The world's cities occupy about 2 per cent of global land surface, but they use 75 per cent of the world's resources and release about the same percentage of global wastes (Giradet, 1992). It has already been noted that more than 50 per cent of the world's population will soon be living in cities, contributing to a massive consumption of global resources. The modern city is an 'open system'. That is, cities are not self-contained, they are maintained by exchanges of materials, energy and information with areas beyond their periphery. The concept of 'metabolism' can be used to form an understanding of this process. As applied to people, metabolism refers to the processes which we use in producing food and energy to conduct our daily lives. 'Urban metabolism refers to the material and energy inputs needed to meet the living and nonliving components of urban systems____
When we have used these inputs, we have what is commonly referred to as waste' (Keen, in Birkeland, 2002). In natural ecosystems such as the rainforest, the waste from one process becomes a resource input for another process. For example, animal droppings and rotting vegetation serve as nutrients for plant life. The wasteful process associated with city metabolism is linear in form. That is, the city consumes goods, energy and food at high rates and pollutes the environment heavily with organic wastes, noxious fumes and inorganic wastes (Figure 4.10). It has been suggested that this linear urban metabolism should be converted to a form of 'circular metabolism' through the actions of design and management (Figure 4.11) (Giradet, 1996; Roelofs, 1996). Circular metabolism approximates to the systems found in nature where waste products are integrated into the wider ecosystem - that is, new inputs of energy and output of waste are minimized through the process of recycling.
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