Microplastics in recovered wastes

Plastics are used in a wide range of products, including construction materials, food packaging, and personal care items such as wet wipes. These products can break down into microplastics and enter waste streams that are applied to land. Measuring microplastics in recovered waste is challenging because they are small, diverse, and structurally complex. Furthermore, measurement is made more difficult by interfering materials, such as natural organic matter.

Currently, several microplastic measurement techniques are used, each with varying abilities to measure particle numbers, size, shape, structure, polymer type, and mass. The use of microplastic measurement techniques with varying capabilities, accuracy, and precision has led to differences in published data, making it difficult to properly understand their risks.

To better understand the nuance of different methods, the NSW EPA commissioned CSIRO to measure microplastics in recovered waste using 3 leading measurement techniques.

Forty-six samples were collected from 5 wastewater treatment plants, 3 composting facilities, and 2 recovered fines facilities. Samples were measured using a gas chromatography method, which separates mixtures into individual components for analysis, and two optical techniques. The type of gas chromatography used was:

• pyrolysis gas chromatography mass spectrometry (pyGC-MS); while the optical measurements were performed with
• laser direct infrared imaging (LDIR); and
• Fourier transform infrared spectroscopy microscopy (FTIR).

The optical techniques directly measure the number of microplastic particles, allowing for the assessment of the particle’s size, shape, plastic type, and structure. In contrast, pyGC-MS breaks down the microplastic into smaller chemicals, providing an indirect measurement of the total mass of each plastic type within the sample. 

Equivalent samples were provided to multiple laboratories to enable a comparison between methods and between laboratories.

Results generated from all methods indicated that biosolids contain the highest concentration of microplastics, followed by composts, then recovered fines.

The proportion of microplastic polymer types was consistent between the gas chromatography measurements of the two laboratories, with polyethylene typically the most prevalent plastic type. However, the total measured mass of microplastics differed between laboratories.

Using optical methods, the abundance of microplastics was found to be similar between the two laboratories. However, while particle counts were comparable, the LDIR method identified polypropylene as the most common polymer, whereas the FTIR method identified polyethylene as most prevalent. 

Discrepancies between polymers identified across laboratories may be due to the use of different reference materials and identification criteria. Optical measurement of plastics can be challenging because many polymers have similar chemical compositions and, consequently, similar spectral characteristics.

When comparing the optical and gas chromatography measurements, the mass of microplastics estimated by optical methods was 10 to 1,000 times lower than that measured by gas chromatography. This significant discrepancy could potentially be due to differences in sample preparation, identification criteria or interference from naturally occurring materials in the sample. 

Measuring microplastics in the environment is a challenging task, as different methods can produce vastly different results.

Currently, macro-plastic contamination is managed in many recovered waste streams by requiring the material to meet permissible thresholds before it’s applied to land. To appropriately regulate the smaller end of the spectrum, more research is needed to understand the quantity and types of microplastics in recovered wastes.

To address this gap, the NSW EPA has invested in further research projects through the Plastic Research Program and the Solving Plastic Waste Cooperative Research Centre, aiming to develop a scalable, cost-effective, accurate, and precise methods for testing microplastics in recovered waste.