The instantaneous transmembrane pressure needs to be continuously increased to compensate for foulant accumulation during constant flux microfiltration. Herein, we compare predictions of a unified mathematical model and conventional blocking laws with laboratory data obtained during constant flux operation. Foulants employed included single species cultures of bacteria and coagulated natural colloids, both of which are known to form compressible cakes. The first principles model unifies fouling arising from pore blocking by individual cells, cake formation, as well as bacterial secretion of exopolymers. It also incorporates non-uniform spatial deposition patterns that have been observed during unstirred dead-end filtration. Mechanistically, these heterogeneities arise either from non-uniform membrane surface porosity or stochastic initial binding of foulants. Previous studies have shown that the initial patchy or uneven deposit morphology is magnified over longer time-scales during bacterial filtration by differential extracellular polymeric substances (EPS) secretion through quorum sensing. However, in this study, we are primarily interested in the averaged behavior (mainly flux and pressure). By spatially averaging the microscale variables we are able to compare with classical blocking law models. We show that blocking laws and the unified model both accurately model fouling under our experimental conditions. The unified model provides mechanistic insights into (bio)colloid deposition and associated fouling during constant flux microfiltration particularly since it is obtained excellent predictive agreement with experimental data using parameters taken exclusively from our recent study of constant pressure filtration. (C) 2011 Elsevier B.V. All rights reserved.