Dietary shifts and nitrogen losses to water in urban China: the case of Shanghai.

China's extraordinary economic development has provided the country's growing population with easier access to animal food products, especially in densely populated urban agglomerations. Increased consumption of such products translates in a higher amount of nitrogen (N) excreted in the form of human manure. Depending on the connection to a sewerage system, or lack thereof, and the N removal efficiency from wastewater treatment plants (WWTPs), a share of the excreted N gets ultimately discharged to water bodies, causing eutrophication. In heavily urbanised areas, N losses from household food consumption account for a dominant portion of total N losses to water. In this study, we firstly estimate dietary N intake, excretion and consequent N losses to water from the residents of Shanghai in 2012. We then explore different scenarios to 2030, in terms of further dietary modifications and different levels of development of the city's sewerage system and WWTPs. In 2012, Shanghai's residents excreted a total of 148.4 Gg N, 54% of which ultimately reached the city's water bodies in diffused N form. The urban population contributed for the majority of the N losses (93%) and showed a higher per capita N load, due to limited N removal efficiency from WWTPs and the significant portion (27%) of residents not connected to the sewerage and directly discharging their excreta to water. The vast majority of the scarce rural population were not connected to the sewerage system and showed a much lower per capita N load, mainly due to the common practice of recycling excreta for agricultural practices. We identify two main approaches to reduce dietary N losses: (1) improving N removal efficiency and sewerage connection rates towards the levels of OECD countries; (2) managing the increase of dietary N intake by promoting healthy and sustainable consumption, as recommended by recent dietary guidelines. According to our scenario analysis, technological improvements can potentially achieve a more significant reduction of total N losses and are easier to implement. Managing demand of animal food and consequent N intake would only stabilise N losses around 2012's levels. On the other hand, a dramatic increase of animal food consumption could have detrimental effects on the city's water bodies, more so if the expected population growth will not be met by an adequate development of a more capillary sewerage system. This study provides valuable insights on dietary N losses in one of China's most developed mega cities, strongly advocating for the necessity of improving N removal efficiency from WWTPs and reducing the percentage of urban residents directly discharging their waste to water bodies.