing cake resistances.