Carbon source and temperature effects on nitrogen removal performance in a novel adaptive activated sludge process

Low temperatures and carbon limitation pose significant challenges to the performance of wastewater treatment processes, particularly in rural regions with limited economic and technical resources. In this study, the effects of three external carbon sources-glucose, acetate, and a glucose-acetate mixture-on nitrification and denitrification performance were systematically investigated in a novel adaptive activated sludge (AAS) system operated under ambient (24 °C) and low (12 °C) temperature conditions. At 24 °C, the ammonia-nitrogen removal rates were 1.57 mg/L·h, 4.51 mg/L·h, and 5.47 mg/L·h for glucose, acetate-glucose mixture, and acetate, respectively. The corresponding aerobic denitrification rates were 1.53, 3.72, and 4.50 mg/L·h. The results show that acetate as a sole carbon source achieved the highest nitrogen removal rate, with a total nitrogen (TN) removal efficiency of 65.52 %. The glucose-acetate mixture demonstrated the lowest carbon consumption (14.33 g COD/g TN). At 12 °C, the mixed carbon source system maintained a reduced carbon consumption of 12.56 g COD/g TN. Microbial community analysis revealed that glucose promoted the dominance of Nakamurella, Micropruina, and Zoogloea, whereas the mixed system enriched Flavobacterium, which is associated with heterotrophic nitrification. Gene expression analysis indicated that acetate metabolism via the glyoxylate cycle and tricarboxylic acid (TCA) cycle enhanced denitrification efficiency. Additionally, polyhydroxyalkanoate (PHA) synthesis pathways were upregulated at the lower temperature (12 °C), suggesting microbial adaptation to carbon scarcity via endogenous carbon storage, contributing to enhanced denitrification under cold conditions. These findings underscore the energy-saving benefits of strategic mixed carbon source selection in AAS systems for cold environments, which significantly reduces operational energy demand while maintaining treatment efficiency.