A model for improving sustainable green waste recovery

Green waste, consisting of leaves, wood cuttings from pruning, and grass collected from parks and gardens, is a source of biomass that can be used for material and energy valorization. Until recently, the EU-Waste Directive 2009/28/EC allowed green waste to be used as feedstock only for compost. This paper presents a framework for examining the most sustainable processing options for green waste valorization in terms of the triple bottom line, People-Planet-Profit. A mathematical model is presented that optimizes profit, as well as environmental and social impact. Four processing options are compared and analyzed: composting, partial separation of wood cuttings prior to composting, partial separation of chopped wood cuttings in the sieve overflow after composting, and a combination of the last two options. Computational results for a Belgian case demonstrate that the optimal sustainable recovery solution is to separate a fraction of the wood cuttings in the sieve overflow for use as green energy feedstock. Additionally, if sufficiently large subsidies are available to separate wood cuttings prior to composting, the optimal solution shifts to one of partially separating the cuttings both prior to composting and in the sieve overflow, and then using the combined cuttings for energy valorization. Whenever cuttings are partially separated the remainder of the green waste is composted.