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A method to determine the Total Nitrogen Emission Limits for Wastewater Treatment Plants Based on the Coastal Water Quality Objectives and Cost-efficient Analysis

total nitrogen; wastewater treatment plants; emission limits; allowable discharge; marginal abatement cost curves; offshore eutrophication

Session 18 - The River-Estuary-Bay Continuum: Unveiling the Carbon and Nitrogen Cycles Under Global Change

The anthropogenic total nitrogen (TN) is a major inducer of coastal eutrophication. With the continuous elevation of pollution control measures, the input of TN from wastewater treatment plants (WWTPs) has become the primary pollution source of TN discharge in the watershed. Determination of TN emission limits for watershed WWTPs is essential for reducing terrestrial TN discharge and controlling eutrophication in nearshore waters. A method to determine the TN emission limits for WWTPs based on the coastal water quality and cost-efficient analysis was proposed in this study. First, the maximum allowable discharge of TN to coastal water was calculated by utilizing the water quality model of the sea area and the linear programming. Under the constraint of the maximum allowable discharge of TN, the TN reduction target and original emission limits of WWTPs were calculated by the scenario analysis of the Soil and Water Assessment Tool (SWAT). Next, the TN reductions of different scales of WWTPs were optimized according to Marginal Abatement Cost Curve-Nonlinear Programming (MACC-NP). Finally, the total nitrogen emission limits of WWTPs were obtained. Qiantang River- Hangzhou Bay was selected for the application of the method. The near-shore waters of Hangzhou Bay are worse than grade IV sea water quality, which is the most prominent eutrophication problem of China's bay, and the Qiantang River is the main river into the sea of Hangzhou Bay from Zhejiang province; the WWTPs are the primary pollutant source of the watershed. The research obtained the maximum allowable discharge of TN in the watershed was 39,600 t/a; the TN in the WWTPs should be reduced by 12.83% from the current situation to meet the constraint. Based on the emission reduction target, the emission limits for WWTPs of the four cities in Qiantang River Watershed, Hangzhou, Quzhou, Jinhua, and Shaoxing, were 7.65±1.74 mg/L, 6.97±1.97 mg/L, 8.34±1.60 mg/L, and 10.08±1.21 mg/L, respectively. Considering the marginal abatement cost curves for total nitrogen removal in different sales of WWTPs, it was found that the marginal abatement cost of large WWTPs was higher than that of medium-sized WWTPs, and allocating all the TN load reduction targets to medium-sized WWTPs was the optimal load allocation scenarios, and the cost could be reduced by 32.98% compared with the original scenario. Correspondence, the emission limits for WWTPs of Hangzhou, Quzhou, Jinhua, and Shaoxing were 6.27±2.47 mg/L, 6.61±2.75 mg/L, 7.80±2.52 mg/L and 10.66±1.24 mg/L, respectively.