PERFORMANCE AND COST MODELING OF ULTRAFILTRATION

Models describing permeate flux, rejection, and cost are coupled to ev aluate the performance and cost of ultrafiltration as a function of ra w-water quality. The model for permeate flux extends a previous model for colloidal fouling based on shear-induced diffusivity to include Br ownian diffusion. Contaminant removal is modeled as mechanical sieving and molecular diameter is regressed against weight to describe remova l of natural organic matter (NOM). Time-dependent permeate flux is con sidered in estimating operating times required to achieve a specified recovery. Costs are calculated as a function of particle-size distribu tion in the raw water. Particles with diameters on the order of 10(-1) mu m display minimum diffusivities, which lends to maximum system cos ts with respect to particle size. Fine materials (<0.5 mu m), with hig h cake resistance, demonstrate pressure-independent permeate flux for conditions typical of hollow fiber ultrafiltration. In some cases, a m inimum in system costs as a function of recovery is observed due to a trade-off between operating time and time-averaged permeate flux. Simu lations for four scenarios of variable particle and NOM concentrations suggest that irrespective of adsorptive fouling, permeate flux may be limited by reversible accumulations of NOM on ultrafiltration membran es.