Microplastics in Biosolids Treatment Stream: Abundance, Fate, and Methodologies

Introduction and Objectives Water Resource Recovery Facilities (WRRFs) passively collect various contaminants which has led to concern about potential environmental impacts downstream. Of particular interest, microplastic (MP) contamination has remained a concern due to various sources of MPs into wastewater, including municipal waste, roadway runoff, and industrial discharge. Conventional wastewater treatment processes have been effective in separating MPs driven by physical separation technologies, consequently accumulating MPs in the solids stream and biosolids [1,2]. Land application of biosolids presents avenues for MPs in biosolids to enter and accumulate in landfills, agricultural soil, plants, soil biota, and further transport into the environment. Understanding MPs' occurrence, fate, and transformation in biosolids treatment is crucial to promote the beneficial and sustainable use of biosolids. Information on MPs across the solids stream is yet to be synthesized to assist stakeholders in understanding risks associated with MPs, and there is still a lack of consensus regarding reproducible analytical methods to quantify MPs by mass and count. To this end, a comprehensive critical review, capturing data and knowledge from 51 studies including peer-reviewed journals articles, reports, and conference proceedings, was conducted to 1) inform the state of knowledge of MPs in solids stream, including their occurrence, fate and characteristics; 2) identify knowledge gaps of MPs in solid unit processes (e.g. digesters); 3) inform the development standard operation procedures for empirical analysis. This study is part of the Water Research Foundation project #5221. Findings

*Abundance and Fate of Microplastics in Solids Stream The literature review found that the abundance of MPs in different solids process units varies by several orders of magnitude (Fig. 1). Moreover, the number of facilities studied, and concentrations reported vary dramatically across the world. The lack of consistent and representative sampling methods across the solid stream poses challenges in developing an understanding of how different sludge treatment processes affect MPs and quantifying the transformation of MPs. Based on a literature review of intra-facility variation of MPs in solids treatment stream (post-primary treatment), the quantity of MPs does not significantly change from sludge generation, thickening to stabilization, but decrease after dewatering, which indicates potential accumulation of MPs in side streams (Fig.2). These side streams often return to the headworks of WRRFs; therefore, these streams may represent an additional 'catch and release' point. A total of 31 types of polymers were reported in literature. Polyester, polypropylene, polystyrene, polyethylene, polyamide, polyethylene terephthalate are the most frequently reported and abundant polymers. Polyester and polyamide are commonly used for synthetic fabric and often appear as MP fibers in both wastewater and sludge.

*Accumulation of Microplastics in Biosolids Amongst 51 studies reviewed, a median of 14,900 pieces of MPs (Std. Dev = 476,498) were found to be deposited into soil per dry kilogram of biosolids applied. Maximum sludge value is significantly higher at 169,000,000 particles per kg. In comparison, MPs in soil are observed to range from single digits per kg to approximately 40,000 particles per kilogram. With land application of biosolids, MPs content in soil can increase hundreds of pieces per kg according to literature [4].

*Methodologies for Microplastics Quantification in Biosolids Sampling, sample processing, and analytical methods from literature reviewed is summarized in Table 1. The methods of the studies reviewed utilize automated sampling system and/or pumps for collecting samples with low solids content, e.g. secondary sludge, digested sludge, whereas high solids content samples are manually taken using buckets or shovels. The studies' sample processing procedures typically follow: density separation, chemical digestion, and vacuum filtration. The quantification of MPs in literature is count-based and relies on vibrational spectroscopy, which is faced with technical challenges including laborious sample preparation and analysis, spectral identification errors, higher size limit, miscounts due to fragmentation, and reduced number of samples due to the high cost associated with these types of analysis methods. To address these challenges in the later empirical analysis of WRF 5221, a dual quantification approach will be adopted (Fig. 3). Pyrolysis-gas chromatography /mass spectroscopy will quantify MPs in biosolids by mass, which is advantageous due to highly accurate quantification of polymers without time-intensive sample preparation or analysis, whilst Raman spectroscopy will provide the MPs characteristics for comparison with literature, including polymer type, size distribution, and morphology. Conclusion This literature review showcases the state of knowledge on MPs in solids treatment stream. It informs utilities of the abundance and fate of MPs in different process units and in relation to facility characteristics and location, providing information that utilities can use to assess potential risks. The project also addresses shortcomings associated with commonly used analytical methods by measuring repeatability as a core element of the study design which will be useful for future studies.