Cost-benefit analysis of introducing technology to rapidly degrade municipal solid waste

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Abstract
Due to an increasing awareness of the true economic and environmental cost of conventional landfilling, recent interest has developed in Australia in technologies that accelerate the degradation of the organic fraction of municipal solid waste (MSW). The management of the organic waste stream, which typically makes up 60 to 70% of MSW in Australia, is central to reducing the reliance on landfill space. One option is to digest the organic fraction prior to landfilling, or preserve landfill space by accelerating the decomposition of the organic fraction within the landfill. This paper quantifies the benefits of digestion as a function of the degradation time, td. The analysis considers both invessel and landfill-based bioreactor technologies and calculates net economic impact, expressed as $US t-1 MSW, as the sum of enhanced and more rapid biogas retrieval, saved landfill space, reduced environmental disamenity and reduced postclosure costs, minus the capital and operating costs to implement the technology. The benefits, on a per tonne basis, are shown to be insensitive to the size of the waste stream while costs diminish as the waste stream size increases. A conventional landfill with td = 20 yr is used as a basis of comparison. At 1800 t day-1, the maximum level of benefit is 13 $US t-1 at td = 2 months, diminishing monotonically to zero at td = 20 yr. The cost of achieving td = 2 months is the cost of invessel digestion, estimated to be 99 $US t-1, resulting in an overall increase of 86 $US t -1 in waste management costs. Similarly, degradation times of 2 < td < 5 yr provide a benefit of 8 to 11 $US t-1 for a waste stream of 1800 t day-1. This rate of degradation can be achieved with landfill-based bioreactor technology. The cost of additional infrastructure needed to implement accelerated degradation, such as pretreatment pads, internal leachate distribution networks and additional gas wells and generators, is estimated to be less than 1 $US t-1. Bioreactor technology is therefore appealing, although their is uncertainty about the additional operating costs needed to run a landfill as a bioreactor. Additional operations would mainly revolve around segmenting the waste mass to tightly control leachate distribution. The cost of these additional operations are unlikely to exceed the operating cost of a modern conventional landfill, estimated at 9 $US t-1. Therefore, landfill bioreactor technology should be cost effective.

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