Biodegradability is the ability of organic substances and materials to be broken down into simpler substances through the action of enzymes from microorganisms. If this process is complete, the initial organic substances are entirely converted into simple inorganic molecules such as water, carbon dioxide and methane.
Biodegradation is part of the earth’s natural life cycle, which is based on carbon. Thanks to photosynthesis by plants and algae, and the inexhaustible energy of the sun, carbon dioxide is absorbed from the atmosphere to synthesise sugars and other substances used by plants to grow and develop. The flow of substances and energy passes through the food chain from plants to herbivores and from herbivores to carnivores. When plant and animal organisms die, microorganisms present everywhere in the environment feed on organic material through biodegradation processes and release water and carbon dioxide into the atmosphere, thereby closing the cycle.
By imitating and improving upon these natural processes, organic waste from human activities can also be removed through biodegradation.
For this operation to be effective it is first necessary to identify the ideal environment in which the process can reach maximum efficiency, and an ‘industrialisable’ duration must be established that is compatible with the pace at which organic waste is produced.
In nature all organic waste takes a certain amount of time to biodegrade, for example straw and wood take longer than starch and cellulose. Similarly, in cold dry environments the processes of biodegradation are slower than in hot humid environments.
This means that biodegradation is strongly influenced by the chemical nature of the substance or material to be biodegraded and by the environment in which this process takes place.
The environments in which biodegradation occurs at a consistent pace, and in which it can be managed industrially, are those of composting and anaerobic digestion. In these systems it is therefore possible to process solid organic waste, including man-made substances (such as biodegradable plastic) for which the speed of biodegradation is compatible with these processes. Composting will produce mature compost (which is a fertiliser) while anaerobic digestion (followed by stabilisation through composting) will produce biogas (and therefore energy) as well as compost.
Another biologically active environment is soil: some materials can be completely biodegraded in soil, and this property can be exploited in specific applications such as mulching.
Compostability is the capacity of an organic material to be transformed into compost through the composting process. This process exploits the biodegradability of the initial organic materials to transform them into a finished product called compost. Compost is therefore the result of disintegration and aerobic biodegradation (occurring in the presence of oxygen): mature compost is similar to fertile soil and its high proportion of organic substances means it can be used as a fertiliser.
Composting can be conducted at a domestic-amateur level on a very small scale, or at an industrial level. Industrial composting uses domestic organic waste and waste from agricultural processing or from other sectors; it is carried out in dedicated facilities that ensure the process is managed correctly. The stages followed in a composting facility bear many similarities to the processes in the countryside: piles of organic matter (waste, animal droppings, sawdust, wood shavings, etc.) produce heat and give off steam, as though they were burning without a flame. In fact biodegradation is occurring in these piles through the action of microorganisms that consume nutrients and transform the initial waste into a series of organic substances called compost. In industrial plants this product is sanitised and stabilised to remove pathogenic microbes or material that decomposes. Quality control procedures must be conducted before this compost can be sold, in order to ensure compliance with a series of legal requirements.
The advantages of composting
Composting is the ideal solution for the disposal of the organic component of urban waste (also known as the “wet part”). Substances such as leftover food, kitchen scraps, garden waste, waste from canteens and restaurants are rich in water and decompose quickly. Consequently they are not suitable for recovering energy through incineration because heat is lost in evaporating water rather than in producing electricity. In landfill sites, wet organic waste causes serious environmental problems, such as methane production, and can contaminate water tables through contaminated percolates. The production of compost and its use in agriculture represent the closure of the environmental cycle and constitute a simple way to address the problem caused by the removal of organic substances from agricultural soils, reduced soil fertility and the onset of desertification, particularly in western countries. After being taken from the fields to our supermarkets, organic matter is returned to its place of origin in the form of compost, a substance that helps maintain soil fertility, prevents erosion, reduces the leaching of inorganic fertilisers and obstructs the development of micro-organisms that are pathogenic for plants, to mention just some of its positive aspects.
Composting: why bioplastics.
Composting is currently applied to selected waste that only contains biodegradable organic matter. Traditional plastics are not included in composting, and when present are discarded because they resist biodegradation and therefore contaminate the finished compost. In contrast, biodegradable plastics can be included in composting, but only if they satisfy the criteria established by the standards that define compostable materials. Incompatible materials were composted in the past in the absence of rules and in a context of unregulated definitions and test methods. This caused significant damage, not least to the trust of users and technicians responsible for composting facilities. This is no longer possible thanks to EU standard EN13432 concerning organic recycling of packaging through composting, and its twin standard EN 14995 which applies the same criteria as EN13432 to the more general field of plastics.
Bioplastics that comply with these standards can play a fundamental role in the valuation and optimisation of the composting process and in the production of high quality compost. The most important example is that of the compostable bag, which enables the creation of homogenous wet waste, where both the container and the contents have the same biodegradation properties.
This increases the efficiency of separate wet waste collection, because it speeds up the decomposition of waste, it can encourage domestic separation of wet waste and therefore increase the quantity of organic waste received, thereby ensuring the quality levels required for the finished compost.