Evaluating the quality of Shiraz wastewater treatment plant effluent for agricultural uses in drought conditions utilizing IRWQI, NSFWQI, and OWQI indicators

The utilization of non-conventional water sources, such as saline water, drainage, and sewage, has proven to be highly effective in managing droughts given the limitations of the country's water resources. The primary advantage of utilizing effluent from urban wastewater treatment plants is the stability of these sources, their accessibility, and their low cost for irrigating agricultural lands, which ultimately addresses the issue of water scarcity. In this study, the quality of Shiraz urban sewage treatment plant effluent was evaluated for agricultural purposes by calculating the National Sanitation Foundation of America water quality index (NSFWQI) and the Oregon water quality index (OWQI) using daily data from 2018 to 2019. Nine parameters were selected to calculate the NSFWQI index for the Shiraz treatment plant, including turbidity, temperature, phosphate, nitrate, coliform, biological oxygen demand (BOD), total dissolved solids (TDS), dissolved oxygen (DO), and pH. The OWQI quality index is similar to the NSFWQI index but includes eight parameters: temperature, dissolved oxygen, biological oxygen demand, pH, nitrogen, total phosphorus, total dissolved solids, and fecal coliform. The results showed that the average NSFWQI values varied between 28.73 and 31.54 in different months, indicating poor quality. The monthly OWQI values ranged from 4.38 to 7.95, indicating very poor quality. Therefore, given the low quality of the effluent, it is recommended for use only in irrigating non-productive plants, forest parks around Shiraz, and a limited part of urban green space.