An innovative MABR (membrane-aerated biofilm reactor) membrane technology was demonstrated at the O9Brien Water Reclamation Plant (OWRP) of the Metropolitan Water Reclamation District of Greater Chicago (Chicago MWRD). The demonstration unit was equipped with one full-scale membrane cassette. The technology was evaluated for its potential to improve performance for total suspended solids (TSS) and ammonia removal during stressed conditions (specifically cold temperature peak flow periods) and to meet future effluent phosphorous limits. Over a period of 9 months, the MABR oxygen transfer rate was stable and ranged between 8 and 12 g-O 2 /d/m 2 of membrane surface area. The nitrification rate varied between 0.5 and 2.5 g-N/d/m 2 and was affected primarily by the ammonia loading rate and the feed carbon to nitrogen ratio. Most of the oxygen transferred was accounted for by nitrification. The MABR hybrid process enables important process improvements while reducing plant energy consumption.