Microplastic Contamination in Compost – Composted Green Waste vs Digestate

Microplastic contamination in compost is a problem now, and in geological timescales, all plastic in soils will be washed into the oceans, leading to yet more microplastics in the oceans. But which source of compost is worse for containing microplastics? Composted green waste or the digestate from anaerobic digestion? In this article, we compare both.

Ocean plastics and microplastic contamination are probably the worst unanticipated consequence of man's actions after climate change. As the world continues to grapple with the problem of plastic waste, attention has turned to the presence of microplastics in all types of compost.

Microplastic pollution of the environment, defined as particles smaller than 5 mm, has emerged as a global challenge because it may endanger biota and public health. Current research is primarily focused on aquatic systems, while little is known about the sources, pathways, and potential accumulation of plastic particles in terrestrial ecosystems.

The use of urban compost (from garden green waste and the organic fraction of “black bag” (residual) mixed waste) in agroecosystems improves soil fertility but is also a source of (micro)plastics that are not removed during the composting process. Knowledge of the fate of these plastics in regularly amended soils is thus an issue for the environmental management of these soils.

Research shows that the plastic content accumulates over time in soils, with the main process thought to be active being particle size reduction through physical effects such as abrasion and pressure. There may be some decomposition of the plastic by bacteria and other organisms, but this has yet to be identified by the research¹.

Read on to find out why we should all be concerned. But first, let us go back to basics to explain microplastics.

In this article, we'll compare the quantity and characteristics of microplastics found in the following types of compost:

1. Household Green Waste compost that is collected by local councils
2. Compost made from the Organic Fraction of Municipal Solid Waste (OFMSW), in other words from the food and other putrescible wastes we put in our non-recyclable bins
3. Digestate compost – the compost produced by anaerobic digestion at biogas plants as the fibrous output from digesters.

Now let us look at each of those in turn:

1 – Microplastic Contamination in Household Green Waste Compost

Household green waste compost, also known as source-separated and kerbside-collected garden waste (curb, US spelling), is a popular method of managing organic waste at the household level.

Green waste from the gardens of homes is collected as source-segregated waste and transported to commercial composting facilities. At these facilities, the green waste is shredded into smaller pieces, allowing for faster decomposition. The shredded waste is then mixed with other organic materials, such as food waste, to create a balanced mixture for composting.

The mixture is then placed in large piles, known as windrows, and turned regularly to ensure proper aeration and moisture levels. This process allows for the breakdown of organic matter by microorganisms, resulting in nutrient-rich compost.

After the composting process is complete, the resulting material is screened to remove any large debris and graded to produce a high-quality product suitable for sale in supermarkets and garden centres. This compost can be used for a variety of applications, such as enriching the soil for gardens and crops, improving water retention, and reducing the need for chemical fertilizers.

As of 2023, the use of this material by gardeners has increased dramatically in the UK and, no doubt, in other nations as well. The reason is that the extraction of natural peat deposits previously sold with added nutrients as garden and potting compost has been banned to save the peatland ecology and protect the wildlife of dwindling natural peat bogs.

Composted green waste has thus become the major source of commercial compost products bagged and sold in garden centres and supermarkets. OFMSW may also be used, and most nations have standards² that limit the permissible amount of plastic below 5mm in particle size.

But regulation is patchy, and the standards for limiting the maximum permissible plastic are in most places very relaxed. Thus, this can be an example of when “green” isn't so green, and there are still too few people who understand the hidden dangers of microplastics in compost.

Small but Mighty: The Troubling Presence of Microplastics in Compost

Studies have shown that household green waste compost routinely contains microplastics¹, with concentrations ranging from several hundred particles per kilogram of compost to many thousands. Some nations, such as Germany and France, wisely place strict limits on the permissible surface area of these tiny plastic pieces.

Others, such as the UK and some US states, are far more lacking in environmental protection and place a weight limit on the allowable amount of plastic spread on land.

However, a weight limit is largely meaningless when the pieces are very thin bits of plastic bags (known generically as flexible plastic film).

The types of microplastics found in household green waste compost are mostly fibrous in nature and originate from items such as synthetic textiles from rags and items such as wet wipes thrown into garden bins.

Plastic bags, plant pots, plastic ties, labels, bulb nets, etc., thrown in with prunings, hedge cuttings, grass mowings, etc., are also very common. The size of these microplastics can range from a few micrometres to 5 millimetres.

How Does Microplastic Get Into Greenwaste Compost?

The straightforward response is that the operator of the compost facility makes the majority of the microplastic in the green waste compost on purpose. Yes. You read that right!

They shred or macerate the incoming compost until the plastic, which compost plant operators consider would be expensive to dispose of if removed, is too small to be noticeable.

We think that it is about time that this situation was put right! Microplastic placed on the land in compost will quickly get washed into rivers and, from there, into the oceans. Once there, it does not rot but builds up into vast floating reefs of plastic, eventually also appearing on bathing beaches globally.

Even the Best PAS100 Compost Contains Microplastic

To generate a market for anaerobic digestate as a renewable fertiliser, WRAP developed the BSI PAS100: Quality Protocol for Compost.  It went into effect on July 31, 2014, and was written under a licence from the British Standards Institution (BSI), but it is ineffective in reducing microplastics in the compost output to a safe level.

Composting household waste allows for the optimization of this organic material recycling process and lowers the amount of biodegradable organic waste dumped in landfills. Nevertheless, in addition to organic matter, municipal solid waste composts also contain so-called inert elements like plastics, glass, and metals and can therefore be a source of these unwanted components in soil (Brinton, 2005; Zubris and Richards, 2005).

Image Source (CC): The Australian Institute of Marine Science: Marine microplastics | AIMS

Since 2006, composts used in agriculture in France must adhere to the standard for organic soil improvers (NFU 44 051). To sort them, the 2004 method XP U44-164 limits the amount of compost in inerts to 0.3 MS for films and expanded polystyrene (SPE) bigger than 5 mm and 0.8% MS for other plastics bigger than 5 mm.

In all the current standards, these numbers for the allowable plastic contents fall within an acceptable range that is supported by the technical limitations on the gathering and sorting of raw materials.

None are based on research to establish the level of plastic contamination that the environment can tolerate because:

• Until the last few years, there has been no equipment (known as depackaging and separation machines) that has been designed specifically to remove the plastic and other rejected materials (cans, bottles, etc.)
• The sole treatment used to process incoming composting materials at composting facilities has until now been shredding machines. These machines do just that. They shred the plastic bags, water bottles, and drink bottles into tiny pieces, making microplastics so small that they will not be seen in the compost when it is sold later.

So, let us tell it as it really is:

• Shredding machines manufacture microplastics while also encouraging composting by reducing particle size overall.
• The existing standards are set at a level that, in the past, was a pragmatic approach to accepting the best that the shredding machines were able to accomplish.
• But there is now a new generation of depackaging and separation machines that can remove the plastic, and other rejects (such as the Twister depackager and separator) can enable the recycling of the plastic and avoid producing microplastics.
• It's time for the composting industry to add a new generation of depackaging and separation machines to their processing and stop the microplastics by setting more stringent standards. This, if applied universally, could stop the microplastic hazard to the global ecology.

Organic Fraction of Municipal Solid Waste (OFMSW) Compost

The organic fraction of municipal solid waste (OFMSW) is the biodegradable portion of municipal waste, and composting is a popular method of managing the disposal of OFMSW. Studies have shown that OFMSW compost contains microplastics, with concentrations similar to or above green waste compost. Quantities found ranged from a few to several hundreds of thousands of particles per kilogram of compost.

The types of microplastics found in OFMSW compost can vary in size and shape below 5 mm. They originate from a huge variety of sources, such as packaging and textiles. They can range from a few micrometres to several millimetres in size and include shreds of flexible plastic film and hard plastic shreds and fragments.

The level of microplastic in OFMSW is high and would logically always be higher than green waste-derived compost, due to the nature of its source.

However, the danger to the environment of OFMSW compost is low, given that it should rarely, if ever, be spread on land. If it is for landspreading, the locations are limited to, for example, the surface of a landfill as a capping material.

Organic Waste Compost from MBT Facilities

Mechanical Biological Treatment (MBT) is a process that combines mechanical and biological treatment to manage waste. A rapid form of composting is a common method of managing the organic fraction of MBT waste.

The purpose of MBT composting is to reduce the readily decomposable organic content to a point at which the waste can be landfilled, with a low potential for landfill gas production and water contamination once it has been deposited in a landfill.

Studies have shown that organic waste compost from MBT facilities contains microplastics, with concentrations ranging from a few hundred to several hundred thousand particles per kilogram of compost.

The types of microplastics found in MBT organic waste compost can be diverse, including fibres, fragments, and films. The size of these microplastics can range from a few micrometres to several millimetres. The source of MBT waste is the same as for OFMSW, and microplastic content can be assumed to be similar.

Microplastic Contamination in Compost Made from Anaerobic Digestion Plant Digestate

Digestate is a byproduct of anaerobic digestion, a process that converts organic waste into biogas and a nutrient-rich liquid or solid residue. Composting in open piles or windrows of less than 10% as the fiber that is left after it leaves the digester. This is a popular method of managing digestate, particularly for agricultural spreading as a soil improver.

Digestate compost does normally contain microplastics, both in the liquid and in the fibre, with concentrations ranging from several hundred to thousands of particles per kilogram of compost.

As elsewhere, the types of microplastics found in digestate compost can be diverse, including fibres, fragments, and bits of film. These microplastics are not likely to be any different in composition to those present in the other sources previously described.

However, there is much stronger pressure on the anaerobic digestion plant operator to reduce microplastic content by the use of the most efficient depackaging and separation equipment for contamination removal in all of its forms before digestion.

All forms of inert contamination entering a digester tank have negative impacts on biogas production and may reduce the profitable yield of the plant, as follows:

1. Inert materials tend to accumulate over time, replacing the reactive volume.
2. The same materials tend to raise the viscosity of the reactor, requiring a higher sacrificial use of mixing energy to keep the reactor operating as time goes on.
3. If, over a period of time the accumulation continues, there may be no remedy other than to drain the digester tank and dig out the accumulated material.
4. The loss of income then continues for weeks or months after refilling the tank until the anaerobic biomass has been re-established and biogas production returns.

Green Waste Compost vs Digestate Compost: Which is the Worst for Microplastics?

It's difficult to determine which type of compost is the worst in terms of microplastic contamination.

As previously stated, studies have shown that all four types of compost contain microplastics when analysed, with concentrations varying widely.

However, it's worth noting that logically:

• OFMSW and BMT-derived composts contain the highest microplastics due to the nature of their fixed MSW sources.
• Household green waste compost is likely to contain the next highest concentration of microplastics compared to the other types of compost due to the manner in which the compost industry uses shredders without a prior removal step.
• The amount of microplastics in the digestate output increases despite the up to 50% removal of carbon molecules from the methane gas output. However, the microplastics in biogas plant output are expected to be lower than in household green waste compost. Due to the need for digester operators not to allow their reactor tanks to fill with plastic bits.

AD Plant and compost plant operators and regulators are becoming more aware of the problems with microplastics in the environment, and from now on, they are expected to adopt the new generation of depackaging and separation equipment that is designed on the principle of not creating microplastics.

Summarising – From Household Waste to Digestate: Comparing Microplastic Contamination in Compost

The presence of microplastics in compost is a serious environmental concern.

All types of compost, other than natural peat, contain microplastics, with concentrations ranging widely.

To reduce the presence of microplastics in compost, it's essential to manage plastic waste more effectively and avoid contamination of organic waste streams. Furthermore, additional research is needed.

The Plastic Problem in Compost: A Closer Look at the Treatment Process Used

Microplastics are produced during waste processing in recycling centres, material recovery facilities (MRFs), energy recovery facilities (ERFs), and mechanical biological treatment (MBT) plants. These facilities use equipment such as macerators, shredders, mills, chippers, and crushers to process waste materials.

Here's how microplastics are produced during each of these processes:

Maceration:

Maceration is a process that uses water to break down waste materials. During maceration, plastic items such as bags, packaging, and other plastics can break down into smaller pieces, most of which can be classified as microplastics.

Shredding:

Shredding is a process that involves breaking down waste materials using rotating blades. During shredding, plastic items are broken down into smaller pieces, the majority of which are classified as microplastics.

Milling:

Milling is a process that involves grinding waste materials into smaller particles and includes hammer milling. During milling, plastic items are broken down into smaller pieces, the majority of which can be classified as microplastics.

Crushing:

Crushing is a process that involves compressing waste materials into smaller sizes. During crushing, plastic items are broken down into smaller pieces, some of which can be classified as microplastics.

Top Brands Use Particle Size Reduction

The top brands of equipment that produce microplastics during waste processing are difficult to determine since many different brands of equipment are used in recycling centres, MRFs, ERFs, and MBT plants worldwide. However, some well-known brands that produce waste processing equipment include Andritz, Bollegraaf, Komptech, and Mavitech.

All but a few of the newer waste treatment methods described employ “particle size reduction” and now a new technology is available that does not employ any particle size reduction. This means that the old outdated equipment should be as soon as possible.

Models of waste separation and debagging that don't produce microplastic particles should now take their place.

Instead, the new generation of depackaging equipment will pluck out the flexible films and hard plastic items largely whole. That way, the rejected material can be placed on the MSW and commercial waste recycling lines with:

• Source-segregated kerbside-collected recyclable household and commercial waste
• The large/ high calorific fraction of mixed MSW (black bag waste) from the initial sorting of household and commercial mixed waste.

Allowing recycling to take place, including plastic recycling via automatic picking of the valuable resins in items such as milk and drinks bottles recognized by AI and blown off the conveyor lines using such equipment as ballistic separators.

An Innovative Range of Separators which Do Not Reduce Particle Size

The Drycake Twister is a type of solid-liquid separation equipment that is designed to avoid making microplastics during the separation process. The Twister operates on a unique suction vortex principle and “counter-current washing,” which involves the use of a rotating vertical spindle to knock, bat, and pluck up the rejected materials and mesh screening to separate the organic solids from the liquid.

The Twister's mesh screen is designed to resist abrasion and avoid the creation of microplastics. The mesh is also designed to allow for the efficient separation of solids and liquids while minimizing the production of fine particles. And, further reducing the potential for microplastics to be created.

Additionally, the Drycake Twister's unique design allows for the separation process to take place without the use of chemicals or additives. The control system also minimises water usage during the separation process.

Overall, the Drycake Twister is designed to minimize the production of microplastics during the solid-liquid separation process. Its unique design and use of specialized materials make it an effective tool for separating solids and liquids without creating microplastics.

So, the Drycake Twister range can be an important part of efforts to reduce microplastic pollution in all wastewater streams.

Article Conclusion: Composted Green Waste vs Digestate

We have discussed the tiny plastic hidden in the waste from a wide variety of sources from household waste to digestate comparing microplastic contamination in compost.

This is a serious matter. Plastic trash poses a real threat to the environment by starving and suffocating wildlife, spreading invasive and possibly dangerous species, absorbing hazardous chemicals, and disintegrating into ever more tiny plastic particles to be consumed later.

We have found that the degree to which microplastics are produced during waste processing varies depending on the type and quality of the equipment used, as well as the specific waste materials being processed.

Ultimately, reducing the production of microplastics during waste processing should be a very high priority for all involved. It will require a comprehensive approach that involves improved waste management practices and increased efforts to reduce plastic waste at its source.

However,  we have shown that sometimes “green” isn't so green due to the hidden dangers of microplastics in compost and no source of organic waste, now that the use of peat is prohibited in the UK, that makes a completely microplastic-free product.

However, the opinion of the author is that green (garden) waste compost can be the worst offending compost source, when considering green waste compost versus OFMSW, MBT (rarely if ever used for landspreading), digestate, and Source-Separated Food Waste. The reason is the universal use of shredding at aerobic composting sites in the UK.

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Another useful resource when it comes to unwittingly causing pollution in our daily lives is to be found in “A Consumer's Guide to Reducing Pollution“.

References:

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