Direct microscopic observation of forward osmosis membrane fouling by microalgae: Critical flux and the role of operational conditions

► Optical microscopy was applied to study forward osmosis (FO) membrane fouling by microalgae. ► Critical flux was observed in FO fouling by using the concentration stepping method. ► The presence of divalent ion Mg2+ in FS or DS adversely affected FO fouling by microalgae. ► FO fouling by microalgae was partially reversible by simple cross flow flushing. ► The presence of Mg2+ either in FS or DS led to reduced fouling reversibility.