Producing a valuable chemical through diversion of wastes can simultaneously resolve the problems associated with increasing wastes and greenhouse gas emissions from conventional chemical production processes. In this work, we investigate the life-cycle greenhouse gas emissions, water, and fossil-fuel consumption for waste-derived polylactic acids (PLA) from three different waste feedstocks, or namely wastewater sludge, food waste, and swine manure, using the Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies (GREET) model. The GHG reduction potential of replacing conventional resins (corn-based PLA, fossil-PET, and -HDPE) with the waste-derived polymer is also investigated. The results show that swine manure-to-PLA pathway is the least carbon intensive (-1.4 kgCO2e/kg) among the three waste-to-PLA pathways on a cradle-to-grave basis, followed by the food waste case (-1.3 kgCO2e/kg) and then by the wastewater sludge case (0.6 kgCO2e/kg). In the baseline scenario, all three waste-to-PLA pathways were less carbon intensive than both fossil-based PET and HDPE on a cradle-to-grave basis: 66% (vs. PET) and 56% (vs. HDPE), 171 and 192%, 181 and 205% reduction in GHG emissions for wastewater sludge-, food waste-, and swine manure-to-PLA pathway, respectively. For all sensitivity cases investigated, the food waste- and swine manure-to-PLA pathways were significantly less carbon intensive than their fossil-counterparts. In terms of the annual decarbonization potential of replacing fossil-based PET or -HDPE, the wastewater sludge- and food waste-pathway showed higher mitigation potential than the swine manure-pathway: i) 18 (vs. HDPE) and 28 (vs. PET) kilotons of CO2e-reduction per year for wastewater sludge pathway; ii) 23 (vs. HDPE) and 26 (vs. PET) kilotons CO2e-reduction/yr for food waste pathway; and iii) about 5 (vs. HDPE or PET) kilotons CO2e-reduction/yr for swine manure pathway depending on the type of conventional resin replaced. However, given the abundant availability of the swine manure feedstocks across the United States, the decarbonization potential of swine manure-based pathway can also increase as the plant capacity or the number of plants grow.

Life cycle analysis of polylactic acids from different wet waste feedstocks

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