Ambient particles in water are a much more severe threat to fouling a spiral-wound membrane element than natural organic matter(NOM). Particles deposit continuously and at a constant rate. Deposition rates, which are generally high ranging between 10(6)-10(8) particle per hour, increase with increasing particle size. The presence of NOM increases the rate for particles >1.0 mum in size while decreasing the rates for particles 0.5-1.0 mum in size. Whereas for NOM adsorption approaches a steady-state level after a period of about 2 weeks, adsorption is at most 12.6% of the available NOM mass in the feed water. Plotting of steady-state adsorption levels vs. feed water NOM concentrations on a log-log scale showed data conforming to a classic Freundlich isotherm model. Particles both >1.0 mum and 0.5-1.0 mum in size interfere with NOM adsorption, reducing the amount of NOM adsorbed.