The Water Recovery System (WRS) is one of the necessary subsystems in an Advanced Life Support program. It regenerates potable water by processing wastewater generated on a space shuttle or on a space station. The Biological Water Processor (BWP) is one of the complex subsections of WRS that involves processes described by continuous time as well as discrete event dynamics. Such systems can be modeled as hybrid dynamical systems in MATLAB/Stateflow. The following state variables of the BWP have been simulated in MATLAB: Pressure drop across Organic Carbon Oxidation Reactor (OCOR), Pressure drop across nitrification reactor, nitrifier flow rate, total flow to OCOR, Gas-liquid separator level, feed pump modes. The developed model of the BWP can be used for the simulation of the complete WRS and also for synthesis of a distributed control laws for the BWP subsection.