By Alisa Reid – 

Plastic pollution is a critical long-term problem with ~400Mt produced annually [1]. Microplastics are an important contributor; classified as <5mm in diameter. There are two classes; primary (manufactured at size) or secondary (eroded to classification size) [2]. This pollution is challenging to successfully mitigate as their size often enables them to enter natural environments unnoticed. The durability of plastic indicates microplastic pollution will be a lasting problem, with the additional danger of unknown health risks. By preventing the flow of microplastics into marine environments it may be possible to mitigate microplastic pollution in oceans. One method involves using wastewater treatment technologies (WWTPs). The Talvitie (2017) study [3] verifies that advanced final-stage WWTPs significantly reduce the number of microplastics discharged into marine environments, but is filtration of this type a viable solution to microplastic pollution?

Labelled microplastics from the San Francisco Bay area. Image: Cole Brookso

Problems

Being small increases the likelihood of microplastics being ingested by organisms. Studies on terrestrial environments have revealed microplastics can reduce soil viability [4]. This has the potential to affect food production and security, emphasising how pressing an issue microplastic pollution is.

Moreover, microplastics can become coated in toxic chemicals, heavy metals, and persistent organic pollutants [3]. Research into whether there are adverse human health effects from ingestion of microplastics (through contamination of food) is ongoing. Some concerns are based around the possibility of biomagnification of these harmful substances [1].

Cleaning microplastics (known as nurdles) from Arniston beach in Western Cape, South Africa. Image: Tom Camacho/Science Photo Library

Solutions

Many technologies which filter microplastics from effluents are available, but with varying efficacy. The Talvitie (2017) study compares several types, including a membrane bioreactor (MBR), which it concludes is the most effective: 99.9% removal rate of microplastics. No other technology achieved this percentage of microplastic removal:

• rapid sand filtration: 97%
• dissolved air flotation: 95%
• discfilter: 40-98.5%

Using MBR decreased the concentration of microplastics from ~6.9 MP/L to ~0.005 MP/L, a drastic difference and the lowest microplastic concentration of all methods tested. The key to MBR success is due to the filters having the smallest pore size (0.4 μm) of all the technologies, meaning it could catch particulates that slipped through other filters. The most common microplastics found in effluents (both before and after treatment) were the smallest size classification (20–100 μm), and the MBR is more suited to filtering out these tiny pollutants [3].

However, the performance of MBR filtration decreases over time as materials slowly build-up on the membrane. On-going research into solving this started when the first MBRs were developed, but at present it is still a major drawback.

Advanced final-stage treatment technologies. Figure: Talvitie et al. 2017

Conclusion

Although further research is required to find viable large-scale removal techniques of microplastics that have already entered natural environments, using widespread WWTPs (particularly MBRs) we can prevent further contamination from microplastics.

Additionally, the Talvitie (2017) study found the majority of primary microplastics originated from personal care products and most secondary microplastics were from synthetic textile fibres or fragmented plastics [3]. Using this knowledge may allow for a more effective approach by using technologies targeting these specific items.

References and further reading:

1. Lim, X. (2021) Microplastics are everywhere — but are they harmful? Nature, 593, 22-25. 10.1038/d41586-021-01143-3
2. UK Centre for Ecology & Hydrology (n.d.) Microplastics factsheet. Available at: https://www.ceh.ac.uk/sites/default/files/Microplastics%20factsheet%20310120.pdf [Accessed 2 December 2021].
3. Talvitie, J., Mikola, A., Koistinen, A., Setälä, O. (2017) Solutions to microplastic pollution – Removal of microplastics from wastewater effluent with advanced wastewater treatment technologies. Water Research, 123, 401-407. 10.1016/j.watres.2017.07.005.
4. Lahive, E., Walton, A., Horton, A. A., Spurgeon, D. J., Svendsen, C. (2019) Microplastic particles reduce reproduction in the terrestrial worm Enchytraeus crypticus in a soil exposure. Environmental Pollution, 255 (2), 113174. 10.1016/j.envpol.2019.113174.