Predation by protozoa plays an important role in activated sludge. In this work, the kinetics for protozoan predation of active bacteria (X-H), extracellular polymeric substances (EPS), and intracellular storage products (X-STO) are added into a previously expanded unified model that describes the dynamics of EPS, X-STO, and soluble microbial products (SMP). The new biomass growth-decay-predation model describes the biomass fractions, soluble organic components, and oxygen-uptake rates considering EPS, X-STO, and predators during dynamic operating conditions in activated sludge. Model calibration using batch experimental data provides the new parameter values for predation processes and insights into mechanisms involving predators. The calibrated value of the maximum specific growth rate for the predators is much slower than for the bacteria, confirming that predators are relatively slow growers. However, the predators and bacteria have similar decay rates and dissolved oxygen affinities. Model testing with results independent of the calibration data shows two things. First, the model and calibrated parameters accurately simulate the independent results when predators are present. Second, eliminating predation by high salinity significantly lowers the OUR, and this is captured by the model. Biotechnol. Bioeng. 2010; 105: 1021-1030. (C) 2009 Wiley Periodicals, Inc.