The effect of aeration on treatment efficiency and bioenergy generation of septic-tank effluent in constructed wetland-microbial fuel cell

This study examines the effect of different rates and modes of supplementary aeration on the treatment of synthetic septic tank effluent in a lab-scale unplanted constructed wetland-microbial fuel cell (CW-MFC) along with bioenergy generation. The experiment was conducted in 4 sequential phases; 1) the cell was operated with no aeration (NA), 2&3) the cathode layer was continuously aerated under different aeration rates (CA-I and CAII), and 4) the cathode layer was intermittently aerated (IA). Results showed that the dissolved oxygen gradient caused by IA formed aerobic and anoxic zones in CW-MFC and promoted total nitrogen removal. The best performance was observed with intermittent aeration where the maximum removal efficiencies of COD (96.8 +/- 3.3 %), TN (62.3 +/- 2.1 %), TKN (68.3 +/- 2.0 %), and NH3 (64.8 +/- 2.3 %) were achieved. Furthermore, this treatment efficiency corresponded with enhanced bioenergy generation under the IA regime, achieving maximum power output (7.5 mW/m2), highest average voltage output (0.399 +/- 0.02 V), Coulombic efficiency (1.94 %), and normalized energy recovery (1.4 Wh/m3). Continuous aeration, however, induced an adverse effect on energy generation due to excessive oxygen penetration into the anode region. Nitrate accumulation was also observed as a sign of an incomplete denitrification process under the continuous aeration regime. Overall, it can be concluded that the intermittently aerated CW-MFC not only lowers operational costs but improves effectiveness in terms of the nitrogen removal process in the treatment of septic tank effluent.