We conducted this study to investigate the physico-chemical characteristics of air-scouring-membrane (microfiltration; MF) backwash water produced by an actual drinking water treatment plant. Visual observations were made on the settling behavior (settlability) of backwash water obtained from both an air-scouring membrane and a rapid sand filter. The settling velocity of backwash water was much slower for the membrane-derived backwash water than the rapid sand filter-derived backwash water. Further, zeta-potential measurements indicated that even though polyaluminum cation coagulant was injected and mixed with the raw water before the MF membrane process, the zeta-potential value of the backwash water was reduced to that of raw water. Particle analysis revealed that the average particle size of the MF membrane backwash water was smaller than that of the rapid sand filter backwash water. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.