This research aimed to develop an alternative to soluble BOD-based corrections for the design of facultative lagoons. The correction process is conventionally applied to account for the soluble fraction of BOD within total BOD. Facultative lagoons are low-cost wastewater treatment systems widely used in regions with average temperatures above 14 ℃, explaining their prevalence in tropical zones. Their design has traditionally relied on empirical methodologies, with the McGarry and Pescod model standing out as one of the most recognized and applied frameworks, which served as the basis for this study. The proposed alternative leverages the relationship between non-removed total BOD and influent BOD, offering a practical and system-derived solution. This approach eliminates the need for empirical estimations, a common feature of traditional methods.