Under the background of rapid urbanization process, cities need more and more substance and energy to maintain. The direction of material flow changes along with the urban development. Urban metabolism studies the material and energy flows arising from urban socioeconomic activities as well as regional and global biogeochemical processes. Food consumption takes an assignable part of substantial input and continues to grow with rapid urbanization process. The unutilized part of CNP (Carbon, Nitrogen and Phosphorous) in food may cause environment problems. The imbalance of urban metabolism is the root of environment problems. It is urgent to investigate the process of food-sourced CNP and to find out the ways to reduce environmental load.
Substance flow analysis (SFA) method focuses on the flow of a certain chemical element and considers all related transformation and transition of this element. Based on SFA method, food-sourced CNP metabolism processes in urban system were characterized systematically in this paper. We focused on three main metabolic processes (food consumption, waste disposal and human metabolism) to analyze the similarity and difference among the urban metabolism of food-sourced CNP substance. We found that CNP share many links in metabolic path but have its unique characteristic independently. After considering about these, we built up the framework of food-sourced CNP metabolism process and make it possible to study these three elements simultaneously and systematically.
We took Xiamen as a case city to trace the urban metabolism of food-sourced CNP from 1991 to 2010. We calculated the flux of food-sourced CNP metabolism in Xiamen, which starts from food consumption and ends in human body or emission to the environment.
The results showed that: 1) The most increasing environmental load of food-sourced CNP are carbon in air (from 74859 t/a to 149763 t/a), nitrogen in soil (from 3897 t/a to 9323 t/a) and phosphorous in soil (from 406 t/a to 1049 t/a); 2) The path with maximum flux is "food-food eating-human fecaluria-wastewater treatment-sludge-sludge landfill-soil"; 3) The urban metabolism of food-sourced CNP mainly resulted in the increasing of loading in soil and water body; 4) The part of food-sourced CNP that stored in human body is extremely low with the proportions of 0.3%, 0.5% and 1.6% in 2010; 5) The proportions of CNP in kitchen waste to that in food source were 13.7%, 32.2% and 70.3% respectively, which had great potential of reduction management.
Some advices to reduce CNP environment loading and optimize the metabolism process systematically were provided. The advices are: 1) increasing the edible rate of food and encouraging consume food in economy way; 2) utilizing sludge from wastewater treatment to produce bricks; 3) producing fertilizer with urban kitchen waste for rural use.