Effluent quality-based ranking of sewage treatment plants using multicriteria decision making technique

Water pollution control agencies worldwide face the complex challenge of ensuring the efficient operation of sewage treatment plants (STPs) to protect water bodies receiving their effluent as well as for reuse. However, evaluating the performance of these plants is difficult due to multiple criteria involved. Further, while previous studies have focused on evaluating the efficiency of individual plants, it remains unclear how the effluent quality of STPs affect the water quality of receiving bodies without relying on the complex simulation-optimization models (such as waste load allocation) to understand the cause-and-effect relationships. The present study addresses this gap by developing a novel multicriteria decision making framework for ranking the performance of 100 STPs located across the country by coupling effluent quality with the prescribed regulatory standards within a single decision matrix. A modified Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) MCDM method has been used. The approach incorporates ten effluent criteria (in terms of quality) and their prescribed discharge standards (as prescribed by the CPCB) within the same decision matrix. The effluent quality criteria considered includes: biochemical oxygen demand (BOD), chemical oxygen demand (COD), BOD removal efficiency, COD removal efficiency, total dissolved solids (TDS), total suspended solids (TSS), fecal coliform (FC), total coliform (TC), nitrate, and phosphate. These 100 plants were categorized on the basis of their design capacity into four classes: 1-20 MLD, 21-50 MLD, 51-100 MLD, and greater than 100 MLD. Results revealed that small-capacity plants with utilizing oxidation pond technology performed more effectively, while larger-capacity plants showed better performance with Activated Sludge Process technology. The best-performing small-capacity plant was Burhi ka Nagla in Agra (0-20 MLD range), while the top-performing large-capacity plant was in Madurai, Tamil Nadu (above 100 MLD). To the best of the authors' knowledge, this is the first study to have employed a modified TOPSIS approach that coupled STP effluent quality with regulatory standards within a single decision matrix. The findings of this study will aid pollution control authorities in developing effective water quality management plans.