Microplastics in drinking water treatment – Current knowledge and research needs

Highlights

• Microplastics (MPs) are being detected in freshwaters and also in drinking water.

• Drinking water treatment plants pose a barrier for MPs to enter drinking water.

• Conventional treatment processes have a potential to remove a part of microplastics.

• Efficiency of distinct treatment steps versus MPs character should be understood.

• Regarding water treatment, special focus should be put on small-sized MPs (< 10 μm).

Abstract

Microplastics (MPs) have recently been detected in oceans, seas and freshwater bodies worldwide, yet few studies have revealed the occurrence of MPs in potable water. Although the potential toxicological effects of MPs are still largely unknown, their presence in water intended for human consumption deserves attention. Drinking water treatment plants (DWTPs) pose a barrier for MPs to enter drinking water; thus, the fate of MPs at DWTPs is of great interest. This review includes a summary of the available information on MPs in drinking water sources and in potable water, discusses the current knowledge on MP removal by different water treatment processes, and identifies the research needs regarding MP removal by DWTP technologies. A comparison of MPs with other common pollution agents is also provided. We concluded that special attention should be given to small-size MPs (in the range of several micrometres) and that the relationship between MP character and behaviour during distinct treatment processes should be explored.

Introduction

Currently, microplastics (MPs) are emerging globally distributed pollutants that receive considerable attention, both from research communities and the public audience. MPs are defined as plastic particles not exceeding a 5 mm size limit (Andrary, 2011; Eerkes-Medrano et al., 2015; Koelmans et al., 2015), while the smallest size classes of plastic particles (< 100 nm or < 1000 nm, definition is unclear) are instead referred to as nanoplastics (NPs) (Koelmans et al., 2015; da Costa et al., 2016; Hahladakis et al., 2018). Contrary to NPs that thus far have been almost impossible to credibly identify in natural matrices, there are numerous studies on MP abundance in the environment. Regarding aquatic environments, much more effort has been put into investigating seas and oceans compared to freshwater, and despite significant abundance variations, MPs have been detected in marine waters worldwide (Cole et al., 2011; Auta et al., 2017). Nevertheless, freshwater bodies are the predominant drinking water sources for human consumption and are therefore suspect as potential sources of MPs to humans. Some raw water samples from selected drinking water treatment plants (DWTPs) have been already investigated for MPs and their presence was confirmed (Pivokonsky et al., 2018; Mintenig et al., 2019), with MPs reaching up to > 4000 items per litre (Pivokonsky et al., 2018). MPs have also been reported as present in lakes, rivers and dams globally (Dris et al., 2015; Su et al., 2016; Anderson et al., 2017; Leslie et al., 2017; Di and Wang, 2018; Triebskorn et al., 2019), even in remote areas (Free et al., 2014; Waller et al., 2017; Green et al., 2018). Thus, DWTPs seemingly have to face the presence of a “new” polluting agent – MPs, at least in some areas.

Notably, the risk associated with toxicological effects of MPs (and NPs) is still not fully described. Numerous studies have investigated these pollutant effects on (predominantly aquatic) organisms and some negative impacts have been observed (de Sá et al., 2018; Triebskorn et al., 2019); however, the tested MP concentrations often greatly exceeded the values currently detected in nature, as reported by Triebskorn et al. (2019). Nonetheless, considering the growing production of plastics (Plastics Europe, 2018), increasing environmental concentrations of MPs may be expected. Recently, some sporadic studies on microplastic interactions with human cells have also been conducted. In vitro experiments by Schirinzi et al. (2017) revealed that MPs (mainly 10 μm polystyrene, accompanied by 40–250 nm NPs) induced oxidative stress to human cerebral and epithelial cells. Triebskorn et al. (2019) reported observing MPs (polystyrene, 0.25 ± 0.06 μm) taken up by human keratinocytes. Another concern may be a possible synergism with hydrophobic organic pollutants that tend to adsorb onto MPs and may be subsequently transferred to respective organs and tissues at elevated concentrations (F. Wang et al., 2018). Although the possible effects of chronical exposure to MPs on human health are still more of a subject of discussion (and future research) than a clearly defined issue (Barboza et al., 2018; Revel et al., 2018), the presence of MPs in potable water should not be neglected.

The aim of this review paper is (1) to summarize and discuss the available information on microplastics in drinking water sources, (2) to encapsulate the evidence regarding MPs in potable water and (3) to provide insight into a current overview of MP removal by water treatment processes, including comparison to other common pollutants. This research also leads us (4) to identify research gaps and needs in the field of microplastics versus drinking water sources and treatment.