Discharges containing plastics, especially microplastics, come from various sources, even uncommon ones. Microplastics are everywhere, spreading through the entire ecosystem. They might be at the bottom of the sea (high-density polymers) and at its surface (low-density polymers), in rainwater, in food, in drinking water, in the air as well as in wastewater.
One of the most challenging and pressing issues is the footprint of microplastics across water bodies, which has led to the global marine plastic debris problem. Table 1 lists sources of microplastics in water bodies.
Table 1. Sources of Microplastics in Water Bodies
Waste Management: Preventing Discharge from Specific Sources (Table 1)
|Run-off from land-based sources||Road surface run-off from the breakdown of road-marking paints and wear and tear of tyres|
|Fibres from textiles due to wear and tear and washing|
|Abrasion of objects such as synthetic soles of footwear and artificial turf|
|Agricultural run-off, particularly due to the use of sewage sludge or plastic materials for mulching|
|Wastewater effluent||Synthetic textile fibres from clothes washing|
|Cosmetic microbeads and disintegrated parts of larger consumer products flushed down toilets and sinks|
|Effluent from wastewater treatment plant|
|Combined sewer overflows||Overflow due to storm and heavy rainfall that bypass wastewater treatment|
|Industrial effluent||Leakage of pellets from plastic industries|
|Fragmentation and degradation of macroplastics||Fragmented and degraded macroplastics due to ultraviolet radiation and high temperatures|
|Atmospheric deposition||Dry and wet deposition, precipitation, and run-off|
|Production and distribution of drinking water||Erosion or degradation of plant components and distribution networks made from plastics|
|Microplastics in drinking water from bottles and caps|
Source: WHO (2019).
Artificial turf, especially that used in sports, is a significant potential source of microplastic leakage to water bodies. Artificial turf is required to absorb impact and prevent injury as well as to maintain the feel of natural turf. It uses infill materials spread throughout the surface below the turf pile. In a 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 shape of turf, while performance infill is usually small polymeric particles (<5 mm in size) laid on top of 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 could 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).
Microplastics leakage into water bodies might come from landfill. A preliminary study by He et al. (2019) validated landfill as not only a source of plastic but a potential source of microplastics, especially from its leachate. The microplastics identified are mostly polyethylene and polypropylene, in fragments and flakes of 100–1000 µm. Microplastics in leachate can be carried out from leachate leakage or from effluent of leachate treatment systems. Microplastics from landfill may also be moving to the airstream through landfill ventilation and/or running off through soil surface to water bodies. The microplastics pathway means that landfill leachate is a challenge. Composting integrated into landfill sites might also be responsible for microplastics leakage as the compost generated from unsorted mixed waste contains microplastics in varied forms. Microplastics could leak into water bodies as agricultural run-off.
1. Optimised Waste Water Treatment Plant (WWTP)
WWTP is fundamental in removing microplastics from discharge in water bodies. 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 microplastics removal (Sun et al., 2019). Sun et al. (2019) recommend advanced techniques such as Raman spectroscopy and thermo-analytic to remove tiny microplastics (<20 µm) remaining 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 some 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 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, about 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. The difference is primarily caused by different technological interventions and by characteristics of microplastics treated. In terms of technology, the Long et al. (2019) research highlighted the current 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%. In terms of microplastics characteristics, the research revealed that some physical and chemical properties of microplastics affected their removal rate. These include the size of microplastics (the smaller the size, the higher the removal rate), the type of polymers (the higher the density of polymers, the higher the removal rate), and shape of microplastics (higher removal rate on fragments and granules than fibres and pellets).
The Republic of Korea (henceforth, Korea) has 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 removal rate of 99.2%. The WWTP that mainly treated microbeads (70.4%) used membrane disc-filter, resulting in removal rate of 99.1%. The WWTP that mostly treated fragments (53.4%) applied rapid sand filtration, resulting in a removal rate of 98.9%.
Investment in advanced technologies is critically needed to prevent microplastics discharge. Identifying the appropriate technology based on the characteristics of microplastics is key to ensure the efficient removal of microplastics. Japan, China, and Korea have proven that an optimised WWTP can be achieved by utilising the appropriate technology and considering microplastics’ characteristics 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 landfill. Unlike Japan, China, and Korea, most ASEAN countries’ treatment plants have limited technological intervention and treatment coverage. As a result, the level of microplastics leakage remains high even after treatment in WWTPs, while microplastics from landfill leachate, tyre wear, artificial turf, or polymer-coated fertiliser are released 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 (1) easy-to-adjust fertilisation type and rate for different crops, (2) better fertiliser-use efficiency, (3) less nutrient pollution in wastewater, (4) no rinsing requirement after fertilisation, and many more (Landis and Dumroese, 2009). However, the thin polymer coating in fertilisers is transformed into trapped microplastics in the soil, which could potentially be leaked into water bodies. This side effect could 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 polymer blend in various combinations with natural polymers, especially lignin, starch, chitosan, alginate, cellulose, or their modified forms (Majeed et al., 2015).
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He, P., L. Chen, L. Shao, H. Zhang, and F. Lü (2019), ‘Municipal Solid Waste (MSW) Landfill: A Source of Microplastics? – Evidence of Microplastics in Landfill Leachate’, Water Research, 159, pp.38–45.
Hidayaturrahman, H. and T.-G. Lee (2019), ‘A Study on Characteristics of Microplastic in Wastewater of South Korea: Identification, Quantification, and Fate of Microplastics During Treatment Process’, Marine Pollution Bulletin, 146, pp.696–702.
Landis, T.D. and R.K. Dumroese (2009), ‘Using Polymer-coated Controlled-release Fertilizers in the Nursery and After Outplanting’, Forest Nursery Notes, pp.5–12.
Long, Z. et al. (2019), ‘Microplastic Abundance, Characteristics, and Removal in Wastewater Treatment Plants in a Coastal City of China’, Water Research, 155, pp.255–65.
Majeed, Z., N.K. Ramli, N. Mansor, and Z. Man (2015), ‘A Comprehensive Review on Biodegradable Polymers and their Blends Used in Controlled-release Fertilizer Processes’, Reviews in Chemical Engineering, 31, pp.69–95.
Nakao, S., A. Ozaki, and K. Masumoto (2019), Fate of Microplastics in a Japanese Wastewater Treatment Plant and Optimization of Microplastics Treatment. https://www.researchgate.net/publication/336995196_Fate_of_Microplastics_in_a_Japanese_Wastewater_Treatment_Plant_and_Optimization_of_Microplastics_Treatment (accessed 10 January 2020).
Sun, J., X. Dai, Q. Wang, M.C.M. van Loosdrecht, and B.-J. Ni (2019), ‘Microplastics in Wastewater Treatment Plants: Detection, Occurrence and Removal’, Water Research, 152(1), pp.21–37.
World Health Organization (2019), Microplastics in Drinking-Water. Geneva: WHO.