Spatial Stratification of Cathodic Biofilm on Nitrogen Removal in the Microbial Fuel Cell Under Different Carbon/Nitrogen Ratios

Abstract

The aim of this study was to investigate nitrogen removal in the microbial fuel cell (MFC) related to spatial stratification of bacterial community in cathodic biofilm under different carbon/nitrogen ratios.  Single-chamber air-cathode MFC was tested with three initial ammonia concentrations and 1 g/L acetate as substrates, resulting in different carbon/nitrogen ratios, which were characterized with chemical oxygen demand/nitrogen (COD/N) of 40, 16, and 10.  With the freezing microtome method, matured cathodic biofilms after 13 operation cycles under the COD/N ratios were sliced into 5 layers (layer 1 close to anolyte and layer 5 close to cathode) with a thickness of 20 ~ 30 μm each layer.  Results showed that high removals of NH4+-N and total nitrogen were achieved in the MFC with stratification of bacterial communities in the cathodic biofilms.  Relative abundance of Paracoccus was > 30.0% in the layers under COD/N of 10, but < 1.5% under COD/N of 40.  The expressive abundance of ammonia monooxygenase under COD/N of 10 was much higher than that under COD/N of 40, indicating different nitrification.  Ammonia oxidizing bacteria (AOB) preferred to grow in layer 5 (i.e., close to the cathode), while anaerobic ammonia oxidizing bacteria (AnAOB) was more abundant in layer 1 (i.e., close to the anolyte).  High COD/N ratio increased the abundance of AnAOB but decreased the abundance of AOB, resulting in low NH4+-N removal rate.

Keywords: microbial fuel cell, Nitrogen removal, stratification, cathodic biofilm, freezing microtome method