Material flow analysis of phosphorus through food consumption in two megacities in northern China.

The key stocks and flows of phosphorus (P) through food consumption in Beijing and Tianjin, two megacities in northern China, were explored using a material flow analysis (MFA) approach to construct a static model of P metabolism. A total of 4498 t P has accumulated with 72% of P flow imported through food consumption eventually remaining in Beijing in 2008. Around 64% of the total inflow of P (2670 t) remained in Tianjin in 2008. P in the uncollected sewage from both urban and rural residents and the effluents from sewage treatment plants has significant negative effects on water quality. An average of 55% the P flow remained in the sewage sludge through urban food consumption. The key problems in P metabolism and management in megacities are identified based on the quantitative analysis of P cycling through food consumption. Relevant solutions for improving P recycling efficiency are also discussed. It is important to link P flows with environmental regulations and to establish a strong coordination between urban and rural areas for nutrient recycling to attain sustainable development of megacities.