Preventing Discharge from Specific Sources

Artificial turf, especially used in sports, is a significant source of microplastic leakage into bodies of water. It absorbs impact and prevents injury, while maintaining the feel of natural turf, and uses infill materials spread through the surface below the turf pile. In typical third-generation turf composition, the infill materials are divided into stabilising infill and performance infill (Hann et al., 2018). Stabilising infill consists of a layer of sand to help retain the turf's shape, while performance infill is usually small polymeric particles (<5 mm in size) laid on top of the stabilising infill. Performance infill mainly uses rubber crumbs from recycled tyres, although some organic materials such as cork and coconut husks are recommended. Contact sports, such as soccer, rugby, and American football, which cause frequent abrasion on infill materials lead to microplastic leakage. The amount of microplastic leakage can be measured by the rate of loss of infill materials. The annual rate is 1–4% of total infill materials applied, which is equivalent to a loss of 18,000–72,000 tonnes (Hann et al., 2018).

Microplastic leakage into bodies of water can come from landfills. A preliminary study by He et al. (2019) validated landfills as not only a source of plastic but also a potential source of microplastic, especially from its leachate. The microplastics are mostly polyethylene and polypropylene, in fragments and flakes of 100–1000 µm. Microplastics in leachate can be transported from leachate leakage or the effluent of leachate treatment systems. Microplastics from landfills may also move to the airstream through landfill ventilation and/or running off through the soil surface to bodies of water. The microplastic pathway means landfill leachate is a challenge. Composting integrated into landfill sites might also be responsible for microplastic leakage as the compost generated from unsorted mixed waste contains various forms of microplastics. It could also leak into water bodies as agricultural run-off.

1. Optimised Waste Water Treatment Plant (WWTP)

WWTP are fundamental in removing microplastics from discharge in bodies of water. Limited research has proven more than 90% removal of microplastics from effluent through WWTP, with the highest removals found after tertiary treatment such as filtration (WHO, 2019). The grease removal stage in preliminary and primary treatment is the most significant stage in the removal of microplastics (Sun et al., 2019). Sun et al. (2019) recommends advanced techniques such as Raman spectroscopy and thermo-analytics to remove tiny microplastics (<20 µm) found in effluent. However, the study was conducted mostly on WWTPs in high-income countries with comprehensive waste water treatment systems – only 33% of low- and middle-income countries have sewer systems (WHO, 2019). Most low- and middle-income countries, especially in ASEAN, have limited technologies for WWTP application and often bypass key stages of wastewater treatment, leading to non-optimal removal of microplastics.

WWTPs in Japan can remove up to 99.6% of microplastics, mainly from primary settlement tanks, with a 78.9% removal rate; followed by a combined reactor and final settlement tank (97.8%); and rapid filtration equipment (58.9%) (Nakao et al., 2019). In seven WWTPs in Xiamen, a coastal city in China, 79.3–97.8% (90.52% on average) of microplastics are successfully removed (Long et al., 2019). The removal rate in China is lower than in Canada, Scotland, and Finland, but higher than in Australia and Italy, and the difference is primarily caused by the varied technological interventions and characteristics of the microplastics treated.

In terms of technology, the Long et al. (2019) research highlighted secondary treatment in Xiamen, which is not specifically designed to eliminate microplastics, and compared it with other countries’ advanced tertiary treatments such as membrane bioreactor, post-filtration, and rapid sand filtration, which have removal rates of up to 99.99%. The research revealed some physical and chemical properties of microplastics affect their removal rate, including the size of microplastics (the smaller the size, the higher the removal rate), the type of polymers (the higher the polymer density, the higher the removal rate), and the shape of microplastics (higher removal rate for fragments and granules than fibres and pellets).

The Republic of Korea achieved a removal rate of more than 98% after tertiary treatment (Hidayaturrahman and Lee, 2019). Different WWTPs use different technologies for the tertiary treatment of different types of microplastics. The WWTP that mostly treated fibres (46.7%) and fragments (31.4%) applied ozone treatment, resulting in a removal rate of 99.2%, while the WWTP mainly treating microbeads (70.4%) used a membrane disc-filter, resulting in a removal rate of 99.1%. The WWTP mostly treating fragments (53.4%) applied rapid sand filtration, resulting in a removal rate of 98.9%.

Investment in advanced technologies is critically needed to prevent the discharge of microplastics. Identifying the appropriate technology based on the microplastic characteristics is key to ensure efficient removal. Japan, China, and Korea have proven an optimised WWTP can be achieved by utilising the appropriate technology and considering the characteristics of microplastics in the comprehensive treatment of wastewater. An optimised WWTP could be a model for an optimised leachate treatment system to prevent microplastics from leaching into landfills.

Unlike Japan, China, and Korea, most ASEAN countries’ treatment plants have limited technological intervention and treatment coverage. As a result, the level of microplastic leakage remains high even after WWTP treatment, while microplastic from landfill leachate, tyre wear, artificial turf, or polymer-coated fertiliser isreleased without treatment.

2. Biodegradable polymer-coated controlled-release fertiliser

In agriculture, microplastics might be leaked from fertiliser application run-off. Polymer-coated controlled-release fertilisers, such as Osmocote®, Apex®, and Multicote®, are utilised for: easy-to-adjust fertilisation type and rate for different crops, better fertiliser-use efficiency, less nutrient pollution in wastewater, no rinsing requirement after fertilisation, and much more (Landis and Dumroese, 2009). However, the thin polymer coating in fertilisers is transformed into trapped microplastics in the soil, which can leak into bodies of water. This side effect can be addressed by applying biodegradable polymer-coated controlled-release fertilisers (Majeed et al., 2015). Non-biodegradable synthetic polymer should be replaced with biodegradable natural or synthetic polymers derived from renewable natural resources. Biodegradation is induced by the polymer blend in various combinations with natural polymers, especially lignin, starch, chitosan, alginate, cellulose, or their modified forms (Majeed et al., 2015).