Analysis of energy footprints associated with recycling of glass and plastic—case studies for industrial ecology

Abstract

Roundput [Int. J. Sustainable Dev. World Ecol. 8 (2001) 29] is one of the most important principles of the development of both natural and industrial ecosystems, and is especially important for analysis of an ecosystem’s dynamics and overall functioning, as it is related to an extent to which energy and matter are recycled and used in a cascade-type operation. Here we argue, using two modelling case studies from the UK and Switzerland, that increasing recycling rates for plastic and glass would improve the energy budget of waste management programmes, and, therefore, benefit the corresponding industrial ecosystems. In the first case study we show that the major source of energy savings from glass recycling is through increased use of cullet in glass manufacture (5.4% reduction in total energy consumption with 100% glass recycling when compared to the present-day situation). In terms of energy consumption, recycling is the preferred waste management option, even if a large proportion of the recycled glass is diverted for use as aggregates. Further energy savings could be achieved by introduction of a city-wide kerbside collection scheme, which would result in an estimated maximum reduction (100% recycling rate) of 7.6% in energy consumption for processing of the Southampton household glass wastes. In the second case study we compare the situation in which all wastes are burnt at a MSWI plant with two scenarios assuming that 8.1% of the plastic is diverted into a cement kiln (mixed plastics; scenario 1) or a mechanical recycling plant (polyethylene, polypropylene, polystyrene; scenario 2). The resulting net primary energy consumption values for both scenario 1 (5.85E8 MJ or 60% relative to the reference scenario) and 2 (7.46E8 MJ or 76.6% relative to the reference scenario) use less primary energy than the reference scenario (9.74E8 MJ). This means that, from the point of view of resource consumption, the diversion of plastics waste away from the MSWI plant has a beneficial effect. Therefore, the increased recycling of glass and plastic would benefit the industrial ecosystems in terms of energy savings. This is similar to the patterns observed in most natural ecosystems, and a careful consideration of this similarity within a framework of industrial ecology should help to reduce the conflict between the two systems.