Environmental impacts of conventional plastic and bio-based carrier bags

Abstract

Background, aim, and scopeWorldwide, the production of biodegradable and compostable plastics has steadily grown. In Part 1 (Khoo et al. 2010), life cycle assessment (LCA) was applied to compare the production stages of a bio-based bag (made from polyhydroxyalkanoate or bio-plastic (PHA)) with polyethylene plastic bag. The scope of the study is within the context of Singapore and does not include other types of conventional or bio-based polymers (e.g., polylactic acid (PLA), thermoplastics, high-density polyethylene (HDPE), EPS, etc). This article (part 2) proposes to investigate the end-of-life options of both bags.Materials and methodsFor part 2, the same LCA methodology is used for the investigation. The LCA system for part 2 starts with disposal options: (1) land filling at Singapore’s offshore Semakau Island, (2) incineration, and the (3) composting of bio-bag. Two useful products, energy and compost, will be produced from options 2 and 3, respectively. While the energy from the incineration of both bags are fed back into the LCA production stage, compost from bio-bags can be used as a peat substitute, thus generating carbon dioxide savings from reduced peat production. The end-of-life environmental impacts were generated for global warming potential, acidification, and photochemical ozone formation. A landfill impact, based on Singapore’s offshore landfill capacity, was also generated. Next, the environmental impacts of the entire life cycle of both products are calculated for a few scenarios—from cradle-to-grave.ResultsThe highest end-of-life impacts are observed from the land filling of bio-bags. Next highest disposal impacts are from incineration, and least of all (minimal) from the composting of bio-bags. The greenhouse gas savings from peat substitutes derived from the compost material is rather insignificant. Overall, the cradle-to-grave results demonstrates that the environmental burdens generated from any of the disposal options are less significant compared to those from both products’ life cycle production stages.Discussion and conclusionsUnless plans for energy recovery systems are in place, the least preferred route for the disposal of bio-bags are at landfills. From the trend of the final cradle-to-grave results, it can be claimed that the life cycle production of bio-bags from PHA can only be considered as environmentally friendly alternatives to conventional plastic bags if clean energy sources are supplied throughout its production processes. This claim was in agreement with other LCA work carried out for the life cycle production of PHA, with the supply of energy by corn stover waste or the consideration of wind power supply in the replacement of grid electricity. It was also observed, however, that some of the results in this article vary from other LCA work carried out by other authors. Some of the reasons included variations in LCA scope and the different range of materials investigated (PLA, HDPE, and thermoplastic starch).Recommendations and perspectivesPresently, the wide range of LCA work carried out on biodegradable polymers differs considerably in the amount of reported background data and the level of detail concerning the LCA system and production methods. A globally accepted as well as concerted effort to describe in detail the life cycle production steps involved, disposal options, type of energy supplied to the production chain, for a well-selected range of polymer materials should be conducted. Meanwhile, it is recommended that a conservative approach is required in introducing bio-based carrier bags as a solution for solving plastic waste issues. Future LCA investigations should also look into the reuse of carrier bags, which is anticipated to bring much greater environmental and sustainable benefit than the replacement of bio-bags with plastic ones.