Enhancement of surface water quality using trading discharge permits and artificial aeration

Water quality trading (WQT) is a strategy for surface water quality management. This study focused on finding the optimal waste load allocation by WQT framework based on the concentrations of BOD and dissolved oxygen (DO) in Sefidrud River. It also assessed the consequences of using artificial aeration on trading discharge permits. For these purposes, river was simulated by Qual2kw. Two checkpoints were considered for monitoring the upstream (Sangar) and downstream (Caspian). Sensitivity analysis was carried out to find the impact factors of each emission sources. The required total maximum daily loads (TMDLs) were determined through simulation results. It predicted that the concentrations of ultimate BOD in both Sangar and Caspian stations may decrease from 3.6 and 8.1 mg/L, respectively, to 2.1 mg/L. Conversely, DO may increase from 5.8 and 0.9 mg/L to 7 and 3.7 mg/L, respectively. This could solely save 7 % of total treatment costs in comparison with the conventional command and control (C&C) policy. WQT may increase the revenues to 21 %. However, this could not provide enough flexibility and robustness in regard to the permit pricing and profits attained by stakeholders. Artificial aeration was considered as an improvement solution in the second scenario. The experimental results of hydroponic systems and hydraulic river simulation by CCHE2D showed that 0.25 day⁻¹ can be added to the aeration coefficients. This modified TMDLs and impact factors thoroughly. Results verified that this approach may increase the revenues of TMDLs and discharge permit market, respectively, to 17 and 30 %. Permits can also be sold with higher prices and stakeholders may attain considerable profits. Consequently, the integration of artificial aeration with WQT was recommended for surface water quality enhancement.