NOVELTY - A sewage treatment system (10) comprises parallel main process lines (12, 13 and 14) having anoxic (18) and aerobic zones and membrane bioreactor zone (22) in series, with inflow device for receiving influent wastewater into main line; and additional, special process lines (16) having similar zones but operated to receive portion of mixed liquor suspended solids from the main lines. The special process line has device for internal recycling of sludge among the zones during normal operation, and device for recycling mixed liquor suspended solids exiting main line to re-enter main line.    USE - As sewage treatment system (claimed) and in wastewater treatment processes.    ADVANTAGE - The membrane bioreactors thickens sludge in a system thus allowing more than twice design flow during peak or storm flow conditions without excess membrane bioreactor (MBR) capacity during normal flow. The system can be operated in a second, surge or storm mode of operation at more than twice normal design flow. The process line can be operated with parallel streams of wastewater processed serially through the anoxic, aeration and membrane bioreactor zones, thereby maintaining equilibrium in the system by removing water via the membrane bioreactors at a rate equal to influent wastewater flow, without the need to remove waste activated sludge during a period of such surge or storm mode. The configuration of the system with the thickened sludge in the special process line, thickened and digested during normal operation, provides additional biomass to handle the peak flow. The system is versatile and can handle a peak flow event for 5 days.    DETAILED DESCRIPTION - A sewage treatment system (10) comprises series of parallel main process lines (12, 13 and 14) including anoxic (18) and aerobic zones and membrane bioreactor zone (22) in series, with inflow device for receiving influent wastewater into main process lines; and at least one additional, special process line (16) having similar zones but normally operated to receive portion of mixed liquor suspended solids from main process lines. The special process line has device for internal recycling of sludge among the zones during normal operation, so as to produce a further thickened and stabilized or digested waste activated sludge, to be removed from the special process line during normal operation; and device for recycling majority of mixed liquor suspended solids exiting main process lines to re-enter main process lines to mix with wastewater in the main process lines. The system can be operated in second, surge or storm mode of operation at more than twice normal design flow. The process lines can be operated in the same manner, with parallel streams wastewater processed serially through the anoxic, aeration and membrane bioreactor zones thereby maintaining equilibrium in the system.    An INDEPENDENT CLAIM is included for operating the sewage treatment system involving: in normal operation mode in which influent wastewater is at design flow, operating system to deliver portion of mixed liquor suspended solids to special process lines, and internally recycling sludge among the zones of special process line during normal operation to produce further thickened and stabilized or digested waste activated sludge; in the normal operation mode, removing the sludge; recycling effluent mixed liquor suspended solids from main process lines back to re-enter main process lines; and in a surge or storm mode of operation when influent wastewater flow rate is more than twice design flow, receiving influent wastewater in all process lines and operating all process lines in the same manner, with parallel streams of influent wastewater being processed serially through anoxic, aeration and membrane bioreactor zones similarly and in parallel through all process lines including special process line; and in the surge or storm mode of operation, recycling effluent mixed liquor suspended solids from all process lines back to enter process lines to mix with wastewater in the process lines.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram indicating the system with multiple process lines, each including three tanks or zones, one of which is membrane bioreactor.    System (10)    Normal process lines (12, 13 and 14)    Additional special process line (16)    Anoxic zone (18)    Pre-aeration zone (20)    Membrane bioreactor zone (22)    Thickening membrane bioreactor zones (22a)    Recycle line. (24)