From uniform control to scale-based asymmetric regulation: A precision governance framework for watershed nutrient discharge permitting.

Excessive land-based nutrient inputs have caused widespread coastal eutrophication globally, triggering harmful algal blooms, hypoxia, and biodiversity loss. The pollutant discharge permitting system (PDPS) serves as a cornerstone instrument for point source control; however, current approaches to establishing discharge limits for Wastewater Treatment Plants (WWTPs) face dual challenges. Spatially, conventional methods lack systematic consideration of marine ecological objectives, resulting in disconnect between terrestrial pollutant reduction and coastal water quality improvement. Economically, these approaches overlook the "scale-based duality" prevalent in urbanized watersheds-where large centralized WWTPs coexist with numerous small-to-medium dispersed facilities-failing to capture heterogeneity in marginal abatement costs. Using the Qiantang River-Hangzhou Bay (QTR-HZB) system as a case study, this research develops a WWTP management framework integrating dual land-sea environmental capacity constraints with facility-specific Marginal Abatement Cost Curves (MACCs). Results reveal that the wet season is the sole critical period when Total Nitrogen loads exceed marine environmental capacity, with pollutant transport amplification in the downstream estuarine region. By implementing "scale-based asymmetric regulation" that reallocates reduction burdens from small-to-medium WWTPs to large-scale facilities benefiting from economies of scale, the optimized scheme achieves 12.15% reduction in watershed treatment costs while satisfying marine water quality objectives, thereby mitigating eutrophication pressure and ecological risks in Hangzhou Bay. This study establishes a multi-tier discharge limit system comprising Maximum Daily Limit (MDL), Average Weekly Limit (AWL), and Average Monthly Limit (AML), bridging optimization and practical permit implementation. The framework supports a paradigm shift from "uniform concentration control" toward "differentiated precision governance," applicable to estuarine systems worldwide.