This research article was published by MDPI, 2022In this study, a fuzzy-based indexing approach (FIA) is developed based on a conceptual framework regarding social, environmental, economic, and technical dimensions to evaluate the overall sustainability potential of existing septic systems (SSs) of around 200 Tanzanian residential buildings in Mwanza city. FIA required the following six steps: selecting, measuring, normalizing, weighting, and aggregating the sustainability indicators (SIs) or dimensions, as well as interpretation of the indices similarly to conventional sustainability indices to aggregate the four sustainability dimensions. In total, 18 SIs were selected based on a literature review. Input data obtained for each indicator were from the social survey and laboratory analysis. The results showed that the entire SSs in the city had a general sustainability index (GSI) of 0.42. The index fell on the verge of the “danger” category, indicating that corrective measures are needed. In conclusion, FIA is simple and transparent, it provides a both theoretical and practical basis for sustainability evaluation, does not require vast quantities of data, and does not demand an advanced computer software package. Moreover, FIA is a proper method to evaluate and improve SS sustainability in the city or provide the information to decision makers, designers, and researchers to scrutinize the decision possibilities in a multidimensional manner.