Experimental data on the transient operation of the extended aeration wastewater treatment plant at Pennsylvania's Prince Gallitzin State Park were collected and the system has been mathematically modeled. Time-variant inlet hydraulic flows were obtained through continuous measurement, and organic loading data were based on grab samples analyzed for biochemical oxygen demand (BOD) and volatile suspended solids concentration. Effluent BOD was used as the criterion for comparing the predicted with the observed time-variant plant response to loadings. Three mathematical models were developed for the simulation.