Recent research indicates that European agricultural land could be the largest global source of microplastic pollution, caused by the spreading of microplastic-laden sewage sludge fertilisers, derived from wastewater treatment plants.
The impacts of this extreme pollution to the soil’s physical properties, the soil ecosystem, and crop growth remain unclear. This restricts the ability to change environmental policies in Europe and the UK to limit the amount of microplastic in fertilisers and to assess the risk that this pollution poses to the local and surrounding biodiversity, as well as to human health. In order to conduct such environmental impact assessments, monitoring programmes must be undertaken to understand the transport and fate of microplastics in wastewater treatment plants and generated sewage sludge. Therefore, it is becoming increasingly important that appropriate techniques for detecting microplastics in treated wastewater and fertiliser are developed and standardised to address this large-scale pollution issue.
The microplastic problem
Our marine, freshwater and terrestrial environments are littered with small plastic particles less than 5mm in size, which are referred to as microplastics. They are derived from the fragmentation of large plastic waste, the shredding of textiles, or from purposely manufactured small plastic particles, and can enter the environment through mismanaged waste, urban runoff and from the effluent of wastewater treatment plants. They have been discovered in most remote rivers of the Himalayas and in the deepest trenches of the Pacific Ocean. More worryingly, microplastics have been found to be ingested by organisms from lower trophic levels, such as zooplankton, to top predators, such as orcas, suggesting that the impact of microplastics in terms of exposure and ingestion affects the entire food chain. This has sparked public concern over the dangers of microplastics to wildlife and human health.
“it is increasingly important that appropriate techniques for detecting microplastics in treated wastewater and fertiliser are developed”
The majority of research concerning microplastic pollution has been conducted in the marine and freshwater environment. However, a new study conducted at Cardiff University and the University of Manchester has suggested that concentrations of microplastic on European farmland may mirror the concentrations found in ocean surface waters, with between 31,000 and 42,000 tonnes of microplastics (or 86 trillion – 710 trillion microplastic particles) spread onto agricultural soils annually (Lofty et al., 2022), (Figure 1).
The culprit of this extreme pollution is microplastic-laden fertilisers derived from sewage sludge generated at wastewater treatment plants. It was found at a wastewater treatment plant in Wales, UK, that up to 650 million microplastic particles between 1mm and 5mm in size entered a wastewater treatment plant every day (16 ± 7 microplastic particles per litre of incoming sewage) (Lofty et al., 2022). 100% of these particles were separated from the incoming sewage and diverted into the sewage sludge rather than being released with the clean effluent. At the facility in Wales, UK, each gram of sewage sludge contained up to 24 microplastic particles, which was roughly 1 % of its weight. In Europe, an estimated 8 – 10 million tonnes of sewage sludge is generated each year, of which around 40 % is used for nutrient enrichment of soils in arable farming. The spreading of sewage sludge is promoted by European Union directives that encourage diverting sewage sludge away from landfill and incineration, and towards energy production and agriculture, contributing to goals that lead to net-zero waste and a circular economy. The relative input of sludge fertiliser onto farmland is different per European nation, determined by local agricultural policy, population density or the availability of agricultural areas for sewage sludge spreading.
“our marine, freshwater and terrestrial environments are littered with plastic particles less than 5mm in size”
Microplastics have the potential to harm soil health, by leaching toxic chemicals during degradation and potentially transporting hazardous pathogens into the groundwater and rivers. Furthermore, they are easily consumed and absorbed by animals and plants (Azeem et al., 2021). Studies have revealed that the occurrence of microplastics in soils can reduce the abundance and diversity of organisms and microorganisms that live there, for instance, the presence of microplastics stunted the growth of earthworms and caused them to lose weight (Boots et al., 2019). Additionally, microplastics in soils can change the acidity, water holding capacity and porosity of the soil, which has a direct effect on plant development and performance by altering plants root growth characteristics and the nutrient uptake process (Rillig et al., 2019). The loss of soil biodiversity and change in soil properties can have a detrimental effect on soil performance and quality for growing crops, which in turn may disrupt future food production.
The lack of a regulations surrounding microplastics in sewage sludge leads to undesired recycling management in which these contaminants are transported back into the soil to eventually return to the natural watercourse via surface water run-off or infiltration into the groundwater. Therefore, diverting of sewage sludge to farmland is effectively shifting microplastics around the environment, as well as undoing the benefits of removing microplastic from wastewater. This raises the question of whether the present circular waste economy for sewage sludge is safe for the organisms which inhabit agricultural soils and the crops which are grown in them.
Microplastic detection in wastewater treatment
In order to monitor microplastics as they enter a wastewater treatment plant in the incoming raw sewage, during the treatment stages, and after treatment either in the effluent or generated sewage sludge, sample testing needs to be undertaken at each stage of the flow process. Assessing the concentration of microplastics in a liquid or solid samples would typically require the following four steps: sample collection, pre-treatment, separation and identification/characterisation (Figure 2). Various techniques at each step have been used by studies as the methods involved in each step are not yet standardised. Techniques also vary according to the type of sample collected (e.g. water sample, sludge sample), sampling equipment (e.g. net sample, grab sample) and the size range of microplastics being collected. The techniques used in each step can have an impact on the final concentration of microplastics observed in the sample, therefore, choosing the most suitable sampling equipment and methods is crucial in analysis.
The first step in assessing the concentration of microplastics is to choose sample locations and to collect a sample using equipment that is suitable for the study’s research objectives, such as determining the most effective treatment stage for microplastic removal. For sampling liquids, buckets, pumps with filters, nets and autosamplers have all been used to collect samples at different stages of a wastewater treatment plant. Buckets have the advantage of being quick and easily obtainable, however, can only sample a limited volume. Pumps with filters and nets can sample huge volumes of wastewater depending on the filter mesh sized used and amount of suspended solids in the liquid. A pump or net with a small filter mesh size will more likely become clogged up with suspended solids, decreasing the sampling time and sample volume size. However, a smaller filter mesh size will allow smaller microplastics to be captured for analysis. Autosamplers can take a sample periodically for analysis, which means they have the advantage of being able to collect a specific amount of sample per time interval without being clogged up, allowing for uniform temporal sampling of wastewater.
The sampling equipment used will be specific to the location of the sample. For example, nets and pumps may be more suitable for clean water effluent since samples should contain very little suspended solids, whereas buckets or autosamplers may be more appropriate for sewage influent because samples will contain high amounts of solids and will not clog up. Solid samples from wastewater treatment plants, such as settled material at primary settlement or sewage sludge, are usually collected by buckets or grabs.
After the sample has been collected, it is usually dried and sieved to remove large particles and help concentrate the microplastics. This is an opportunity to determine the minimum size limit of microplastics that will be identified in the sample, as there is presently no agreed upon lower size limit for particle detection. A finer filter and hence a lower minimum size limit of detection, allows for the identification of more microplastics. However, a smaller sieve size is more likely to become clogged with organic matter and other debris, thus precautions must be made when choosing suitable sieve sizes.
Both liquid and solid samples from wastewater treatment plants can contain high amounts organic matter. Microplastics can clump together with this organic matter via biofilms produced by microorganisms. Without the pre-treatment stages, the aggregated microplastics and organic matter are difficult to characterise and identify using visual or spectroscopic techniques, which may lead to an underestimation of the microplastics in the sample.
There are various methods to digest the organic matter from the sample. One of the most popular methods is wet peroxide oxidation using H2O2 in the presence of an iron catalyst, commonly known as Fenton’s reagent. This method is efficient in oxidising the organic matter in the sample without degrading or changing the characteristics of the microplastic particles. Another emerging technique is the use of enzymatic digestion where samples are placed in a mixture of enzymes such as proteinase, lipase and amylase which degrades the organic matter, leaving the microplastic particles unaffected. This is thought to be a gentler technique using less harsh chemicals compared to Fenton’s reagent, however, the technique is expensive and time consuming, taking up to 13 days to digest a sample (Sun et al., 2019). Other techniques which have been used in previous research studies include acid (HCl, NHO3, H2SO4) or alkaline (KOH, NaOH) treatment which are highly effective in degrading organic matter but have been observed to melt and fragment the microplastic particles in the sample. Typically, sewage sludge and influent samples will contain more organic matter, compared to clean water effluent, and therefore, it may require sewage sludge samples to spend longer in the presence of the pre-treatment solution or to repeat the pre-treatment to ensure that all the organic matter has been removed from the sample.
The next step is to separate the microplastic particles in the sample from inorganic particles and the sediment matrices. This is usually undertaken using density separation with a salt solution with densities ranging from 1.2 – 1.8 g/cm3 and utilising the density difference between sediment particles (density: 2.65 g/cm3) and microplastic particles (densities: 0.80 – 1.45 g/cm3). The sample is usually placed into a large container containing a salt solution, stirred, and left until all the microplastics have risen to the top and the denser inorganic particles have sunk to the bottom. This top layer of microplastics is then skimmed off and dried ready for identification (Figure 3).
Different salt solutions can be used for density separation, each with their own advantages and disadvantages in terms of extraction efficacy, cost and toxicity. Sodium chloride (NaCl) salt solution (density 1.2 g/cm3) is commonly used due to its low cost and low toxicity. However, microplastic particles with a high density such as polyethylene terephthalate (PET) (density: 1.32–1.41 g/cm3) and polyoxymethylene (POM) (density: 1.3 – 1.4 g/cm3) will not rise to the surface when in the presence of NaCl salt solution, resulting in poor extraction efficiencies and an underestimation of the microplastics in the sample. Other salt solutions have been used, such as zinc chloride (ZnCl2) (density: 1.6 g/cm3) which has recovery rates of microplastics of up to 96-100%, however, concerns about the solution’s environmental impact have been raised, thus recovery and reuse of the solution is recommended. Sodium iodide (NaI) (density: 1.6 g/cm3) also has high recovery rates of microplastics of between 65.8% -100% but is expensive and needs cation when handing. Calcium Chloride (CaCl2) (density: 1.3g/cm3), sodium metatungstate (SPT) (density: 1.4 g/cm3) and even canola oil have also been used as separation solutions for microplastic particles with high extraction efficacies.
Microplastic identification techniques
The final step in detecting microplastics from wastewater treatment samples is to identify the microplastics, quantify them, and characterise them in terms of their physical characteristics (size, shape, colour) and chemical characteristics (polymer). Visual inspection using a microscope is the simplest way to identify and quantify the microplastics in a sample, as well as determine their physical characteristics in terms of size, shape and colour. However, this method depends on the magnification strength and operator of the microscope. For example, natural fibres and textile fibres are difficult to distinguish between and items which are clear or the same colour as the background may be missed. Up to 70% of visually identified microplastics by microscope were not confirmed as plastics particles by FT-IR spectroscopic analysis, and this increased with decreasing particle size (Hidalgo-Ruz et al., 2012).
Submerging the sample in dyes such as nile red and rose bengal which stain polymer particles a red colour can aid microplastic identification and improve identification error (Figure 4).
Chemical characterisation of the microplastics in the sample can improve the accuracy of microplastics identification and allow further investigation into their composition, which can give an insight into the plastic’s density, polymer and source. The most common techniques for this are Fourier-transform infrared spectroscopy (FT-IR) and RAMEN spectroscopy. In FT-IR, infrared radiation is exposed to the microplastic particles and a spectrum is returned with unique characteristic peaks corresponding to particular chemical bonds between atoms that can be compared to a library of known polymers. There are several FT-IR methods available, including attenuated total reflectance (ATR) FT-IR which is useful for characterising irregular microplastics, and micro-FT-IR which is suitable for characterising small-sized microplastics down to 20 µm. Alternatively, RAMEN spectroscopy is a vibrational spectroscopy technique which provides information on the molecular vibrations of a particle in the form of a vibrational spectrum. RAMEN spectroscopy uses sub-micron wavelength lasers on the microscopes, which is capable of identifying particles down to 1 µm in size. One less commonly used technique is pyrolysis-gas chromatography–mass spectrometry (Pyro-GC-MS) which is a destructive method which characterises the chemical composition of the microplastics in a sample by analysing the mass spectrometry of their thermal degradation products. Unlike spectroscopic techniques, there is no size limit to the microplastics that may be analysed, however, the technique does not provide information on the size, shape, or amount of microplastics in the sample.
During each step of the detection of microplastics in samples there is a risk of contamination from the air, clothing and tap water. Therefore, control measures must be undertaken to avoid contamination of samples, which include, using glass and metal equipment instead of plastic wherever possible, wearing 100% cotton clothing instead of synthetic textiles which can shed into the samples, using filtered ultra-pure water when cleaning equipment and surfaces and keeping samples covered as much as possible with non-plastic lids to avoid airborne contamination. In addition, procedural blanks and controls, using open petri dishes, should be used for capturing background airborne microplastic concentrations. Further control can be added to samples by using a fume hood which can reduce 50% of contamination to samples (Wesch et al., 2017).
Implications for the future
At present there are no policies or solutions which address the extreme amount of microplastic pollution contained in sewage sludge from wastewater treatment plants. This is because our current understanding and quantification of the risk that microplastic-laden sewage sludge may cause to crops and organisms in the soil is limited. Without knowing the full impact that microplastics may have on soil biology, food production and on the boarder environment, such as the natural watercourse leading to our oceans, policymakers might not be required to update legislation that would have a significant impact on large-scale microplastic pollution.
Therefore, there is a requirement to monitor the microplastic concentrations in different wastewater treatment flows and in the produced sewage sludge to generate data that can be used as baseline concentrations of microplastics input and output from wastewater treatment facilities, as well as raise awareness of the issue. To do this, there needs to be standardised methods for detecting microplastics from wastewater treatment samples so that studies are comparable. At present, however, each method has its advantages and disadvantages, as well as providing varying results
in terms of microplastic concentration, size, shape and colour. This restricts the comparison of current and future dataset across the scientific community, leading to difficulties in accurately determining microplastics concentrations in sewage sludge and agricultural soils worldwide, as well as assessing the severity of the associated ecological risks of microplastic pollution. Environmental impact assessments, which are composed of an exposure assessment, an effect assessment, and risk characterisation, as well as effective environmental policy are all underpinned by data that is comparable, for which harmonised detection methods are crucial. Therefore, the development of standardised microplastic detection methods which can produce comparable results across scientific disciplines are crucial for future policy change for this ever-increasing microplastic problem.