Fe0-induced multifunctional effect of aerobic denitrification, autotrophic denitrification and chemically enhanced phosphorus removal in oxic/anoxic process for low C/N wastewater

For conventional anoxic/oxic (A/O) system, exogenous carbon sources supplement and higher nitrifying liquid reflux ratios were necessary to guarantee efficient total nitrogen (TN) removal for low C/N wastewaters, which increased operational costs and energy consumption. To remove total phosphorous (TP), another anaerobic stage was required (i.e., A2/O), but the competition for organics and the inhibition of anoxic phosphorous uptake from nitrite generated during denitrification limited TP removal. Thus, this study presented an Fe0-enhanced O/A process. The results showed that when Fe0 added in both stages, best TN and TP removal performance (58.3% and 96.5%) were achieved. When Fe0 added in single stage, the efficiencies decreased to 37.5% and 93.6% (added in oxic stage) or 41.9% and 90.1% (added in anoxic stage), respectively. When no Fe0 added, the efficiencies were reduced to 18.9% and 46.9%. In oxic stage, Fe0 accumulated aerobic denitrifiers (Zoogloea) and nitrogen-cycling genes (amoA, nxrB, napA and nirK), thereby initiating simultaneous nitrification and denitrification (SND). In anoxic stage, it facilitated the growth of autotrophic denitrifiers (Brevundimonas, Ferruginibacter, Thermomonas and Hyphomicrobium) and up-regulated denitrifying genes (napA, narG and nirS) that induced autotrophic denitrification (ADN). Fe0 also made major contribution to TP removal via chemical precipitation. Besides, a synergistic effect existed that Fe0 in oxic stage not only further accumulated Brevundimonas and denitrifying genes in anoxic stage, but also indirectly enhanced intracellular and extracellular electron transfer by improving metabolism activities, extracellular polymeric substances contents and the abundance of mtrC in anoxic stage, which facilitated Fe0 utilization resulting in elevating TN removal. In this study, the understanding of enhancement mechanisms of SND and ADN by Fe0 were deepened, providing the basis for low C/N wastewater treatments.