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

Abstract

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.