Respirometry is a widely used tool for analyzing and optimizing of biological wastewater treatment processes. However, existing respirometers are often complex and expensive, which limits their applicability in settings with limited resources. This study presents a simple and cost-effective respirometer based on standard laboratory equipment that can be used to monitor and evaluate microbial activity in wastewater treatment plants. The proposed methodology involves constructing a respirometer with a beaker and a magnetic stirrer. The device has two operating modes: intermittent and continuous aeration. It is suitable for open-vessel experiments. A mathematical model was adapted to estimate microbial respiration rates from oxygen consumption data while accounting for passive oxygen transfer. The intermittent aeration mode demonstrated good reproducibility. However, the continuous aeration mode resulted in a lower error rate, suggesting that it may be more effective for detecting subtle changes in respiration activity. The respirometer described in this study is simple, cost-effective and provides a practical solution for evaluating microbial respiration in wastewater treatment plants. The methodology is highly accessible, requiring only equipment that is widely available, and it can be used for routine monitoring and model calibration. Intermittent and continuous aeration modes offer different advantages and limitations that should be considered depending on the desired application. Overall, this study underscores the importance of simplifying respirometry techniques to enhance their accessibility to researchers and practitioners in wastewater treatment.