Potential impacts of microplastic-derived dissolved organic matter in water and wastewater treatment systems: A critical review and future research needs

Microplastics (MPs) are widely recognized as particulate pollutants in water and wastewater treatment systems (WWTSs); however, their transformation into microplastic-derived dissolved organic matter (MP-DOM) and its potential impacts remain poorly understood. This review synthesizes current knowledge on the generation, composition, and treatment relevance of MP-DOM, an emerging fraction of dissolved organic matter (DOM) formed during photochemical, oxidative, mechanical, thermal, and biological aging of MPs within treatment processes. Available evidence suggests that WWTSs can act as active hotspots for MP-DOM formation, producing low-molecular-weight oligomers, oxidized fragments, and additive-derived compounds that persist beyond particulate removal. Compared with background DOM, MP-DOM exhibits distinct reactivity, bioavailability, and transformation pathways. We evaluate how these physicochemical properties may influence coagulation, adsorption, oxidation, membrane filtration, biological treatment, and sludge processing. Emerging studies indicate that MP-DOM can alter flocculation, compete for adsorptive and oxidative capacity, contribute to disinfection by-product formation, modulate microbial activity, and exacerbate membrane fouling. However, the magnitude and operational significance of these effects in full-scale systems remain poorly constrained due to limited field observations and reliance on simplified laboratory studies. We identify key knowledge gaps related to MP-DOM occurrence, polymer-specific generation pathways, microbial interactions, and differentiation from background DOM. Collectively, MPs should be considered not only as particulate contaminants but also as potential sources of reactive dissolved organic fractions that may influence treatment performance under increasing plastic inputs. Accordingly, monitoring strategies and process models may need to incorporate MP-DOM as a distinct compositional component of the DOM pool.