Robust and high-efficient nitrogen removal from real sewage and waste activated sludge (WAS) reduction in zero-external carbon PN/A combined with in-situ fermentation-denitrification process under decreased temperatures

Despite the advantages of the combined anammox and fermentation-driven denitrification process in nitrogen removal and energy consumption, stable performance at decreased temperatures remains a challenge. In this study, a robust and high-efficient nitrogen removal efficiency (95.0-93.1 -86.8-93.4%) with desirable effluent quality (3.0-4.1 -7.9-4.9 mg/L) under long-term decreased temperatures (30 degrees C -> 25 degrees C -> 20 degrees C) was achieved in a zero-external carbon Partial Nitritation/Anammox combined with in-situ sludge Fermentation-Denitrification process treating sewage. Excellent sludge reduction averaged at 14.9% assuming no microbial growth. Increased hzsB mRNA (2.2-fold) and reduced Ea (80.9 kJ/mol) proved resilient anammox to lower temperature. RT-qPCR tests revealed increased NarG/NirK (5.1) and NarG/NirS (4.9) mRNA at 20 degrees C, suggesting higher NO3  -> NO2  over NO2  -> N2 pathway. Metagenomics unraveled dominant anammox bacteria (Candida-tus_Brocadia, 2.27%), increased denitritation bacteria containing more NarG (Hyphomicrobium, 0.8%), fatty acid biosynthesis and CAZymes genes. Enhanced denitritation with recovered organics from sludge reserved nitrite for anammox and facilitated higher anammox contribution to N removal at 20 degrees C (42.4%) than 30 degrees C (39.5%). This study proposed an innovative low-temperature strategy for in-situ sludge fermentation, and demonstrated stability of advanced municipal wastewater treatment and sludge disposal through energy savings and carbon recovery under decreased temperatures.

Automated summary

This study achieved stable and efficient nitrogen removal through a zero-external carbon Partial Nitritation/Anammox combined with in-situ sludge Fermentation-Denitrification process under decreased temperatures. The findings show the resilience of anammox to lower temperatures and highlight the importance of sludge reduction and organic recovery for enhanced denitrification.