Compostable plastics in green waste: a low down on their breakdown
Compostable plastics may appear to be a straightforward solution for challenging-to-recycle waste streams. However, their performance in real-world composting conditions needs further scrutiny.
An experiment was conducted to determine how well AS 4736-2006 certified compostable bags and forks composted in a commercial composting facility and to assess if the compost produced has ecotoxicological impacts.
The compostable forks remained visible in the compost for 117 days, while the bags were visible for 70 days. The forks also left a fine, white, powdery residue throughout the remainder of the composting process. Additionally, both items negatively impacted the growth of earthworms and plant roots.
The EPA used these findings to inform the development of its FOGO position. Under this position, AS 4736-2006 certified compostable kitchen caddy liners are the only compostable plastics permitted to enter the composting waste stream.
Composting is often thought to be a simple solution for our single-use plastic item addiction. Compostable plastic standards (such as AS 4736-2006 and AS 5810-2010) require >90% biological transformation (biodegradability) of the plastic with no associated toxicity. However, some manufacturers make questionable claims of ‘greenness’ with limited evidence to support them.
The NSW EPA commissioned CSIRO to examine some of these claims by assessing the real-life compostability of 2 AS 4736-2006 certified compostable items in a commercial composting facility:
- kitchen caddy liners made from polybutylene adipate terephthalate (PBAT) and thermoplastic starch blends
- compostable forks made from polylactic acid (PLA).
The experiment was set up with 2 controls and 2 treatment groups using 2 of the most commonly available compostable plastic polymers.
Four 50-tonne FOGO samples were prepared with:
- a treatment stockpile containing compostable forks
- a treatment stockpile containing compostable kitchen caddy liners; and
- two control stockpiles containing only FOGO.
To be representative of a potentially realistic scenario, each treatment stockpile contained 250 kg (0.5% by weight) of the certified compostable material. The experiment ran for a 181-day composting period, with regular sampling performed to examine the breakdown of the compostable plastic items over time. At the end of the composting period, toxicity assessments were conducted to evaluate the compost's comparative impact on earthworms, nematodes, wheat, soil respiration, and soil nitrification.
After 75 days, the compostable forks had reduced to small white particles with a texture similar to that of the bulk compost, which persisted throughout the experiment. Individual polylactic (PLA) particles could not be distinguished from the rest of the compost after 117 days.
In contrast, the compostable caddy liners exhibited signs of significant degradation after 14 days and became indistinguishable from the rest of the compost after 70 days. Both polymers' molecular weight was reduced by over 75% after 6 weeks of composting.
Ecotoxicological findings from the completed mature compost samples were mixed. Soil respiration and nitrification were variable across both control and experimental treatments. The compostable items didn’t impact the reproduction and growth of nematodes, nor the survival of the earthworms. The caddy liner treatment group showed greater wheat shoot mass and growth than the control group. However, both the caddy liner and fork treatment groups had negative impacts on wheat root growth and worm growth.
The AS 4736-2006 compostable plastic standard requires the worm mass of the treatment group to be within 10% of the mass of the control. In this case, neither of the certified compostable forks nor the certified compostable bags would have achieved compliance with AS 4736-2006.
At the time this study was conducted in 2020, the NSW government was in the process of navigating the future rollout of food organics collection as part of the FOGO waste stream and developing future plastics actions, with questions surrounding potential FOGO inputs and plastic bans at the forefront of mind.
Currently, and in part as a result of this work, the NSW EPA exclusively permits food organics, garden organics, AS 4736-2006 certified compostable plastic kitchen caddy liners and fibre-based caddy liners to enter the FOGO waste stream. The NSW EPA has also taken action to ban the supply of unnecessary plastics, such as the PLA forks used within this study.