Impact of waste processing byproducts on the carbon footprint of integrated waste-to-energy strategies.

Resource recovery is a promising solution for multiple waste and economic challenges worldwide. Although the life cycle environmental performance of resource recovery systems has been widely studied, the impacts of their resultant waste processing byproducts remain unassessed. Depending on their management techniques and destinations, such large waste streams, such as non-recovered recyclables and anaerobic digestate, may significantly alter the overall environmental footprint of the integrated solid waste management (ISWM) strategy. This research aimed to assess the life cycle impacts of various ISWM strategies incorporating material recovery and waste-to-energy (WTE) systems. The examined strategies were based on combinations of incineration (INC) and anaerobic digestion (AD) plants, in addition to material recovery facilities (MRFs) and landfills. The study used eighteen scenarios to explore the effect of the potential pathways of waste processing byproducts on the overall carbon footprint of the strategies. The proposed lifecycle assessment framework was applied to an emerging metropolis. The highest energy recovery was achieved in the dual AD and INC scenarios, particularly when the digestate was incinerated, followed by the mono INC scenario. The highest material recovery, specifically recycled materials and digestate, was accomplished in the MRF+AD and MRF +AD+INC scenarios, in which digestate was marketed and non-recovered recyclables were incinerated. It was found that alternative pathways of waste-processing byproducts can reduce the overall global warming potential (GWP) of the base scenarios. Combining MRF and INC as well as MRF, AD, and INC while incinerating non-recovered recyclables, disposing ash, and marketing digestate (if any) were the optimum environmental options in terms of GWP. Incineration of non-recovered recyclables reduced carbon emissions by 29% compared to disposal, whereas marketing and incinerating the digestate positively impacted GWP compared to landfilling (83 and 77%, respectively). A breakdown of the sub-processes revealed that the energy recovery, recycling, and landfilling processes had the highest contributions to the total GWP (up to 88, 92, and 98%, respectively); the collection and transportation processes had the least impacts (2 and 13%, respectively). The more waste-processing byproducts were incinerated, the greater GWP contribution of energy recovery.