Chain-length-associated response patterns of chlorinated paraffins on activated sludge systems driven by microbial...

Chlorinated paraffins (CPs) are emerging contaminants detected in wastewater treatment plants, yet their impacts on activated sludge systems remain poorly understood. In this study, parallel sequencing batch reactors were employed to comprehensively evaluate the effects of short-chain (SCCP), medium-chain (MCCP), and long-chain (LCCP) CPs on pollutant removal performance, sludge properties, and microbial ecological responses. Under the tested nominal loading, the C₂₄-LCCP standard led to a clear reduction in nitrogen removal efficiency, whereas MCCP and SCCP maintained stable or even enhanced phosphorus removal performance. CP exposure generally increased oxidative stress and cytotoxicity, while SCCP and MCCP further stimulated extracellular polymeric substances secretion, consistent with an enhanced floc/cell-interface protective phenotype. Metagenomic analysis revealed that SCCP and MCCP enriched genera (Acinetobacter, Dechloromonas, Zoogloea) associated with phosphorus removal and increased the abundance of key nitrogen transformation genes, whereas the C₂₄-LCCP standard exhibited comparatively weaker shifts in functional gene profiles. Metatranscriptomic profiling indicated treatment-associated differences in transcriptional responses under the tested nominal loading, with SCCP showing the largest DEG set (>30,000 genes) in this dataset. Integrated metagenomic and metatranscriptomic analyses revealed a coordinated stress‑response program under SCCP, characterized by activation of efflux pumps, DNA repair, redox regulation, environmental stress responses, and biofilm-associated functions, together with elevated energy metabolism and ABC transporter signals. These molecular and community-level patterns aligned with the observed variations in treatment performance and sludge properties, providing convergent evidence for a chain-length-associated response framework. These findings provide comparative molecular and phenotypic evidence that may inform future risk assessment and hypothesis-driven mitigation studies on CP impacts in biological wastewater treatment systems.