Sludge Retention Time Governs Ectoine Synthesis and Pollutant Removal in Halophilic Activated Sludge Treating High-Salinity Wastewater

In the treatment of high-salinity wastewater, the removal of nitrogen and organic pollutants remains a challenge, while the production of value-added compounds, such as ectoine from halophilic bacteria, offers a promising resource recovery pathway. In this study, halophilic activated sludge enriched with Thauera as the dominant strain was cultivated in a sequencing batch reactor (SBR) to treat synthetic high-salinity wastewater (30 g/L NaCl) under different sludge retention times (SRTs). The optimal nitrogen and organic carbon removal performances were achieved at an SRT of 10 days, with an ammonia nitrogen removal rate of 77.67% and a total organic carbon (TOC) removal rate of 72.51%. Ectoine production was strongly SRT dependent, as volumetric ectoine concentration was ~2 mg/L at 5 d SRT, almost undetectable at 10 d SRT, ~10 mg/L at 16 d SRT, and peaked at 21.5 mg/L at 22 d SRT. Short SRTs favored dynamic ectoine utilization for osmoprotection and metabolic stability, whereas long SRTs led to passive ectoine accumulation and deteriorated treatment performance. The system realized stable short-cut heterotrophic nitrification with negligible nitrite and nitrate accumulation, indicating direct conversion of ammonia to gaseous nitrogen. These results demonstrate that SRT regulation effectively balances ectoine synthesis and pollutant removal, providing a feasible strategy for resource-oriented treatment of high salinity wastewater.