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The biodegradability and compostability of footwear

Exploring an issue which has a key role to play in the wider discussion on the health of our planet.

by Tom Bayes

Image © Cezary p

‘Sustainability’ is a topic that everybody has heard about. There are many different definitions, depending on the context in which the term is being used. However, it essentially refers to anything that can be maintained at a certain level or rate. The majority of people would agree that the current rate of growth of the human population, our use of resources and man-made pollution levels are not sustainable in the long term. Consideration must be given to the resources and energy used in the manufacture of a product and this, divided by its expected life, will give a fairly accurate estimation of the product’s ‘footprint’. Durability, longevity, reparability and restorability can all influence and improve sustainability credentials. It is also important to consider what happens to the product at the end of its life and, of current interest, is the possibility of making biodegradable (capable of being decomposed by microorganisms) and compostable materials and products. This article discusses these options and explores what they actually mean.

Most often, the term ‘sustainability’ in environmental terms is used to describe an idealistic and utopian way of life humans must adopt in order to continue our current lifestyles. The word is connected with the long-term endurance of ourselves and our species, and the maintenance of current lifestyles in terms of economy, health and life span is a particularly important feature we are keen to pursue. Solutions such as returning to the population levels and lifestyles of many centuries ago are not attractive. While the condition of our planet is often used in debates, history has shown that the Earth survives catastrophes and environmental pressure quite well.

We are rampant consumers – far more than our forebears were. At the turn of the 20th century, the world’s population was around 20 per cent of what it is today, at approximately 1.6 billion. Footwear sales were relatively low, with the majority of people having only one pair that they regularly repaired. Today, we buy on average three pairs of shoes or boots each a year compared to, for example, one pair every four years in 1900. While the world’s population has also exploded to around 7.9 billion, we consume footwear as if there were 90 billion, and this exponential growth in consumption per head also applies to most resources we use – including food and water. It has been estimated that around 7 per cent of all humans that have ever existed are in fact alive today.

Current estimates are that 85 per cent of footwear ends its life in landfill. This is not a sustainable disposal strategy, as it essentially means burying our waste for future generations to deal with. Apart from having new hills composed of rotting rubbish, the process means that any microbial degradation that takes place is anaerobic, which generates methane gas (CH4). In contrast, aerobic digestion releases carbon dioxide (CO2). These are both greenhouse gases, with methane being the more potent. Landfill is therefore a major source of methane in the atmosphere. In modern landfill sites, the gas is collected and used to generate power, but it is clearly recognised that these places do contribute to greenhouse emissions. Incineration is an option used globally, which allows energy to be collected and used. Some of the resulting gases are captured, but most systems result in the flue gases being released. While this a more efficient method than landfill, it is still not an ideal solution.

There are many ‘biogas’ power stations in operation now. Relatively small operations producing around 0.5 MW of electricity, these accept biodegradable waste such as food waste from retailers, food and beverage manufacturers, caterers and kerbside recycling schemes, as well as certain packaging and animal farm waste. This is digested anaerobically (see the box ‘With or without oxygen’) to produce methane which powers gas turbines to generate electricity. Fundamentally, the resultant digestate is not classed as waste. Instead, it is certified as ‘PAS110’ fertiliser – a valuable product that can be spread on farmland to replenish the nutrient levels of the soil and increase productivity, as well as the nutrition value of the crops. In turn, this helps with another issue that faces us – the production of nutritious food. The biogas power stations often incorporate many energy-saving systems and are extremely efficient. However, they can only process waste that falls into a specific classification – the substance must be biodegradable.

With or without oxygen

The term ‘anaerobic’ in the case of biodegradability refers to organisms that digest without the need for oxygen – in fact, the presence of oxygen can be poisonous to the microorganisms involved. The result of this process is the production of methane gas, which is a potent ‘greenhouse’ gas (one that contributes to warming of the planet). However, this gas is flammable, and can be easily collected for another purpose – such as driving turbines to generate electricity. This produces carbon dioxide, which is also a greenhouse gas. It is anaerobic digestion that formed the coal and oil deposits and consequently natural gas, which is mostly methane. This is the process that is predominantly present in environments such as landfill sites, under water (for instance, marshland) and bio-based power stations.

‘Aerobic’ in this instance refers to organisms that digest in the presence of oxygen and utilise that gas to break down the materials to form mostly carbon dioxide gas. This is the type of digestion that occurs, for example, in home compost bins and in nature with leaves in autumn.

Carbon is said to have been created in stars around 14 billion years ago, and some of it can be found on earth where levels are thought to have actually remained constant for four billion years. It is therefore a ‘closed system’. Carbon is an element and is not created or destroyed on our planet. Instead, 99 per cent of all carbon on Earth is cycled by living things. Coal, oil and natural gas (so-called fossil fuels), as well as certain rocks such as limestones, all owe their existence to biological processes. However, these deposits largely remained permanent sinks of captured carbon and take many millions of years to form. The rest of the carbon, in biomass and in the form of atmospheric CO2, has remained in constant equilibrium in this cycle for many millions of years, occasionally being upset by volcanic activity, wildfires and seasonal events such as autumn (‘fall’).

During autumn, the amount of CO2 in the atmosphere increases rapidly due to leaves falling, grasses dying and the reduction in photosynthesis. The US National Aeronautics and Space Administration (NASA) has actually monitored this process from orbit. The leaves and other plants aerobically biodegrade more or less completely back to simple chemicals and compost. These essential components replenish the soil and, with the arrival of spring, aid regrowth and nourishment. Importantly, this process is not considered as pollution. We are all familiar with this natural process of biodegradation and composting, which represents one of the desired outcomes of any such process – whether artificial or natural – for materials used for footwear.

Leaves and other plants aerobically biodegrade more or less completely back to simple chemicals and compost

Compostability actually stands out as the most desirable solution. Compost is not about the disposal of waste, but rather its conversion to nutrient-rich organic materials in order to combat another problem associated with the 21st century, that of growing nutritious food in depleted, intensively-farmed soils. Organic content is important in soil – it helps to retain moisture and prevents the soil being windblown. However, the plants and crops do not use the carbon, which remains in the soil. Plants synthesise atmospheric carbon to produce tissues, sugars and the proteins essential for life to grow. Crops must be nutritious, which is the purpose of the resulting compost – a higher purpose than simply the biodegradation of waste. | fotokostic

The nutrient rich organic materials in compost are used to enrich intensively farmed soils

Problems arise when humans start taking very large amounts of the captured carbon in fossil fuels and introducing it to the biosphere in a relatively short time, thus upsetting the equilibrium in the long term. The formation of fossil carbon is a cycle, on a geological timescale of many millions of years, while the biological carbon cycle is just a few years and, in the case on plants, often seasonal.

Once again considering the landfill issue, the carbon gases originate from two sources. With natural materials such as leather and cotton, the source is from atmospheric carbon dioxide. The carbon gases from synthetic products originates from fossil fuels (mainly mineral oil) and is newly-released carbon. The digestion process is essentially slow oxidation which releases the same amount of gas as if the product were incinerated. This is the same for biodegradation, in which carbon gases from synthetic products is new carbon released into the atmosphere. Uncontrolled or unconstrained biodegradation of synthetic materials directly into the environment will release new greenhouse gases and add to the problem.

Test methods

Most current test methods rely on the measurement of these gases. The mass of carbon present in the specimen is measured or estimated, and the biodegradability potential of the product is determined by the amount of carbon dioxide or methane it produces. Given the previous rational discussion presented in this article, on the source of the carbon, and the fact that total degradation of the organic content is not always desirable for compost, it is not surprising that there is considerable confusion over the results and how they should be presented when making honest and genuine claims.

Manufacturers should consider the end-of-life of the product – where it could end up and what the consequences could be – whether this occurs by accident or an intentional end-of-life process. Will it release new carbon? Is it better to collect and use an industrial process such as fermentation or incineration to both capture the energy and gases? If it is a natural product (non-fossil based), will it degrade and result in an additional benefit such as compost (that can be collected and applied) or will it persist and not break down without human intervention?

The choice of materials is important. Not all leather is perfect, many examples have plastic coatings and other chemicals present, but natural materials clearly have a head start. Materials need to be selected based on an end-of-life plan and what claims can be made that allow the end user, the consumer, to make an informed rational choice based on facts. If the resulting product is compost, it needs to undergo plant response testing and toxicity testing to ensure that there are no trace elements that will impact future generations. Materials that are anti-microbial may require specialist processing in order that they can return to the biosphere. Of course, there are political and ideological influences (which are a matter of personal choice) at play in this arena, as well as considerable misinformation from the media, but these should not influence the actual science of these processes.

SATRA has partnered with other companies to commence a project to examine the need for new test methods and will subsequently offer versions that take these factors into account. Science-based methods presenting the facts that are easy to understand and explain can help to support environmental claims of SATRA’s customers and are able to remove confusion.

How can we help?

Please email to discuss the testing of finished footwear or components for their biodegradability or compostability.

Publishing Data

This article was originally published on page 18 of the December 2021 issue of SATRA Bulletin.

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