Microplastics in compost, a new challenge for the circular economy

Today, it is difficult not to have heard of the concept of circular economy. Briefly, it can be summarized in that what was previously considered a waste is now a raw material. Materials that were previously discarded are now used for other things and can be turned into a new product. This is already familiar when thinking about glass, metals or containers; but it is more difficult to assimilate in the case of the organic fraction of household waste.

When everyday waste is managed, things get complicated: typical household waste contains 40% organic matter that can be used if it can be recovered separately, since the resulting material is used in agriculture.

For the collection of domestic organic waste, administrations have developed different methods. The most widespread are the containers on the streets (normally the brown color is assigned to the container intended for organic waste) and door-to-door collection, either at specific collection points or at homes themselves.

After collection, the remains generated in homes are taken to the composting plants where they are mixed with other remains of plant origin such as pruning remains. In this way, the correct balance of carbon, nitrogen and phosphorus is achieved until a product suitable for use in agriculture is generated. The composting process at the plant is completed after several months.

Are there microplastics in the compost?

All composting plants have specialized mechanisms for removing unwanted materials. They can be metal, glass or plastic. Modern composting plants have mechanisms for removing improper materials with sizes as small as 2 millimeters.

The remains of plastic materials are everywhere and our household waste is no exception. Plastic is used in our homes in large quantities in the form of plastic bags, food containers and a huge variety of objects that can end up in the garbage, even in the one that is collected in the organic waste container.

Once in the composting plants, the plastic remains present that have escaped screening, together with the organic matter, are subjected to biological decomposition processes and mechanical treatments that are capable of fragmenting them to generate microplastics. These are the plastics that will eventually find their way into agricultural soils where they can have various effects.

To reduce the arrival of plastic waste in the compost, door-to-door collection strategies have been developed in which, through greater citizen involvement, an improvement in product quality is sought. In addition, the arrival of new compostable plastics seeks to avoid the transfer of conventional plastic waste to compost. The key is that any plastic that goes into the compost will end up scattered in the field.

Microplastics in the compost

In a study carried out by researchers from the University of Alcalá and the Autonomous University of Madrid, the final product produced in five composting plants located in the northeast of Spain over several months in 2021 has been examined. The results have been published. recently in the scientific journal Science of The Total Environment.

In the samples that we have taken throughout this study, the microplastics have been carefully separated from the rest of the compost materials. Once separated and identified, it has been possible to determine the amount of plastic present in the samples.

The main differences between the treatment plants are the amounts of waste they manage, which depend on the population served, the compost collection strategies and the technologies they use to make it. These factors have turned out to be key to explaining the amount of plastic found.

What has been found?

There is presence of plastic in all the samples examined. The size ratio indicates a higher number of microplastics (less than 5mm) than large plastics in the final product arriving in the field. This is a problem, since as the materials get smaller they are more difficult to control and the damage they cause can be greater.

The concentration of microplastics varies between plants, in a range of 5-20 particles per gram of compost, depending on the technology used. Smaller treatment plants, which use door-to-door strategies and prohibit the use of non-compostable bags, obtain better results than larger plants with less pre-composting control.

To get an idea of ​​what these figures mean, even the best performing plant would contribute 4.8 billion microplastics per year and the five plants together would spread 1.4 trillion microplastics each year in agricultural soils.

The main materials found in compost are polyethylene (very common in garbage bags and most everyday items), polystyrene and polypropylene (widely used in packaging), polyvinyl chloride (PVC, together with polyethylene, for being used in the film we use as food wrapper) and polyester and acrylic fibers (generated by the wear of synthetic fabrics). On the contrary, no trace of any compostable material has been found, which indicates that the composting plants are working properly.

Possible solutions

With this work it has been shown that a control prior to the arrival of the materials at the composting plants is necessary. To do this, door-to-door strategies, in which the collection of organic material is individualized, have a positive effect on the reduction of plastic in the compost. However, despite the effort.

At present, individualized procedures are difficult to carry out in large towns since it is hardly feasible to dispense with containers at street level. To solve this problem and achieve a reduction in the plastic that we spread on agricultural soils, there are several ideas:

*Carlos Edo is a doctoral student in Hydrology and Water Resources Management at the University of Alcalá, Francisca Fernández is Professor of Biology at the Autonomous University of Madrid and Roberto Rosal is Professor of Chemical Engineering at the University of Alcalá. This article was originally published on The Conversation and is published here under a Creative Commons license.