Non-sewered sanitation systems reshape microplastics into chemically complex and reactive contaminants

Non-sewered sanitation systems (NSSSs) are gaining global attention as decentralized solutions, yet the transformation of microplastics (MPs) within these systems remains poorly understood. Here, we investigate MP characteristics in the influent and effluent of two NSSSs using laser direct infrared spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Both systems substantially reduce MP abundance (retention: 53.7–77.1%) but also alter MP characteristics. MPs exhibit fragmentation, reduced particle size, increased morphological irregularity, and surface oxidation, evidenced by decreased spectral matching scores and emerging oxygen-containing functional groups. FT-ICR MS reveals molecular-level oxidation enrichment, and over 260 candidate plastic additives are identified. LC-MS/MS confirms elevated phthalate ester concentrations, including DBP and DEHP. Statistical analyses link MP fragmentation, oxidation, and additive release as coupled processes. These findings suggest that NSSSs reshape MPs into more chemically reactive forms, highlighting the need to evaluate transformation-associated risks beyond retention efficiency. Microplastics are known to be present within non-sewered sanitation systems, but how treatment processes alter their physicochemical properties is not well understood. Here, the authors use a range of techniques to study the fate and transformation of microplastics with treatment.