Microplastics: The What, Where, Why And Impact

by Craig Coker (BioCycle Magazine) Among the organics recycling challenges du jour is the potential presence of microplastics in compost and digestate. Two-part article series starts with an overview and ends with findings of current research. Part I

Over the past 10 years, the organics recycling industry (which includes composting, anaerobic digestion, and diversion to animal feed) has come to recognize that plastics contamination from food packaging is a significant challenge to the implementation and growth of these diversion practices. Plastic packaging is ubiquitous in the American food distribution system. Many different types of plastics are used in food packaging, as shown in Table 1.

Recovering packaged food wastes for reuse or recycling requires either mechanical depackagers or human labor for source separation, both of which are likely to achieve variable and imperfect separation efficiency (do Carmo Precci Lopes et al., 2019; Edwards et al., 2018). Depackaged and source separated food wastes may contain missorted plastic packaging with varying levels of contamination (Porterfield et al., 2023). Plastic contamination in organics recycling — especially in food waste feedstocks — has led to concerns about microplastics.

MPs can also be formed during and as a result of food waste depackaging, a separation process. In its simplest form, separation is a binary process, splitting a feed material into two components. These components could be called the extract (or that which you are trying to recover) and the reject (that which you do not want). The objective of a binary materials separator is to split a feed material into two different components by exploiting some difference in the material’s properties.

The research on the health effects of microplastics has focused, to date, on direct exposure. MPs in composts and digestates used as soil amendments are a secondary pathway of exposure, which has not yet been studied to any extent.

Inhalation and ingestion are the two primary routes of exposure to MPs. Inhalation causes physical damage to the lungs and ingestion is thought to have potential impacts on the immune system, liver, energy metabolism and reproduction. There are no comprehensive studies of MPs in the diet, although MPs have been found in seafood/fish, salt, beer, honey, milk, rice, sugar and seaweed (Vose, 2022).

In 2019, the World Health Organization (WHO) commissioned a report to evaluate the evidence of risks to human health associated with exposure to nano and microplastic particles (NMP) in drinking water. A key observation is that MPs are ubiquitous in the environment and have been detected in environmental media with direct relevance for human exposure, including air, dust, water, food and beverages.

The WHO assessed the quality, reliability and relevance of data on both exposure and effects for their possible contribution to a risk assessment of MPs. The assessment scores indicated that the available data are of only very limited use. Several shortcomings were identified, the most important of which was the heterogeneity of the methods used. It is recommended that standard methods be developed and adopted to ensure that the research community can reduce uncertainties, strengthen overall scientific understanding and provide more robust data for assessing the risks of human exposure to NMPs (WHO, 2022).

Microplastics in soils have been found to increase soil aeration, water repellence and porosity but to decrease soil bulk density and aggregate sizes (e.g., de Souza Machado et al., 2018b, 2019; Kim et al., 2021; Qi et al., 2020).

MPs’ impacts on terrestrial plants (particularly crops) are poorly understood.

Overall, the phytotoxicity of MPs varies dependent on their polymer type, size, dose and shape, plant tolerance, and exposure conditions. The accumulation of MPs and subsequent damage in plants may further affect crop productivity, and food safety and quality, causing potential health risks (Wang et al., 2022).