Optimising load allocation and reduction strategies for sewage treatment plants in urban tropical rivers: Modelling with QUAL2K and GIS

The discharge of pollutant loads from sewage treatment plants (STPs) significantly impacts the quality of tropical rivers. The present study evaluates the impact of projected increases in population equivalent (PE) from STPs on the selected water quality parameters (WQP) of the Kuang River basin (KRB) under the influence of climate stress and advanced treatment. Using the QUAL2K model, discharge was simulated, and scenarios were based on population growth rate (PGR) for the next 10 years, with Low (2.1% PGR), Medium (2.2% PGR) and High (2.5% PGR). The findings indicate that elevated PE affects TSS, NH₃-N, and BOD₅ concentrations with a p-value < 0.05 between the seasons. Under high PGR scenarios in the dry season, concentrations for BOD₅ were 7.82 mg/L, 3.02 mg/L for NH₃-N, and 48 mg/L for TSS. This study suggests reducing pollution loads from STPs by 98.88% (35.59 and 7881.5 kg/day) for BOD₅, 99.44% for NH₃-N (7.16 and 1585.27 kg/day), and 83.93% for TSS (35.25 and 1303.08 kg/day) to meet class II. Additionally, the model’s performance was validated with flow calibration results showing R2 = 0.79-0.94 and NSE = 0.80-0.94. Therefore, this study projected the load allocation for WQP for all identified STPs for sustainable KRB management under future urban growth.