Conversion and removal strategies for microplastics in wastewater treatment plants and landfills
Graphical abstract
Introduction
Global annual plastic production currently exceeds 300 million tonnes, and with up to 13 million of them are released into aquatic environments [1], [2]. Reports estimate that 80 to 95% of marine litter is composed of plastics [3], while plastic products in municipal solid waste constitute 8 to 12% [4]. In the United States, the recycling rate of plastics is 8%, indicating that the majority of plastic produced ends up in landfills or the environment [5]. After their useful lifespan and disposal by methods other than recycling, plastic products undergo gradual fragmentation into microplastics (MPs) through weathering, photolysis, abrasion, mechanical and microbial decomposition. MPs are defined as polymer particles < 5 mm in diameter. Additional environmental sources of MPs originate from the use of manufactured MPs as microbeads, capsules, fibers or pellets in cosmetics, personal care products, cleaning agents, paints and coatings. It was estimated that 8 million tonnes entered the ocean in 2010 [6]. The combination of large plasticware degradation and the release of manufactured MPs has led to an estimated over 5 trillion MP particles weighing 243,000 tonnes afloat in the oceans [7]. The small size of MPs prevents their recovery with current practices, which has boosted their ubiquity and persistence in the environment. The presence of MPs in conjunction with their high surface area and hydrophobicity facilitate their ingestion by living organisms and enhance the risk of adsorption/desorption of toxic chemicals and pathogens in water [8], causing concerns for their eventual negative effect on environmental and human health.
The consensus among the prevailing data about potential MPs toxicity to living things suggests that the accumulation of MPs and nanoplastics (NPs) in aquatic organisms would likely have negative health effects, such as inhibition of growth and development, neurotoxic responses, metabolic disorder and genotoxicity [9], [10]. Possible effects of MPs and NPs on the mammalian gut microbiota and host cellular toxicity have been investigated in mouse models, which indicated that the degree of toxicity is less severe compared to fish but did exhibit a potential for low to moderate negative effects including disorders in metabolism and gut microbiota dysbiosis [11], [12]. A previous review demonstrated that polystyrene MPs may act as immune stimulants that induce cytokine and chemokine production in a size- and concentration-dependent manner [13]. Several reviews have summarized the evidence for the potential toxicity of MPs and discussed their toxicity mechanism and chemical reactions involved in humans [14], [15], [16]. These point to a need for additional studies to elucidate cellular and systemic toxicity mechanisms of MPs and NPs in human beings. Wastewater treatment plants (WWTPs) act as barriers but can also be a main entrance route for MPs into the aquatic and terrestrial environments. Studies reported that 25% of the MPs are entering the ocean through WWTP effluent [17]. In addition to water release, current sludge disposal practices could discharge 14,526 MPs particles per person per day [18]. Being the main disposal location for sludge, landfills therefore accumulate a large number of MPs through the sewage sludge and the fragmentation of large plastic waste directly sent to landfills, which has been estimated at 79% of all plastics generated [19]. Several studies have detected MPs in landfill leachates and seawater near landfill sites [20], [21], [22]. The extent of the role of landfills as a land‐based MPs source needs additional investigation. Since chemical, physical and biological processes take place simultaneously in WWTPs and landfills, they present opportunities as locations for actionable intervention to remove MPs and to prevent further environmental and human exposure efficiently.
Until now, no MPs removal strategies have been successfully applied in WWTPs and landfills. Standardization or unification of methods for MPs sampling, detection and characterization are urgently needed, as comprehensively discussed in previous reviews [23], [24]. Several recent review papers have been published regarding MPs in WWTPs. For instance, Zhang and Chen summarized analysis methods for MPs, effects of MPs on wastewater/sewage sludge treatments, and MPs removal methods [25]. Zhang et al. focused on the removal of MPs, removal pathways, and the impact of MPs in anaerobic digestion [26]. Enfrin et al. reviewed the degradation of MPs to NPs, the impacts of MPs on wastewater treatment, and others [27]. Sun et al. reported the detection and characterization of MPs [28]. However, these reviews have not comprehensively summarized the potential strategies or technologies that could be employed nor discussed the challenges associated with incorporating potential technologies into existing wastewater treatment processes for MP removal, degradation, or upcycling. Moreover, landfills, as another land-based pollution source and a hotspot of MPs contamination, have also been overlooked in these reviews. The fragmentation and degradation of MPs in landfills, the concentration of MPs in leachates, and the role of landfills and leachate treatment facilities in removing MPs remain under-investigated. To date, the literature has not fully discussed the interaction among MPs, various contaminants and biofilms in wastewater streams that may deteriorate the removal efficiency of MPs. Therefore, in this review, we will discuss (1) current life cycle of MPs in WWTPs and landfills which are important anthropogenic emission sources of MPs, emphasizing the current data for MPs concentration, size distribution, type, shape, and potential chemical degradation reactions in each WWTPs units and landfills; (2) feasibility of strategies for upcycling MPs; and (3) challenges for implementing MP removal technologies in WWTPs and landfills.
Section snippets
Occurrence of different MP polymer compositions and MPs in WWTPs influents and effluents
The common types of MP polymers in WWTPs are polyester (PES) (up to 90% of total polymers), polyamide (PA) (up to 53%), polyethylene (PE) (up to 17%), polypropylene (PP), alkyd, acrylic, and polystyrene (PS) (Fig. 1) [29], [30]. The variation of MPs composition is closely related to influent sources. Polyethylene terephthalate (PET) constitutes a large percentage of PES, coming mostly from synthetic clothes along with PA [31]. PE is widely used in facial or body cleansers, packing films, and
Separations technologies
Diverse separation technologies have been applied to remove contaminants including plastics in wastewater. These methods are mainly categorized by the principles of separation: (1) size, (2) density, and (3) hydrophobicity. Depending on the characteristics of source materials, the composition of effluent, capacity of the facility, and other circumstances, it is necessary to select proper technologies and design the separation process for effective removal and recycling plastics. In particular,
Complex mixtures
In any treatment process, complexity and diversity in the mixture increases difficulty in treatment. MPs represent a highly complex mixture with different shapes (fragments, fibers, and microbeads), sizes, polymer types and chemical additives, which make recovering and degrading MPs harder (Fig. 4). The input and relative concentrations of MPs in WWTPs or leachate treatment facilities also vary day to day and hour to hour. A sequence of techniques targeting the common features of the diverse
Conclusions and perspectives
To best of our knowledge this is the first comprehensive review discussing the physical, chemical, biological and thermochemical degradation processes of MPs in both WWTPs and landfills. Removal of MPs from marine basins or rivers is not a viable strategy due to their ubiquity and continuous evolution of plastics from larger items to smaller sizes. Thus, MPs pollution source control such as wastewater effluent and leachates that focus on converting MPs into value-added products will be more
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge funding from The Research Foundation for the State University of New York 1156645-2020-85943.
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