Abstract
The management of plastic waste and dairy manure is associated with adverse environmental impacts which drive interest in bioplastic research. Over 460 million tons of petroleum plastics and 33.4 MMt CO₂e of manure are produced annually. To resolve the linkage between the growing waste and associated environmental concerns, polyhydroxyalkanoates (PHAs) were sourced using a dairy manure feedstock, redirecting waste into a bioplastic. This study applied a cradle-to-gate life cycle analysis (LCA) to compare manure-derived PHA with polylactic acid (PLA) bioplastics, petroleum-based polystyrene (PS), and conventional manure management. All data were compiled using peer-reviewed literature and bench-scale models. All systems were converted to the functional unit plastic production from a 1200-head dairy, equivalent to the output of 126 kg of plastic. The LCA was conducted using openLCA software, with impact characterization performed using the TRACI 2.1 midpoint methodology. The LCA focused on acidification, eutrophication, freshwater ecotoxicity, global warming, human health (particulate matter), and smog formation. Comparative results of the LCA indicated that manure management dominated PHA in most impact categories aside from freshwater ecotoxicity, where the PHA emissions had more environmental impact. PLA and PS environmental impacts dominated PHA in all categories aside from eutrophication. Additionally, the inclusion of anaerobic digestion to PHA production was an important factor for calculating environmental impacts, as anaerobic digestion improved environmental performance in most impact categories. While manure-based PHA was less impactful in most categories when compared with its counterparts due to less energy and chemical usage, its freshwater ecotoxicity impacts were greater. These results reflected the challenges in evaluating a product as sustainable due to the nuance of environmental outputs.