An alternative design of the membrane aerated biofilm reactor (MABR) was developed to overcome some of the current technical and economical limitations preventing full scale applications of the process. The ZeeLung system uses a new dense hollow-fibre membrane with an unprecedented thin diameter. Two pilot units treating a synthetic high-strength industrial wastewater (4700 mgCOD/L, 145 mgTKN/L) operated successfully for 16 months. They performed simultaneous COD removal, nitrification and denitrification. The very high specific surface area (810 m2/m3) allowed the surface loading rate to be kept low enough (3.6 gCOD/(m2.d)) to maintain a relatively thin biofilm (200 to 350 microm) and use low-pressure air (41 kPa) instead of high-pressure pure oxygen. Intermittent air and liquid mixing at high frequency and low shear were compared: they were equally effective in enhancing substrate transfer, but failed to stabilize biofilm accumulation. Air sparging additionally prevented the acidification of the bulk by stripping CO2.