Innovative Quantification of Microplastics in Digestate from Ontario Biogas Plants: Implications for Environmental Monitoring and Management

Innovative Quantification of Microplastics in Digestate from Ontario Biogas Plants: Implications for Environmental Monitoring and Management Abstract Objectives: Microplastics (MPs) are persistent contaminants that can threaten soil health and terrestrial ecosystems. Digestate, a byproduct of anaerobic digestion (AD) of organic substrates, may potentially contribute to agricultural MP contamination due to its growing use in Ontario as a soil amendment for nutrient recovery. This study aimed to quantify MPs in digestate derived from digesters processing agricultural waste, source-separated organics (SSO), food waste, and/or mixtures thereof. We observed that MP levels in digestate varied with feedstock composition, particularly SSO, which was associated with higher contamination levels. These findings could be useful to inform standards development and management approaches to minimize MPs in land-applied digestate. Methods: Digestate samples were collected from nine anaerobic digestion facilities across Ontario, Canada by the Ontario Ministry of the Environment, Conservation, and Parks and Ontario Ministry of Agriculture, Food, and Agribusiness. These facilities represented diverse anaerobic digester systems based on feedstock sources and compositions (Table 1) [1]. Microplastics were isolated from fresh or frozen digestate samples using a matrix-specific, optimized multi-step purification protocol. The basic steps included ultrasonic treatment (1 hour) to disaggregate particles, thermal concentration at 105°C to remove excess moisture, enzymatic digestion (24 hours) to degrade organic material, wet peroxidation (75°C) to oxidize residual matter, 20 µm sieve separation to retain MP particles, and ethylenediaminetetraacetic acid (EDTA) treatment to dissolve inorganic aggregates [2]. Recovered MPs were analyzed using microscopy and Fourier transform infrared (FTIR) spectroscopy to classify size, shape, and polymer composition. MP concentrations were quantified using the Most Probable Number (MPN) method [3]. Principal component analysis (PCA) was used to evaluate patterns in MP distributions based on feedstock composition across sampling sites. Main Findings: MPN Method Validation This study validated and utilized the novel MPN method approach [3], traditionally used for microbial quantification, to estimate MP concentrations in digestate. Optimal conditions were established, including three dilution levels, 15 replicates, and a failure probability range of 0.7—0.95. The numbers of MP of different types determined by visual counting were well matched with the MPN values and contained within the 95% confidence (Table 2), demonstrating the method's accuracy and reliability. MP Isolation and Characterization Microplastics were effectively isolated by the combined enzymatic digestion, wet peroxidation, and EDTA treatments. MPs were classified by shape (fragments, films, flat fibers, and round fibers) and color (clear, black, blue, green, orange, red, purple, yellow, and multicolored) (Table 3; Figure 1). MP isolated from agri-food digestates contained a high fraction of MP films (44.2% to 76.3%) and fragments (8.4% to 52.5%), whereas those from SSO-digesters were predominantly fragments (57.7% to 83.1%). PCA revealed that MPs from agri-food digesters (A, B2, C, D) were associated with higher contents of films and flat fibers, while MPs from SSO digesters (E, F1, F2, G1, G2, G3, H) were linked to higher fragment content (Figure 2). FTIR analysis identified diverse polymers, including polyethylene, polyester, polyvinyl alcohol, polystyrene (Figure 1). MP Quantification MP concentrations ranged from 6,200 to 26,000 MP/g-dry solids, with 95% confidence intervals of 5,000—11,000 MP/g-dry solids for agri-food digestate and 11,000—37,000 MP/g-dry solids for SSO digestate (Table 4). Digestate from SSO or mixed feedstocks contained significantly higher MP levels compared to digestate derived from organics such as manure, food processing residues, fats, oils, grease, and agricultural byproducts (e.g. field crop and plant residues). This highlights feedstock composition as a key factor influencing contamination levels. Significance: This study developed an enhanced method for quantifying MPs in digestate, combining the MPN approach with enzymatic digestion and EDTA treatments to improve efficiency and accuracy in isolating MPs from complex matrices. Findings of this study demonstrated that digestate derived from SSO feedstocks contributed most significantly to higher MP contamination, emphasizing the importance of effective source separation to prevent contamination of organic waste streams with plastic wastes. These results may be useful to inform standards development or management actions to reduce MP contamination risks, safeguarding soil health and terrestrial ecosystems. Building upon this research, future research could explore long-term MP accumulation in soils and its impacts, assess the effectiveness of pre-treatment technologies to minimize MP input, and investigate alternative waste processing methods to further reduce contamination risks. This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois. Presentation time 09:30:00 09:45:00 Session time 08:30:00 10:00:00 Session Microplastics In Wastewater Treatment: Insights from Recent Research Session location McCormick Place, Chicago, Illinois, USA Topic Contaminants of Emerging Concern & Trace Organics Topic Contaminants of Emerging Concern & Trace Organics Author(s) Chang, Sheng, Smith, Megan, Liu, Yi, Wang, Bei, Eulett, Tivona, Flemming, Cecily, Wong, Justin, Pileggi, Vince, Crolla, Anna Author(s) S. Chang 1 , M. Smith 1 , Y. Liu 1 , B. Wang 1 , T. Eulett 1 , C. Flemming 2 , J. Wong 2 , V. Pileggi 2 , A. Crolla 3 , , , , Author affiliation(s) University of Guelph 1 , Ontario Ministry of the Environment, Conservation, and Parks 2 , Ontario Ministry of Agriculture, Food, and Agribusiness 3 Source Proceedings of the Water Environment Federation Document type Conference Paper Publisher Water Environment Federation Print publication date Sep 2025 DOI 10.2175/193864718825159950 Volume / Issue Content source WEFTEC Copyright 2025 Word count 17 Purchase price $11.50 Get access Log in Purchase content Purchase subscription You may already have access to this content if you have previously purchased this content or have a subscription. Log in Need to create an account? You can purchase access to this content but you might want to consider a subscription for a wide variety of items at a substantial discount! 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