Promoting ship sewage recycling in China's inland waterways: A balanced funding approach for advanced disposal fees and subsidies

Ship-generated sewage presents significant environmental challenges in maritime transport, particularly in inland waterways. In China, the existing fee-free delivery policies fall short of providing sufficient incentives for ports and ships, while heavily relying on unsustainable government subsidies. To tackle these challenges, this paper introduces a financially sustainable Advance Disposal Fee (ADF) system paired with a subsidy recycling policy. The proposed approach is built on a tri-level Stackelberg game framework that involves the government, two ports, and a fleet of ships, aiming to optimize recycling policies and stakeholder decision-making. This framework is further formalized into a mixed-integer nonlinear tri-level programming model, where four stakeholders strategically maximize their respective profits. Using two ports along the Pearl River in China as a case study, the findings reveal that the proposed recycling policy substantially improves engagement from both ports and ships, achieving a sewage delivery rate of 89 %. These results provide practical guidance for the development of government recycling policies, the selection of port reception strategies, and the formulation of ship sewage management plans.