Turning excess sludge into a resource: Driving simultaneous nitrogen removal and sludge reduction under low dissolved oxygen

The simultaneous partial nitrification, anammox, denitrification, and fermentation (SNADF) process is highly desirable for sustainable wastewater treatment. However, simultaneously maintaining efficient sludge reduction and nitrogen removal in SNADF systems under low dissolved oxygen (DO) conditions remains challenging. This study investigated the feasibility of using excess sludge as a functional resource to enhance SNADF performance under a low DO concentration of 0.2 mg/L. The reactor achieved efficient nitrogen removal, with an average influent TN concentration of 61.3 ± 4.0 mg N/L and a total nitrogen removal efficiency of 87.9 ± 7.8%, while exhibiting substantial sludge reduction (Y obs = −0.483 kg MLSS/kg COD). Typical cycle analyses and an in-situ test suggested that excess sludge simultaneously enhanced fermentation and partial denitrification/anammox (PD/A). Microbial and metagenomic analyses revealed enrichment of anaerobic ammonia oxidizing bacteria, ammonia oxidizing bacteria, fermentative bacteria, and key functional genes involved in nitrogen conversion and fermentation. Excess sludge not only served as a treatment target but also functioned as a biomass resource by providing microbial biomass and biodegradable organics, thereby potentially reinforcing the interaction between fermentation and PD/A. Fermentation potentially accumulated readily biodegradable organics by remaining at the acidogenesis stage, while PD/A continuously consumed fermentation products and further stimulated fermentation activity. Consequently, efficient nitrogen removal and sludge reduction were simultaneously achieved in a single reactor. This study provides a promising strategy for integrated wastewater treatment and excess sludge management with improved energy efficiency and resource utilization.