Prioritizing the best sites for treated wastewater instream use in an urban watershed using fuzzy TOPSIS

This study developed a new framework that prioritized the best sites for treated wastewater (TWW) instream use using fuzzy Technique for Order of Preference by Similarity to Ideal Solution (fuzzy TOPSIS), a fuzzy-based multi-criteria decision-making (MCDM) technique. We identified key criteria for TWW use based on the Driver-Pressure-State-Impact-Response (DPSIR) framework that considered technical, social, economic and environmental aspects. We also introduced triangular fuzzy numbers and conducted individual interviews to consider the uncertainty of weighting values and input data. This procedure was applied at ten sites in a South Korean urban watershed, where hydrologic modeling exercises were performed. Our simulation results for water quantity (i.e., drought flow, low flow and the days to satisfy instream flow) and water quality (i.e., BOD concentration and the days to satisfy target water quality) showed significant inter-annual variability that could be better represented with fuzzy numbers. Furthermore, the use of fuzzy TOPSIS gave different rankings of the best sites for TWW use compared to those obtained from a weighting sum method, a traditional MCDM technique. Such varied rankings with different MCDM techniques indicate the need for fuzzy-based techniques, considering various uncertainties and thus being less controversial.