Sewage sludge as a source of microplastics in the environment: A review of occurrence and fate during sludge treatment

Highlights

• It is reported that 10⁶-10¹⁴ MPs are released into the environment via sludge annually.

• Limited knowledge exists on the fate of MPs in sludge stabilization, dewatering and drying.

• Degradation potential for some MPs has been observed during stabilization.

• Based on type, size and amount of MPs, they affect digestion process differently.

Abstract

Modern wastewater treatment plants (WWTPs) effectively remove microplastics (MPs) from wastewater and unsurprisingly concentrate them in sludge. Hence through its beneficial use and disposal, sludge causes secondary release pathways of an estimated average amount of 10⁶ to 10¹⁴ wastewater-based MPs to various environmental compartments yearly. Despite these numbers, studies investigating sludge are scarce. Currently, majority of the studies in the field focus on identifying the magnitude of the problem, whereas research investigating the fate and effects of MPs during sludge treatment are very rare. This review aims to bring together and critically evaluate the limited studies conducted about MPs in the sludge treatment line and bring out the key gaps and research needs in the area. Studies conducted so far indicate that depending on the type, size, and amount of MPs, their effects during anaerobic digestion differ, with some studies demonstrating serious negative impact on biogas production. Possible effect mechanisms are also suggested such as formation of reactive oxygen species (ROS) and leaching of toxic chemicals. Moreover, a potential for sludge treatment processes (thickening, dewatering, drying, stabilization, etc.) to change the characteristics and the number of MPs, which may increase surface area available for adsorption and desorption of pollutants, was observed. Review uncovers that, in the broad universe of MPs, some highly abundant ones in sludge such as polypropylene, polyurethane, polycarbonate, and acrylic are not yet investigated in sludge treatment. Future research should focus not only to investigate the fate/effects but to fully understand the mechanisms behind these, which is missing in many studies reviewed. Besides, new studies show that effect of MPs start from the floc formation stage during biological treatment, which in fact determine the final sludge behavior in thickening and dewatering. Therefore, holistic approaches starting from wastewater till sludge exits WWTP seem necessary. Substantiating from polymer chemistry and response of plastics to stress conditions, review suggests possibilities of deterioration during sludge treatment processes. It becomes evident that some totally uninvestigated aspects such as disintegration conducted before stabilization, can change the fate of MPs during sludge treatment and may bring new perspectives to the solution of the problem.

Introduction

Increasing plastic pollution with rapid growth in global plastic production and consumption has started to create scientific and social concerns (Rolsky et al., 2020, Zhang et al., 2020). The rate of global plastic production in 1950 increased from 1.5 million tonnes to 368 million tonnes in 2019 (Plastics Europe, 2020). Despite the growing waste management activities, most of the plastic waste continues to be released into the environment (Bläsing and Amelung, 2018). According to the United Nations Environment Programme's report, keeping the current production patterns and waste management activities will result in 12 billion tonnes of plastic waste to be in landfills and the natural environment by 2050 (UNEP, 2018). Moreover, nearly 10% of all land-based plastics are suggested to end up in the marine environment (Jahan et al., 2019; Thompson, 2006). This situation creates concerns not only with aesthetical issues and environmental pollution but also due to negative impacts on the ecosystem and human health. Furthermore, plastic pollution is a significant problem that causes economic losses exceeding hundreds of millions of dollars annually (Hardesty et al., 2015).

Microplastics (MPs) are plastic particles smaller than 5 mm in size that are easily transported in environmental systems. There are two categories of MPs based on their sources (Gatidou et al., 2019; Mintenig et al., 2017). Primary MPs are intentionally produced in micrometer sizes to be used in cosmetic and personal care products, whereas secondary MPs, which constitute the largest portion of MPs in water, result from the degradation of larger plastic products by exposure to environmental stressors (e.g., water, wind, and sunlight) (Lv et al., 2019, Ngo et al., 2019). Hence, both primary and secondary MPs enter the terrestrial and aquatic environments via effluents from domestic, industrial and agricultural activities, transportation, and wastewater treatment plants (WWTPs) (Gao et al., 2020; Waldschläger et al., 2020). In addition, MPs accumulating in landfills are resuspended by wind and become air-borne, which can spread further in the environment through atmospheric transport (Rillig, 2012; Rocha-Santos and Duarte, 2015). Therefore, MP pollution is encountered in all kinds of environmental systems, including water, soil, air, and sediment. In addition to being ubiquitous in the environment, MPs also raise concern with their potential to block the digestive system of animals and to be the vector for the transfer of hazardous pollutants into biota (Lv et al., 2019, Yang et al., 2021a). Because of their hydrophobicity and high specific surface area, MPs can adsorb organic pollutants and heavy metals on their surfaces and transfer them to the food chain. In addition, providing available hydrophobic surfaces for biofilm formation causes MPs to carry microbes to organisms (Zettler et al., 2013). These factors bring numerous negative impacts on living organisms, such as toxicity (Martínez-Gómez et al., 2017), enzyme inhibition (Guilhermino et al., 2018), and immobilization (Rehse et al., 2016). By the realization of their danger, MPs have started to hold a significant place in environmental research for the last 10 years (predominantly in the last 5 years).

WWTPs, which are the last step of human-made water cycle, receive a high amount of MPs via domestic and industrial wastewaters, storm water, and landfill leachate inputs (Ou and Zeng, 2018). WWTPs have been reported to have MP removal efficiency of up to 99.9%, however they still release a daily average of 10⁵-10⁸ MPs with the effluent, due to highly loaded influent wastewater (Sun et al., 2019; Ziajahromi et al., 2016). Besides, the MPs removed from the wastewater are transferred into sludge, rather than being eliminated during treatment processes (Okoffo et al., 2019). Therefore, WWTPs have been identified as both sources and sinks of MPs pollution in the environment (Mintenig et al., 2017; Zhang et al., 2019).

Realizing the continuous generation of vast amounts of sewage sludge and the beneficial constituents in it, nations have been considering sludge as a resource rather than a waste. As a prerequisite for further handling, high amount of water needs to be removed by thickening/dewatering/drying, so that sludge can be managed more cost effectively. Due to high organic and nutrient content, land application as a soil conditioner or fertilizer has been the major recycling route. In North America and Europe sewage sludge generated is beneficially used up to 50% by recycling on land (Nizzetto et al., 2016). On the other hand, land application of improperly treated sewage sludge creates increasing concern due to the presence of pathogens and more importantly accumulation of some trace pollutants, including toxic organics and MPs in the ecosystem (Chen et al., 2020c). Due to the fact that over 90% of MPs in wastewater are accumulated in sludge that has potential for beneficial use and some to be disposed of, it is clear that WWTPs lead to an additional pathway for MP pollution in the environment (Zhang et al., 2020). An annual average of 10⁶-10¹⁴ MPs have been shown to be released into the environment through incineration, landfilling, and land application of sludge. Strikingly, the annual amount of MPs entering the soil through land application of sludge is greater than that enters the oceans (Zhang et al., 2020). Thus, agricultural use of sludge is pointed out as one of the largest source of MPs entering the environment (Hurley and Nizzetto, 2018).Zhang et al. (2020) has observed 87.6 to 545.9 MPs/kg of soil after annual amendment with sludge-based composts, which also led to MP uptake in earthworms. Similarly, Corradini et al. (2019) reported as high as 3500 MPs/kg of dry agricultural soils which was exposed to 10 years of continuous sludge disposal.

Limited recent studies have investigated the effects of MPs on sludge stabilization processes and found that depending on their physical and chemical properties, MPs may pose different impacts on anaerobic digestion of sludge. The corresponding effects of stabilization on MPs have yet superficially addressed, which only hints on possible deterioration of MPs during these processes. Besides, a couple of studies have shed light on the impacts of MPs on dewaterability, which may further impact sludge disposal practices. Therefore, understanding the fate and effects of MPs during sludge treatment processes is critical to determine the magnitude of the MP-based problem posed on the efficiency of WWTPs.

Previous review articles examining MPs in WWTPs have extensively focused on wastewater, whereas, sewage sludge has been assessed only to a very limited extent. Until now, sludge-related review articles concentrated on either only detection/identification methods and abundance (Rolsky et al., 2020) or a specific sludge treatment process (Mohammad Mirsoleimani Azizi et al., 2021). For example, the sole impact and co-impact of MPs with other pollutants on anaerobic digestion of sludge have been recently investigated (He et al., 2021); on the other hand, no review article critically evaluating the fate and effects of MPs in the entire sludge treatment line is available at the time when this review is prepared. There is a clear need to summarize the limited amount of information in the current literature on MPs in sludge treatment, put forth the prospects and suggest areas of exigent research.

Correspondingly, this article aims to review recent progress about fate of MPs during sludge treatment as a whole, as well as MPs' consequent effects on sludge treatment processes. For the sake of complete portrayal of the problem, review starts by summarizing sampling and detection techniques, reported abundances as well as the characteristics of MPs encountered in sewage sludge. Then it critically discusses the current findings on MPs’ fate and effects in sludge treatment and identifies the important knowledge gaps in the literature by providing suggestions for future research directions.