Nitrogen balance for the urban food metabolism of Toronto, Canada
A nitrogen balance for the urban flow of food and pre- and post-consumption food wastes was developed to determine the impact of municipal waste management policies and programs on the recovery and recycling of imported nitrogen. A simple input, storage and output balance was used, with particular focus on the fate of waste outputs. The balance was done for the City of Toronto, Canada for 3 years in the 15-year period from 1990 to 2004. The balance revealed that at most, only 4.7% (in 2001) of food waste nitrogen in the urban system was recovered and/or recycled, despite several municipal waste diversion programs. The major losses of nitrogen occurred in the management of sewage wastes and potentially through in the landfill disposal of commercial food. The current focus of municipal waste management programs on the diversion of organic wastes from landfill rather than overall nutrient recovery, and the increasingly stringent regulation of land application of treated sewage wastes, has resulted a small decrease (from 3.35% in 1990 to 2.3% in 2004) in the percentage recovery and recycling of nitrogen in the flow of food and food wastes. Given the potential for the prevention of environmental problems caused by excessive nitrogen accumulation or mismanagement, and the opportunity for recycling food waste nitrogen to close the ecological nutrient cycle and reverse nutrient mining, the simple balance method provided a rapid assessment tool based on easily obtainable data to highlight the sources of major loss at which to target future municipal waste management strategies.