A pilot study of Mn redox driven nitrogen removal in municipal tailwater: Nitrogen metabolic pathway and electron transport mechanism

Abstract

Microbial-mediated manganese (Mn) cycle coupled with nitrogen removal in municipal tailwater has received increasing attention. Herein, this study successfully extended the application of this technique to an 800 L biofilter, achieving a daily capacity of 2.4 m3 for treating municipal tailwater with a low C/N ratio. The operation results over 180 days indicated that the MnOx biofilter performed more efficiently than gravel control biofilter, achieving ∼ 40.2 % for nitrate and ∼ 35.9 % for total nitrogen (TN) removal efficiencies, respectively, under the conditions of hydraulic retention time (HRT) of 4 ∼ 5 h and low temperature (6.3–10.5 °C). To better elucidate the potential mechanisms, various functional bacteria involved in nitrogen and Mn transformation were identified, including heterotrophic denitrifying bacteria, such as Denitratisoma, Thauera, and Anaerolineae (cumulative relative abundance: 16.94–23.14 % in MnOx system vs. 9.86–11.90 % in control system) and Mn(II) oxidizing denitrifying bacteria, such as Pseudomonas and Acinetobacter (12.90–18.13 % vs. 3.59–4.64 %). Furthermore, metagenomic analysis indicated that the Mn cycle enhanced the expression of genes responsible for electron generation in organics/Mn metabolic pathways (e.g. TCA cycle and Mn oxidation), promoted electron transfer by enriching the Mn(II)-oxidizing denitrifying bacteria, and improved the abundance of genes associated with electron utilization (e.g. nitrate and nitrite reduction). The enhancement of the electron generation-transport-utilization due to Mn circulation contributed to efficient nitrate removal. This study provides further guidance of large-scale application and expands the understanding of the nitrogen removal mechanism in the Mn cycle-mediated process in municipal tailwater.