Low salinity enhances azo dyes degradation in aerobic granular sludge systems: Performance and mechanism analysis

The biodegradation performance of azo dyes can be enhanced under low salinity conditions, but the internal biodegradation mechanism is still unclear. Aerobic granular sludge (AGS), a salt-tolerant biological wastewater treatment technology, was used in this study to explore the enhancement mechanism of acid orange 7 (AO7) degradation at low salinity level (1.0 %). Results indicated that the AGS structure and reactor performance were almost unaffected by different AO7 concentrations (5-10 mg/L). Compared with salt-free conditions, the AO7 removal efficiency was elevated by 9.9 %-19.0 % at 1.0 % salinity level, owing to the enrichment of AO7 decolorizing bacteria (e.g. Acinetobacter) and functional enzymes (e.g. FMN-dependent azoreductase). The up-regulated genes involving in the key metabolic functions (e.g. carbon metabolism and oxidative phosphorylation) promoted the electron and energy production, thereby facilitating the AO7 decolorization and degradation. These results aid understanding of the enhancement mechanism of AO7 biodegradation under low salinity conditions from macroscopic and microscopic perspectives.