Evaluation of potential environmental benefits from seawater toilet flushing.

Water scarcity has become a global issue that has forced many communities to seek alternative water resources. The majority of water on the earth's surface comes from the sea. Seawater has the potential to mitigate water stress after proper treatment. In Hong Kong, seawater has been used directly for toilet flushing for more than 50 years and the seawater toilet flushing (SWTF) system serves 80% of the residents. However, its environmental feasibility remains unknown. This study is a pioneer evaluating the environmental performances of the SWTF system by comparing SWTF with other alternative water resources including desalinated seawater, desalinated wastewater effluent, centralized wastewater reclamation and on-site greywater reclamation systems, while the conventional long-distance imported water scenario is set as the baseline for comparison. This evaluation is first conducted in the Southern District and North New Territory in Hong Kong, demonstrating SWTF is significantly more environmentally-friendly than other alternative water resources, which is the only alternative water resource application approach with environmental impacts comparable with the conventional long-distance imported water scenario. On the contrary, other alternative water resources application approaches would result in additional environmental impacts. Seaside distances and effective population density are two major geographical impact factors effecting the environmental impacts of different water systems in these two districts. The benefits obtained from SWTF in fourteen cities in South China are further investigated. It is confirmed that SWTF can significantly relieve water stress with the lowest environmental impacts comparing with other alternative options in these cities if alternative water resources must be applied for domestic usage. However, the assessment from different aspects should be further conducted to compare these alternative water resources.