In this study, field tests were performed at the wastewater treatment plant of ITOB-Organized Industrial Zone located in Menderes, Izmir where membrane bioreactor (MBR) process has been employed for wastewater treatment. For advanced treatment of MBR effluent, a mini pilot scale membrane test unit having nanofiltration (NF) membranes was employed. For this study, only NF90 membrane was used for tertiary treatment of different feed mixtures. The feed mixtures were obtained by mixing MBR effluent and concentrate stream of NF90 membrane at different proportions. During field tests, while some amount of concentrate stream of NF90 membrane was sent to the feed tank with MBR effluent to obtain feed mixtures, the remaining part of concentrate stream was discharged to the MBR pool.