Well-controlled membrane filtration experiments were performed to systematically investigate the effect of permeate flux and tangential flow (crossflow) on membrane fouling. Results were analyzed by the resistance-in-series model where the reason for flux decline was subdivided into adsorption, concentration polarization, and reversible and irreversible fouling. A synthesized paper mill wastewater with mainly lignin and 2-chlorophenol, biological suspension (activated sludge), and their mixture were used as feed solutions for ultrafiltration (30 000 Da) and microfiltration (0.3 mu m) at different concentrations. The filtration experiments demonstrated that permeate flux declined faster with increasing feed concentration and membrane pore size and with decreasing tangential flow. The biological suspension rather than wastewater quality was a major cause for permeate flux decline in membrane bioreactors. In the absence of permeate flux, filtration resistance by foulants adsorption was negligible, as compared to total filtration resistance in the presence of permeate flux. It was also shown that tangential flow had almost no effect on the decline rate of permeate flux at pseudo steady state. Membrane cleaning results revealed that, in the absence of tangential flow, permeate flux decline was dominantly caused by reversible fouling. On the other hand, tangential flow caused slightly higher irreversible fouling due to higher permeation drag, as compared to the case of absence of tangential flow. Autopsy of fouled membranes suggested that the irreversible fouling layer was initially formed by pore blocking of small particles followed by strong interaction of fouling layer with mainly dissolved materials and by fouling layer compaction due to permeation drag. (C) 2005 Elsevier B.V. All rights reserved.