Forward osmosis (FO) is an emerging membrane separation process, and it has recently been explored for microalgae separation, one of the key steps in algal biodiesel production. The current study systematically investigated the physical and chemical parameters affecting FO flux performance during microalgae separation. To the best of the authors' knowledge, this is the first study reporting FO fouling by microalgae as well as the effect of solute reverse diffusion on FO fouling. FO fouling was more severe at greater draw solution concentrations and in the active-layer-facing-the-draw-solution orientation, which can be partially attributed to the corresponding higher flux levels under these conditions. Indeed, a critical flux phenomenon was observed for the concentration driven FO process, where significant relative flux reduction occurred only when the water flux level exceeded some threshold value. The presence of Mg(2+) in the feed water had detrimental effect on algal fouling, with more dramatic flux loss at greater Mg(2+) concentration in the feed. Despite that MgCl(2) had superior FO performance (higher water flux and lower solute reverse diffusion) when compared to NaCl as draw solution in the absence of foulants, the use of MgCl(2) as a draw solution nonetheless promoted significant flux loss as a result of severe fouling when algae was present. This was likely caused by the reverse diffusion of Mg(2+) from the draw solution into the feed water, which led to an unfavorable interaction between the divalent ion and the algal biomass in the feed water. Such reverse-diffusion-induced fouling should be explicitly considered for draw solution selection. (C) 2010 Elsevier B.V. All rights reserved.