Against the grain: What can be done about microplastics in soils and digestate?

Microplastics are everywhere: We know that much. But should we be worried? What can we do about it? Some of the speakers at Aqua Enviro’s European Biosolids and Bioresources Conference, in late November, presented findings from a project aiming to understand the levels of this contaminant in compost and AD digestate. A change in the policy landscape seems imminent, as Envirotec found out.

Certification schemes aim to provide some assurance that products like compost and digestate are safe for human, animal and plant health.

One obvious obstacle facing would-be investigators is inconsistent terminology. Even the definition of “microplastics” is slippery. Particles vary widely by size, material type and provenance (see “Particle primer”, opposite). “It is likely that the majority of microplastics come from the fragmentation of larger pieces of plastic litter in the environment,” says a 2019 report from the Royal Society,1 and the tendency of discarded litter to degrade or fragment is helped along by things like UV radiation, physical erosion and temperature changes.

But tiny plastic fragments follow a multitude of pathways into the enviroment. Rivers, for example, though we would expect this to be a catchment-specific issue, said David Tompkins, a soil scientist with Aqua Enviro, during the November event. And not all rivers carry the same load of microplastics. Road run-off is another source – and vehicle tyres are a particular contributor to this pollution. Applying biosolids to land is also a culprit.

At this stage we don’t know much about the relative contributions of each. “Is that an argument to do nothing?” asked Tompkins. “I don’t think so, but we need to be careful what actions we take.” A cautious appraisal was maybe unexpected from a presentation titled (though clearly with some tongue in cheek) “So what if there are microplastics in bioresources?”

What do we know?

Gaps in the evidence base are a striking feature of the topic at present. We don’t know enough to inform policy. Useful efforts have begun to measure and regulate the microplastics content of some bioresources, but we have a very incomplete picture.

When it comes to the UK, “it’s actually not that bad”, and while we’re not the best performers, the UK is one of the best-performing nations in terms of the limits already being applied to plastics content in materials like compost and digestate.

In the UK, the standard rules for environmental permits for AD and composting sites specifically exclude wastes “significantly contaminated with non-compostable or digestible contaminants, in particular plastic and litter”, which must not exceed 5% weight-for-weight (w/w), “and shall be as low as reasonably practicable by 31 December 2025”. Tompkins conceded that this is incredibly high, although in reality the day-to-day amounts are below that. “5% seems to be where L.A. contracts are pegged currently,” he said, and “as low as reasonably practicable” has yet to be defined. Suffice to say regulators are taking a close look at it.

In connection with Defra’s plans to increase the collection of food waste, WRAP published an organic waste roadmap in 2019 specifying a need for quality in relation to the inputs, operations and outputs of processes like AD (the destination for most of it) and composting. It allocates actions to stakeholders including the EA, local authorities and organic processors. Questions WRAP wanted to ask the market included: Is it better to capture food waste in compostable or non-compostable caddy liners?

The impossibility of tracking the fate of such feedstock choices was one motivation for the project on which Laura Fonseca Aponte (also with Aqua Enviro) and Tompkins presented at the November event. The project set out to ascertain what we know at present about plastic contamination in UK source-segregated composts and digestates, and also what evidence exists that these levels might be harmful. It also investigated how to measure plastics levels in composts, digestates and soils, and what process interventions are available to reduce it.

In the UK, the limits applied to plastic contamination in these output materials tends to follow the requirements of certification schemes like PAS100, which applies to compost, and specifies a physical contaminant upper limit of 0.25% mass-for-mass, of which up to 0.12% m/m can be plastic. The equivalent standard for digestate, PAS110, specifies physical contaminant limits based on nitrogen content. The most stringent standard in the UK is applied to digestate in Scotland, and is essentially 8% of the PAS110 limits.

Tompkins said the feedback from Scotland is that these limits are achieveable, although they do mean additional screening following digestion, and therefore extra disposal costs.

In fact he suggested the science behind it is threadbare, citing his own instrumental role in coming up with the 8% figure; it is more of the order of a subjective appraisal, based on a “worst case” calculation derived from the physical contaminant limits of PAS110.

So how much plastic is actually present in product certified with standards like PAS?

Renewable Energy Assurance Limited (REAL) provides quality assurance and end-of-waste schemes that achieve compliance with PAS and the Scottish requirements. The group published study results on 6 December which appear to show that in the majority of compost samples, the level of plastic contamination fell below 0.2% mass/mass. .

Fonseca Aponte’s presentation cited independent studies of composting sites (PAS100-certified), with plastic contamination levels variously cited at 0.08% to 0.48% dry weight (from three AD sites in 2006), 0.1 to 2.1% air-dry weight (a 2011 study of green waste sites in Wales), and an average figure of 0.03% dry weight in a 2017 study. Inconsistent metrics are an obstacle to comparison, she said, and the project presented recommendations for resolving this.

Access to data is also an obstacle. Not much is available from the EA, there’s nothing from SEPA, and NRW didn’t respond to their request. However, a large dataset for PAS100 and 110 materials is available from REAL. This focuses on particles larger than 2mm and covers two years, but requires a fee to access.

Notable blind spots are with discriminating between different types of plastic – and nothing is known about the fate of compostable plastics, the source of an increasing proportion of contamination, and one that will be important if there is a move to collect food waste in compostable liners. These plastics might also present challenges for AD.

Even the definition of “compostable” is still a source of some uncertainty, she said.
Another blind spot is with the sub-2mm particles – and micro- or nanoplastics in general.

Sampling and testing methods (source: “Measuring microplastics in UK source-segregated composts and digestates”, Laura Fonseca Aponte, SUEZ/Aqua Enviro, European Biosolids & Bioresources Conference 22-23 November 2022).
Sharpening resolution

The clarity of the current picture is limited by the methods used for sampling and quantification. PAS100 and 110 testing uses both screening (dry or wet) and visual sorting. Materials are passed through a 2mm mesh, so it only separates out 2mm-and-larger fragments. It also makes it difficult to differentiate fibres made from plastic vs wool. Sometimes organic material gets stuck inside the plastic, distorting the weight measurement.

One immediate prospect for the future, to discriminate between compostable and non-compostable (i.e., oil-based) plastics is to use FTIR spectroscopy, which was offered by two of the labs they approached. This looks like the most commercial option, although “there is still a high cost involved”. Compostable plastic tends to degrade during the extraction process, which complicates matters. FTIR would also depend on having a bespoke library of spectra, presenting a need to know in advance what kind of plastics you expect to find in samples.

Spectroscopy methods like FTIR and Raman offer a way to identify the precise type of polymer.

Spectroscopy methods like FTIR and Raman offer a way to identify the precise type of polymer. They can also be used in conjunction with scanning software, to provide an automated count of the number of particles of each polymer. But they can’t tell you the weight, a metric best acquired using another method such as thermogravimetric analysis.

The ubiquitous plastic bag is a source of PET particles.

Visual identification techniques are best if you want to know the size, shape and colour of plastic particles – so methods like scanning electron microscopy (SEM), light microscopy, and transmission electron microscopy. But they don’t help much with identifying the type of polymer.

So it seems a complex “horses for courses” measurement problem, and if the industry chooses to implement these, it will be “a little bit more expensive than what we have now on the [PAS] standards”.

Quantifying plastics in compost and digestate has tended to focus on particles bigger than 2mm, and there are standards in place. Gaps in the standards picture include quantifying plastics in soil and differentiating different plastic types (see table, above).

She suggested the next phase of work would be to analyze the existing data on particles bigger than 2mm, and to start building a picture of what’s there at sub-2mm.

A 2019 review of microplastics in freshwater and soil suggested that previous studies may have under-estimated the number of particles as they can be easily mistaken for organic particles.

Can we live with it?

Tompkins considered the “so what?” element. While we’re working to establish a clear picture of what’s there, we should also consider what harms microplastics might present in bioresources like compost and digestate, and if there’s a level we can live with – although he was quite clear that his preferred level was “zero”, which is a simple application of the precautionary principle, since we don’t actually know what effects they’re going to have. But there’s plenty of evidence of established harms (see box “Harms: What do we know?”).

A difficulty seems to come up with extrapolating experimental results – which tend to favour excessive dosages of plastics – to what happens in the field. Where we are with standards like PAS110 and PAS100 equates to soil concentrations of plastic starting at 0.006% on a dry weight basis over a decade – simply miniscule compared to what’s been documented in relation to effects like reproductive problems in earthworms. Another challenge is that some of the reported harms depend on the nature of the plastic particle itself. Is it polyethylene? A film? Particle? Are the edges rough or smooth? Could an earthworm ingest it? “So depending on that combination of questions, the result you see experimentally can be really really different. So I would counsel very strongly against a single limit for microplastics in any material.”

He also gave examples showing that “in many cases there is absolutely no experimental consistency in the findings from this research, which again… creates a problem for us when it comes to thinking about limits for these materials on land.”

Nano niggles

Moving down in scale to study smaller particles also gets complicated – and is maybe part of the reason why monitoring to date has stuck with 2mm diameter and larger particles. German researchers have done a lot of work with 1mm. Tompkins’ feedback from people who work at that scale using a standard dry or wet sieving technique was that it was “incredibly challenging”, and involved picking out fragments with tweezers. “Strictly speaking you should be cleaning off any surface contamination before you then dry or weigh [the material].” This also means you have to look at imposing other kinds of limit than weight-based, such as the length of a piece of material when placed on a flat slide – so, another additional test.

Future fertilizer regulations

He also spoke about the regulations for fertilizer products in Europe – they have a limit, which will be coming down in 2026. We have the option to build our own regulations now, based on our own requirements. This might also present an opportunity to incorporate limits in relation to sewage sludges. Cakes that have been derived using dewatering polymers are a cause for concern, and he said we needed to be taking a close look at these polymers and their environmental fate – “they’re not plastics in the conventional sense”, he said, but they are long chain molecules that are slow to break down, and he tends to think about them in relation to this whole issue.

At a recent meeting, where the project solicited the views of stakeholders in England, they were asked whereabouts the limits for microplastics should be set – i.e., stick where we are, go for a market-led approach (like Scotland), or go for zero. Market-led is perhaps easiest, he suggested, with it being a matter of simply aligning practices across the whole of the UK, but they actually said they wanted to go for zero. “Ok, that’s going to be… challenging,” he said. In any case, an evidence-based limit is simply not possible at present given the limitations of the data.

Going for “as low as reasonably practicable” will involve doing some work on the existing data to ascertain just how accurate it is.

So, yes, he summarised, there’s plenty of plastics in the environment, and plenty of evidence of harm under experimental conditions, but no evidence of harm in the field. The market for fertilizers taken from composting and digestate remains buoyant – and was so even before the war in Ukraine, which has caused prices to spike. “So there’s some kind of cognitive dissonance here between the policy regulatory peer group, if you like, and what the markets are actually prepared to live with.” In the meantime, regulators are looking at it closely and we should expect the allowable limits to come down.

1. Microplastics in freshwater and soil: An evidence synthesis. November 2019. The Royal Society


Particle primer

Source: “Measuring microplastics in UK source-segregated composts and digestates”, Laura Fonseca Aponte, SUEZ/Aqua Enviro, European Biosolids & Bioresources Conference 22-23 November 2022.

Plastic particles can be grouped according to size, shape, material and other factors. “Microplastics” most often refers to fragments with a diameter below 5mm and above 0.1 – 0.3mm (though there is some divergence). The definition of nanoplastic is more contentious, and a 2019 paper1 holds it be particles where at least two dimensions are in the size range 1 – 100 nm, whereas the presentation by Fonseca Aponte (see main article) used the same label for sub-1um particles.

What kinds of polymers are found in soil samples? Her presentation noted PE, PET (from grocery bags, for example), PS, PL, PP, PVC and ACR. The complete degradation of compostable plastics in soil and compost has been demonstrated, although no data is available on the fate of these materials in AD and digestate.

Microplastics are commonly categorized as either “primary”, which are purposefully manufactured (the microscopic “beads” present in exfoliants, for example) or “secondary”, which arise when bigger plastic items degrade.


Harms: what do we know

The harms associated with plastic particles seem to grow in seriousness as particles get smaller. This might be another concern in itself, in that the particles already out there are destined to further fragment over time, into tinier and tinier pieces.

At the nanoscale, controlled lab experiments show that very small particles are able to cross cell membranes, impacting cell function and DNA.

Slightly larger particles are known to impact the health and reproductive behaviour of earthworms. Again, this has been observed at laboratory scale. And such ill effects can amplify up the food chain, as birds, for instance, eat those smaller animals.

And then larger particles still, the “macroplastics”, the stuff that can be seen blowing around, can affect things like soil water percolation, and be associated with perched water tables, and impacts on the root penetration of soils.

Another potential pathway of harm is the role microplastics might play as carriers of other contaminants, including microorganisms and things like PFAS, but this appears to be another area where a picture is only starting to be built. One study in November using electron micrography noted that around 200 species of bacteria appear to colonize microfibres found in the Mediterranean Sea.