Emergy indices and ratios for sustainable material cycles and recycle options

Abstract

Emergy (spelled with an m) is the energy required to make a service or product expressed in energy of one form. The emergy used in the life cycles of major building materials as well as the emergy inputs to waste disposal and recycle systems were evaluated. Emergy per mass (expressed as solar emergy per gram [sej/g]) for building materials varied from a low of 0.88 E9 sej/g for wood to a high of 12.53 E9 sej/g for aluminum. Generally, emergy per mass is a good indicator of recycle-ability, where materials with high emergy per mass are more recyclable. Recycling added between 1 (cement) and 234% (wood) to the emergy inputs per gram of building materials. The analysis of materials suggested that recycle of wood may not be advantages on a large scale, but metals, plastic, and glass have very positive benefits. Two types of solid waste disposal systems were evaluated: municipal solid wastes (MSW), and construction and demolition wastes (C&D wastes). Expressed as emergy, the costs of collecting, sorting and landfilling (for 25 years) MSW were 251.0 E6, 8.2 E6 and 37.9 E6 sej/g, respectively. The costs of demolition, collection, sorting and landfilling C&D wastes were 49.0 E6, 21.7 E6, 6.7 E6, and 11.7 E6 sej/g, respectively. Three different recycle trajectories were identified and analyzed: (1) material recycle (the ‘standard’ recycle of a material where it is used again as the same material [i.e. glass bottles recycled and made again into glass bottles]); (2) by-product use (where a by-product from some process is used to make something entirely different [i.e. flay ash in concrete]); and (3) adaptive reuse (where a material after recycle is reused for an entirely different purpose [i.e. plastic milk cartons are converted into plastic lumber]). Three recycle indices measuring the benefits of various recycle systems suggested that materials that have large refining costs have greatest potential for high recycle benefits and that highest benefits appear to accrue from material recycle systems, followed by adaptive reuse systems and then by byproduct reuse systems.