The ability of low frequency ultrasound for cleaning fouled membrane has been proved. In this investigation, various frequencies and mixed wave ultrasound were used separately and together with two concentrations of EDTA as chelating agent in forward flush cleaning of MF membranes which was fouled by milk solution M. A statistical multifactor design was employed to obtain more precise results. The main factors involving several frequencies of 28, 45, 100 kHz and mixed wave ultrasound as well as two concentrations of 1 mMole and 3 mMole of EDTA and their interaction effects were evaluated on flux recovery, cleaned membrane resistance and cleaning efficiency. The results showed that mixed wave ultrasound individually and together with EDTA I mMole had higher cleaning efficiency than other treatments and there is a synergistic effect when ultrasound was used with EDTA as a cleaning factor. Also polynomial regression of flux as a function of time showed that only 5 min forward flushing under irradiation of effective frequency of ultrasound and a low concentration of EDTA are enough to approach the cleaning of fouled MF membranes. (C) 2007 Elsevier B.V. All rights reserved.