What Happens to Compostable Packaging in Home Composting Systems?

This post is greater than 6 months old - links may be broken or out of date. Proceed with caution!

Dr Muhammad Ali CEng, Associate Professor in Materials and Environmental Innovation

School of Civil Engineering and Surveying, University of Portsmouth

Compostable caddy liner, magazine wrapper, cups and cutlery on a white background — Compostable caddy liner, magazine wrapper, cups and cutlery – Anita Carey (PhD Researcher at the University of Portsmouth)

Single-use plastics are widely recognised as a major source of environmental degradation, directly contributing to land and water pollution. Nearly all petroleum-based plastics are derived from fossil fuel hydrocarbons and their production significantly contributes to harmful greenhouse gas emissions leading to climate change. It is estimated that plastic packaging accounts for roughly half of all plastic waste generated in the UK. This amounts to 2.5 million tonnes of plastic packaging each year, equivalent to a staggering 100 billion pieces of plastic packaging. Therefore, reducing the environmental impact of plastic pollution, particularly plastic packaging, is critical to addressing the constantly growing threat to our natural environment.

Many plastic producers, governments and policymakers are being forced to shift towards products such as compostable plastic packaging, bio-based plastic bags, biodegradable food containers, etc. However, it is becoming clear that there is a lack of understanding in public about compostable and bio-based plastics, as these are often considered materials intended for dry recycling. These are usually mixed or disposed of with petroleum-based plastics that are non-compostable, which undermines the concept of biodegradable or bio-based plastics. Further, as more and more bio-based plastic wastes will be generated in the future, there is a need to understand and solve various complex mechanisms associated with the handling and treatment of compostable materials including compostable packaging.

Three black dalek Composters in a courtyard — Dalek Composters - Anita Carey (PhD Researcher at the University of Portsmouth)

Compostable packaging is typically marketed as ‘made from natural resources’, which aims to decompose in composting environments. However, initial research studies conducted at the University of Portsmouth have found that home composting success is inconsistent with packaging frequently remaining intact after the composting process is completed. This issue is compounded by unclear composting instructions and the fact that some "compostable" materials are only suitable for industrial composting facilities, not home systems.

Over the years, the composting process has evolved from being static piles or windrows to more sophisticated in-vessel composting systems that are capable of handling large amounts of various biodegradable wastes. However, there is limited research on composting organic waste (kitchen and garden waste) along with compostable packaging in a home composting system. Composting is a biological process that breaks down organic waste into a stable dark brown crumbly earth-like material. The composting process is influenced by several parameters, including temperature, moisture, aeration, feedstock (organic waste type), and the presence of microbial communities. Maintaining these key parameters within optimal ranges is crucial for effective composting. While achieving this control is feasible in large-scale industrial composting facilities, it can be more challenging in smaller setups such as the home composting bins. A variety of home composters are readily available in the market, including models like the Dalek, modular wooden systems, tumblers, and various insulated bins. It is important to recognise that home composting systems differ significantly from large industrial operations. The composting process is typically much slower due to lower temperatures and limited control over aeration, moisture, and the carbon-to-nitrogen ratio. This lack of control can affect the composting speed and the overall quality of the finished compost.

A tray of young plants on a shelf — Plant growth trial on composted products – Muhammad Ali

Many local authorities in the UK offer paid garden waste collection services. However, many homeowners and families with gardens, along with avid gardeners and composting enthusiasts, prefer to create their compost from garden waste like lawn clippings, hedge trimming etc. Many add organic kitchen waste such as fruit and vegetable peelings, ground coffee, and tea bags along with paper and cardboard to balance the feedstock in their home composter. With the increasing availability of compostable packaging like compostable carrier bags and food waste caddy liners, home compost bin owners are encouraged to add items labelled as compostable to their bins. However, the effectiveness of home composting for breaking down compostable packaging has received mixed reviews. Many users report finding fragments of compostable caddy liners even after a year in the compost bin. These findings highlight the need for a clearer understanding of the complex mechanism associated with home composting systems and how various bio-based and compostable packaging behaves within them.

Raman spectrometer at the University of Portsmouth Environmental Technology Laboratory – Muhammad Ali

The University of Portsmouth is at the forefront of conducting cutting-edge research on plastics pollution. It equips and empowers a global community of plastics researchers to tackle this ever-growing challenge. The research project on compostable packaging aims to investigate the decomposition of a variety of compostable materials under home composting conditions. The main goal is to develop practical strategies for the management of organic waste streams including compostable packaging made of cellulose, starch, sugarcane bagasse, polylactic acid etc.

The work undertaken in such projects involves:

Development and monitoring of composting reactors (test rigs, e.g. setting up Daleks) to treat substrates including compostable packaging, and food/catering waste.
Examining the composting process and undertaking detailed analyses of composted materials using traditional and modern laboratory equipment.
Device user-focused surveys, questionaries’ and interviews to explore home composting practices and experiences with compostable packaging.
Evaluating the quality of composts derived from compostable wastes, producing products that meet the end-of-waste criteria including the Publicly Available Specification (PAS) 100 standard.
Comparing plant growth performance across different treatments using a series of controlled trials in a climate control chamber/greenhouse.

Inductively Coupled Plasma Optical Emission Spectrometer at the University of Portsmouth Environmental Technology Laboratory – Muhammad Ali

To work on this type of project you will need a good first degree from an internationally recognised University or a Master’s degree in an appropriate subject like civil and/or environmental engineering, environmental and/or soil sciences, in exceptional cases, professional experience and/or qualifications can also be considered. A background in waste management, and/or environmental studies is desirable. Full training is provided on the use of state-of-the-art environmental monitoring tools including such as Raman spectroscopy and Inductively Coupled Plasma Optical Emission Spectrometer located at the University of Portsmouth Environmental Technology Laboratory.

You can find details of our research centre here, and search for a suitable research program at the University of Portsmouth website.