An environmental hazard assessment of biodegradable plastic bags
Single-use plastic bags were widespread in the NSW litter stream before the 2022 ban.
The safety of alternative bag materials with a range of marketing claims was assessed by testing 5 bag types, including a traditional plastic bag, for chemical leaching and aquatic toxicity. All bags released metals like copper and zinc above the Australia and New Zealand Environment and Conservation Council (ANZECC) freshwater quality guideline limits. Compostable bags were also found to exhibit toxicity, harming water fleas and certain microbes.
The findings demonstrated that all bag types, regardless of their marketing claims or material composition, pose some environmental risk.
Informed by this evidence, the NSW Government implemented a ban on lightweight plastic bags of all materials in 2022, aiming to reduce litter and the risk posed by single-use plastic bags in the environment.
Before the 2022 plastic bag ban, single-use plastic bags were a common sight in the NSW litter stream.
Concerns were growing about their widespread presence and the potential harm they posed to ecosystem services and the environment. Around the same time, several manufacturers began promoting new types of single-use plastic bags as being environmentally friendly alternatives, claiming they would degrade without causing ecological harm.
These single-use bags were often made from alternative materials intended to compost, biodegrade, or fragment within the environment. To investigate these claims and inform decision-making, 5 bag types were assessed to see if they leach chemicals or exhibit aquatic toxicity.
Of the 5 bag materials tested:
- two were designed to be compostable
- one was degradable
- one was oxo-degradable
- one was a standard high-density polyethylene (HDPE) bag.
The compostable bags were made from thermoplastic starch (TPS) and a polybutylene adipate terephthalate (PBAT) and TPS blend. The degradable bag was made of high-density polyethylene (HDPE) mixed with TPS, and the oxo-degradable bag was made of HDPE but contained chemical additives to assist the material to fracture within the environment.
To determine the impacts of material aging each bag type was aged via exposure to three months of simulated sunlight at 30 °C. Leachate from aged and un-aged bags was extracted by incubating 100 g of each bag in 750 mL of moderately hard water at 50 °C for 72 hours. The leachates were analysed for organic compounds, metals, dissolved organic carbon, and dissolved oxygen. Ecotoxicological tests were also conducted on water fleas, nematodes, and various fungal and bacterial species.
The Australia and New Zealand Environment and Conservation Council (ANZECC) freshwater quality guidelines set acceptable limits for pollutants such as metals, nutrients, and other chemicals to protect aquatic ecosystems.
In this study, there were no major differences in the metal or organic content of leachate between aged and un-aged plastic samples. However, leachate from all bags exceeded the ANZECC limits for copper and zinc. Both compostable bags, the degradable bag, and the standard HDPE bag exceeded the guidelines limits for cadmium, chromium, tin, and zinc. The compostable TPS bag exceeded limits for cobalt and lead, and the PBAT/TPS starch blend exceeded the guidelines for phenol concentrations.
Bags containing starch components (i.e. two compostable and one degradable) released more organic carbon and caused greater reductions in oxygen levels than HDPE and oxo-degradable bags. Leachate from both compostable bags also showed toxicity, harming water fleas and reducing the growth of microorganisms.
At the time this study was conducted in 2016, various Australian states and nations abroad had taken a range of actions to mitigate the environmental impact of single-use plastic film bags. With each jurisdiction defining single-use plastic bags slightly differently and often creating exemptions for compostable materials or materials that meet certain other material specifications.
This study demonstrated that while each of these materials is markedly different, all present a risk of harm to the environment. In 2022, with this information in hand, the NSW government banned the supply of all lightweight single-use plastic bags, regardless of material type.
Read the full CSIRO report: An environmental hazard assessment of biodegradable plastic bags.