The processes of Biological treatment intend to reduce the organic matter content by using microorganisms. Problems, which often occur in the treatment process, include the Wastewater Treatment Plant (WWTP) being planned for treating domestic wastewater only, but in fact the WWTP often receives non-domestic wastewater particularly direct or indirect faecal deposits. There are 13 simultaneous systems of nonlinear differential equations using the method of Runge o angle charts and tolerance ± 20%, respectively for BOD (76.8%), COD (57.7%) and DO (81.9%). The model, as a means for performance evaluation of the WWTP, is appropriate for Class II water quality standards.