This computer program (FORTRAN, IBM 1130) is a steady-state model of the conventional activated sludge process which is flexible enough to simulate the performance of any of the models proposed (complete mix, plug flow, multiple aeration tanks, step aeration, step return flow, contact stabilization, extended aeration, etc.). Two classes of microorganisms are considered: heterotrophs and nitrosomonas. The mathematical model allows the maximum rate constant for synthesis to vary with process loading. Another computer program is also included which simulates a fill and draw bench experiment in which activated sludge is mixed with substrate in any proportion. The program uses Runge-Kutta method of numerical integration to compute the biological activity as a function of time in the batch activated sludge experiment.