Occurrence and Fate of Microplastics in Anaerobic Digestion of Dewatered Sludge

Microplastics (MPs) transferred to sludge during water treatment processes, particularly wastewater treatment, enter anaerobic digesters through sludge treatment prior to its final disposal or reuse. MPs retained in digested sludge confirm the presence of MPs during anaerobic sludge digestion. The abundance of MPs in anaerobic digesters varies considerably from 0.02 MPs/g DW to 169,000 MPs/g DW of sludge. MPs’ variability in digested sludge is partly attributed to the influent quality and treatment capacity of a wastewater treatment plant. Fibrous MPs are the most common MPs detected. The MPs in digested sludge usually have sizes less than 1 mm. Common MPs retrieved are those of acrylic, polyamide, polyethylene, polyester, and polyethylene terephthalate. In anaerobic digesters, MPs could interact with organic matter causing increased solubilization, which leads to higher formation of volatile fatty acids. Contrarily, they could impede the digestion of organic matter. They could interact with emerging pollutants and reduce their negative impacts on anaerobic digestion through adsorption on MPs. MPs could change the microbial profiles of anaerobic sludge digesters, favoring some microbes while inhibiting others. Polyamide monomers were found to promote the growth of certain microbes, causing increased biogas production. Inhibitory effects are often due to the leaching of chemicals, particularly bisphenol A, from MPs. MPs undergo morphological and chemical changes in anaerobic digesters. They have thinner surfaces at certain sites and cleavages after digestion. Their abundance reduces after digestion, implying potential degradation or biodegradation. This makes anaerobic sludge digestion a prospective avenue for MP removal through bioaugmentation and sludge pretreatment.