Synergistic bioaugmentation: Serratia sp. WKD and carboxymethyl cellulose-modified nano-zero-valent iron enhance malachite green decolorization by activated sludge

The activated sludge process exhibits limited efficacy in decolorizing triphenylmethane (TPM) dyes like malachite green (MG) in textile wastewater. This study demonstrates that bioaugmentation with Serratia sp. strain WKD significantly enhances MG removal in activated sludge systems. Within 2 h, the combined system (activated sludge + strain WKD) achieved 98.5 ± 0.1 % decolorization efficiency—a 5.3-fold increase over sludge-only controls (18.5 ± 1.7 %). High-throughput sequencing revealed strain WKD's ecological dominance, reducing microbial alpha diversity (8 h) through competitive exclusion. When coupled with carboxymethyl cellulose-modified nano-zero-valent iron (CMC-nZVI), the hybrid system synergistically enriched stress-tolerant degraders ( Serratia sp. and Pseudomonas sp.), thereby accelerating decolorization. Prolonged incubation (24 h) enabled microbial diversity recovery (Shannon and Simpson indices) as detoxification alleviated environmental stress. Functional profiling identified pathway-specific modulation: critical metabolic functions (e.g., carbohydrate/amino acid metabolism) were transiently suppressed, while membrane transport activity remained intact, preserving system functionality. These findings establish strain WKD and CMC-nZVI as a synergistic bioaugmentation strategy, enabling rapid dye removal while maintaining microbial community resilience through dynamic stress-response mechanisms.