Fe0-mediated mixotrophic denitrification enhanced by carbon fiber for secondary effluent with low C/N ratio

Abstract

Compared with Fe0-mediated autotrophic denitrification (ADN), mixotrophic denitrification (MDN) can alleviate the problem of reduced denitrification efficiency due to Fe0 passivation. In this study, carbon fiber (CF) was added as an additional cathode material to Fe0-mediated MDN to promote Fe0 corrosion and advanced denitrification of secondary effluent with low chemical oxygen demand (COD) to nitrogen (C/N) ratio. The results showed that the average removal efficiencies of total nitrogen (TN) and nitrate nitrogen (NO3--N) in the experimental group with CF coupling Fe0 (CF/Fe0) were achieved as high as 80.08 % and 79.79 % in Phase VI (C/N ratio = 3, hydraulic retention time (HRT) = 48 h), respectively. Mechanistic investigation showed that part of Fe0 corroded during the reaction process and the reaction products (Fe3+, Fe2+) were adsorbed on CF surface, sustaining the reaction of NO3--N conversion and promoting denitrification performance. Furthermore, the iron ions attached to the CF surface will further stimulate the generation of extracellular polymeric substance (EPS) containing redox-active components such as flavins and Cytochrome c (Cyt c). Consequently, the activity of the electron transport system was promoted. CF dosing resulted in a high enrichment of dominant denitrifying bacteria such as Acinetobacter, Comamonas and Pseudomonas in MDN, and expression of narG, napA, nosZ and other microbial metabolic function genes. Based on the above, the efficiency and mechanism of CF enhanced Fe0-mediated MDN was elucidated, which is favorable for enhancing the nitrogen removal in secondary effluent containing NO3--N with low C/N ratio based on Fe0-mediated MDN.