Nano-Zerovalent Iron (nZVI) Shifts Microbial Nitrate Respiration from Denitrification to Dissimilatory Nitrate Reduction to Ammonium (DNRA) for the Treatment of Low-C/N Wastewater

Nano-zerovalent iron (nZVI) as an alternative electron donor could drive nitrate reduction for nitrogen removal or recovery from wastewater. However, whether and how nZVI governs the two competing microbial nitrate-reducing processes, namely, denitrification and dissimilatory nitrate reduction to ammonium (DNRA), remains unknown in activated sludge systems. Here, through batch experiments using denitrifying sludge under varied C/N ratios and nZVI doses, a maximum nitrate-to-ammonium efficiency of 97.0% with a nitrate-reducing rate of 15.2 mg N/L/h was achieved at a C/N ratio of 2 and nZVI dose of 1000 mg/L. While nZVI-driven DNRA dominated microbial nitrate reduction over a wide range of C/N ratios (1–10), high nZVI doses caused cellular damage. Metagenome and transcriptome analyses indicated the proliferation of DNRA bacteria (e.g., Desulfobulbus, Geobacter, Nitrospira) in the presence of nZVI and the predominance of DNRA over denitrification with upregulated nrfA/H and downregulated nirS, norB, and nosZ genes. Enhanced nitrate-to-ammonium interaction might potentially benefit from pili-assembling and iron-respiring bacteria that acquire electrons from Fe(0) via extracellular electron transfer. This work proved that nZVI could regulate microbial nitrate reduction by modulating the activated sludge communities and, therefore, provided a feasible route to recover ammonium from low-C/N wastewater.