Mission compostable

Opinion: The July 2023 ban of many single-use plastics in Aotearoa New Zealand has led to the boom of the compostable products market, as the more ‘eco-friendly’ alternative to plastic packaging. Many of us assume, or at least hope, that ‘compostable’ coffee cups, bags, food trays and so on will be broken down and returned to earth in a way that’s good for the garden, but are they?

As a doctoral candidate in environmental science who is passionate about environmental contaminants, I was one of the team of researchers at the University of Auckland commissioned by the Ministry for the Environment to undertake research into additives in compostable products, and how they affect our soils through the different chemicals present in them.

Our research resulted in three reports published on the ministry’s website, which detail the findings and provide some recommendations to help improve the practices surrounding compostable products in Aotearoa New Zealand, with a focus on soil health. Our research showed that what is and isn’t compostable is very much a work in progress and there’s a lot more to do.We looked at products marketed or perceived as compostable: paper cups, a tea bag, a bin liner, flat brown paper bags, wooden cutlery and a fibre tray. We found that we really don’t know what is being added in these products. The pizza box tested was found to have every contaminant that we tested for, including phthalates (hormone-disruption chemicals) and heavy metals with high levels of copper. Bin liners and brown paper bags showed the presence of BPA, a chemical that has been voluntarily phased out of many baby products and plastic bottles due to evidence they disrupt endocrine systems and affect the function of our body’s natural hormones.

Another chemical of interest we found was PFAS, aka ‘forever chemicals’. These have been in the international news in recent years as it has become apparent that what makes them so valuable to manufacturers is also what makes them potentially damaging to the environment. As their moniker suggests, forever chemicals have remarkable stability, which allows them to last in the environment for extended periods of time.

PFAS are popular with manufacturers because they provide grease and water resistance to materials like paper. We found high levels of them in a sugar cane fibre tray, which had been added intentionally at levels above what would allow a certification. We also found them in the compostable coffee cups, the paper bags, the pizza box, and the tea bag, though it’s likely this PFAS came from contamination during the production process rather than them being added deliberately.

PFAS are unlikely to degrade during the composting process, and they may have detrimental effects on soil ecosystems, from bacteria biodiversity to earthworm weight – those with worm farms would have to be cautious about this.

Gardeners should also be aware; adding the ‘compostable’ products to compost systems is unlikely to improve the nutritional content of the end compost. Instead of providing more instant nutrients (like compost made from food scraps) that would go back into the growth of food, these products will contaminate compost which in turn will contaminate the soil it is applied to and then may be available for plants to uptake.

The actual compostability of products promoted as compostable is confusing for customers, as are labels on products claiming to be ‘biodegradable’ or ‘bio-based’. Much of the labelling fails to give customers the information they need about where and how these products should be disposed of and what specific environmental conditions they need to be able to break down to avoid ending up in landfill.

Our soil is a taonga and should be protected from contaminants given its links to both our physical and mental health. We need tougher policies on labelling and claims – products should have simple wording that provide information on where the product should end up at the end of its life. This includes enforcing certifications so a consumer knows what they are buying without having to decipher confusing or misleading terminology.

While we acknowledge the limitations of our study (we were resourced to analyse only 10 products), we have identified several compounds of concern, clearly showing the need to do more research about which of them have the potential to create long-lasting environmental impacts.

Although compostable products seem like a more environmentally friendly alternative to plastics, they still require the production of a single-use item containing chemicals that can take decades to degrade. It’s better to stick with reusable cups and keep coffee cups out of the compost—save that space for food scraps instead!