Aged microplastic fibers induce activated sludge bulking: A new non-filamentous bulking behavior in sequencing batch reactors

Microplastic fibers (MPFs), due to their fibrous morphology, can escape primary interception and accumulate in activated sludge systems, where they may trigger non-filamentous sludge bulking-a previously overlooked risk. This study comparatively evaluated the effects of pristine and aged MPFs on sequencing batch reactor performance. While pristine MPFs at both 1 and 10 mg L⁻¹ had negligible impact, aged MPFs at 10 mg L⁻¹ caused severe sludge bulking (sludge volume index (SVI)= 280 mL g⁻¹) and up to 40 % loss of contaminant removal. At 1 mg L⁻¹, aged MPFs led to an SVI of ∼110 mL g⁻¹ without inducing bulking, yet indicating potential long-term risks. Accelerated UV-C aging increased fiber roughness, hydrophilicity (contact angle 113° → 82°), and biofilm adhesion (3-6 × higher), extending retention about six-fold. Retained aged MPFs bridged adjacent flocs and selectively enriched Nakamurella (∼85 %), a polysaccharide (PS)-secreting, non-filamentous genus. Leaching tests showed no toxicant release, confirming that deterioration resulted from physical-biochemical interactions, which promoted excessive formation of PS-rich loosely bound extracellular polymeric substances (LB-EPS). Two-dimensional correlation spectroscopy revealed sequential functional transformations (OH→ C = O→ CH→CO/CO-C) within LB-EPS, forming a viscous matrix that impaired floc cohesion and reduced oxygen transfer by over 50 %. These findings provide the first evidence that aged MPFs trigger severe non-filamentous sludge bulking through a mechanism distinct from filamentous bulking and underscore the need for enhancing MPFs interception strategies and further investigation of fibrous pollutants to maintain wastewater treatment efficiency and protect aquatic environments.