Microplastics in Biosolids Treatment Stream: Abundance, Fate and Methodologies

Microplastics is an important class of contaminants of emerging concerns. In scientific literature, microplastics typically refer to plastics particles, fibers, and fragments between 1 µm and 5 mm. Water Resource Reclamation Facilities (WRRFs) are conduits between the waste sources and the environment. WRRFs collect a large quantity of microplastics from various sources, including municipal waste, roadway run off, industrial discharge. Conventional wastewater treatment processes have been effective in separating microplastics driven by physical-chemical properties of microplastics, and consequently, microplastics accumulate in solids stream and biosolids (Blair et al., 2019; Bui et al., 2020). Treated biosolids have a range of disposal pathways, among which agricultural land application accounts for approximately 50% of annual total biosolids disposed. Land application of biosolids presents avenues for microplastics in biosolids to enter and accumulate in agricultural soil. The accumulation of microplastics in agricultural soils can lead to various adverse implications for soil quality by impacting soil structure, water dynamics, and soil biota. Microplastics that enter the terrestrial environment through biosolids can further mobilize to different environments, through storm runoff and aerosolization. Understanding microplastics' occurrence, fate, and transformation in biosolids treatment is crucial to mitigate the above-mentioned adverse effects of microplastics in biosolids and to promote the beneficial and sustainable use of biosolids. Information on microplastics cross the solids treatment steam is yet to be summarized or synthesized to assist stakeholders understand risks associated with microplastics on a high level. To this end, a comprehensive critical review was conducted to inform the state of knowledge of microplastics studies regarding sludge and biosolids and synthesizes data from those studies, to address the lack of understanding of microplastics in those unit processes. This study captures data and knowledge from 51 studies including peer-reviewed journals articles, reports, and conference proceedings. The proposed presentation will cover the abundance, fate and characteristics of microplastics in solids stream and biosolids, as well as methodologies for analyzing microplastics in sludge and biosolids samples. This study is part of a Water Research Foundation funded project (WRF 5221). Abundance and Fate of Microplastics in Solids Stream and Biosolids Wastewater sludge can be a sink of microplastics as a large portion of microplastics in wastewater can be separated during skimming and sedimentation. The abundance of microplastics in each type of sludge varies over multiple orders of magnitude (Fig. 1). The lack of consistent sampling across the entire solid treatment train poses challenges in developing an understanding of how different sludge treatment processes affect microplastics and quantifying the transformation of microplastics. Based on the intra-facility variation of microplastics in solids treatment stream, the level of microplastics does not significantly change from sludge generation, thickening to stabilization, but reduces after dewatering, showcasing potential accumulation of microplastics in sidestreams. Innovative biosolids treatment technologies, such as pyrolysis and hydrothermal liquefaction, provide opportunities to remove microplastics along with other persistent contaminants of emerging concerns. On average, 14,900 pieces of microplastics can be deposited into soil per dry kilogram of biosolids applied, revealing biosolids as a pathway for microplastics to enter the terrestrial environment. Furthermore, maximum sludge values are significantly higher than the average at 169,000,000 particles per kg. Sources in literature indicate microplastics increasing '280 items/kg and 430 items/kg' with sludge application (Yang et al., 2021). Soil concentrations are observed to range from single digits per kg to approximately 40,000 pieces per kilogram. The average concentration of microplastics in soil is approximately within 100 particles per kg to 1,000 particles per kg for a rough order of magnitude comparison. Approximately half of the microplastics found in sludge and biosolids are in the shape of fibers, which indicates laundry discharge is an important source of microplastics in wastewater and biosolids. Microplastics of Different Characteristics: Polymer Types and Morphology A total number of 31 types of polymers were reported in literature. Polyester, polypropylene, polystyrene, polyethylene, polyamide, polyethylene teraphatalate are the most frequently reported and abundant types of polymers. Polyester and polyamide are commonly used for making synthetic fabric (Sun et al., 2019). Thus, they often appear as microplastics fibers in wastewater and biosolids and their prevalence is in line with the abundance of fibers in various types of sludge. The polymer types abundant in biosolids align with the ones in wastewater reported in previous studies (Bui et al., 2020; El Hayany et al., 2022), showing that laundry discharge and storm runoff are major sources of microplastics in wastewater as well as biosolids. Source control to reduce microplastics load in raw sewage could be an effective measure for microplastics reduction in solids stream. Additionally, the similar abundance of polymer types in liquid and solids stream processes also reveals that the accumulation of microplastics in solids stream is not polymer type specific. Methodologies for Microplastics Quantification in Biosolids Sampling and analytical methods used in literature are summarized to inform future analysis of microplastics in sludge and biosolids. The sampling of low solids content samples, e.g. secondary sludge, digested sludge, can utilize automated sampling system and pumps, whereas high solids content samples are manually taken using containers or shovels. The sample processing procedure typically follows: density separation, chemical digestion, and vacuum filtration (Fig. 2). The quantification of microplastics in literature is count-based and relies on vibrational spectroscopy. Mass-based quantification should also be adopted for future studies due to the potential fragmentation of microplastics. Conclusion The literature review conducted by the Project Team showcases the state of knowledge on microplastics in solids treatment stream. It informs utilities of the abundance, fate of microplastics in different process units and in relation to facility characteristics and location, providing information that utilities can use to assess potential risks. It also provides insights into the origins of microplastics in biosolids, which help utilities identify potential entry points into the water treatment and wastewater systems. Currently there is no standard method for microplastics analysis in sludge and biosolids. The summary of reported methods in literature will provide the utilities valuable references and guidance on producing reliable, reproducible data.