Proposal of six indicators of material cycles for describing society's metabolism: from the viewpoint of material flow analysis

The first essential step toward promoting material cycling (recycling) is to understand the current state of the material cycles in a society. However, the types of material cycles that we should document and the methods of their quantification are not standardized. In addition, an important factor in promoting material cycling is the establishment of the indicators that will enable objective measurement of the effectiveness of policies and actions. This paper categorizes the forms of material cycles that we should document, discusses their characteristics, and proposes six indicators of material cycles for describing society's metabolism. We reached the following conclusions: (1) the forms of material cycles fall into three types, according to differences in how they are quantitatively measured and how they contribute to the objectives of material cycling: reuse of used products, recovery of by-products (as material and heat), and recovery of used products (as material and heat). They should be considered separately in terms of measuring material cycles and in establishing indicators of material cycles; (2) six indicators for measuring the material cycles are proposed, on the basis of the types and objectives of material cycling: direct material input (DMI), use rate of recovered used products (URRUP), material use efficiency (MUE), material use time (MUT), recovery rate of used product (RRUP), and domestic processed output (DPO). They reflect the main points in life cycles of materials; (3) three practical problems in capturing material flows and calculating the indicators exist: to capture by-products and used products that are not captured in current statistics, to distinguish between by-products and used products, and to capture product stocks. (C) 2003 Elsevier B.V. All rights reserved.