Implementation of an Microsoft (MS) Excel add-in, called bioXL3, is presented in this paper. bioXL3 is an educational tool for undergraduate- and graduate-level education in the environmental engineering field. The add-in is an open-source tool and is used for the simulation of conventional biological wastewater treatment plants based on Activated Sludge Model No. 3. The software is developed using visual basic for applications (VBA) in MS Excel and is tested with a number of sample model setups. All formulations used in VBA programming are provided here. Also, simulation results of a sample predenitrification system are summarized. The performance of the software in engineering education was tested by a teaching session. Results indicated that the add-in can prove useful in undergraduate and graduate-level education in environmental engineering.