Recycling Thermal-Alkaline Waste Activated Sludge Lysate for Coupled Sulfate Reduction and Nitrate Transformation: Performance, Ammonium Accumulation, and Microbial Insights

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Waste activated sludge (WAS) is a carbon-rich residual biomass that can be converted into a recycled carbon source for biological wastewater treatment; however, the applicability of low-temperature thermal-alkaline WAS lysate to coupled sulfate reduction and nitrate transformation remains unclear. This study optimized low-temperature thermal-alkaline pretreatment and evaluated the resulting lysate as the sole external carbon source in a continuous anaerobic reactor treating simulated sulfate- and nitrate-containing wastewater. The preferred pretreatment condition was pH 11, 90°C, 60 g total solids/L, and 120 min. Across the orthogonal experiments, acetate and propionate accounted for >75% of total volatile fatty acids (VFAs), while verification of the selected condition yielded ∼12,340 mg/L soluble chemical oxygen demand and 594 mg/L total VFAs. During continuous operation, the lysate established reducing conditions and supported sulfate and nitrate transformation. Under Stage II, when both the influent chemical oxygen demand/SO 4 2− ratio and temperature were increased, sulfate conversion and apparent nitrate conversion reached 81.9% and 71.5%, respectively, although their individual effects could not be resolved. Substantial net NH 4 + -N accumulation showed that nitrate disappearance did not represent complete nitrogen removal. 16S ribosomal RNA gene sequencing supported the proposed microbial associations, while ammonium accumulation remained a key concern for lysate reuse.