Microbial fuel cells running on high strength organic nitrogen wastewater - Ammonium reduction and recovery strategies

Abstract

As an alternative to conventional nitrogen removal processes, microbial fuel cell (MFC) systems offer new advantages. To date, several nitrogen-based reactions have been reported in MFCs, both at the anode and at the cathode. Although electricity generation using pure substrates and wastewater has been widely studied, the effect of MFC treatment on ammonia has received relatively little attention. Nitrogen removal from wastewater is an important component of treatment, particularly for high strength wastewater. Nitrogen removal has been difficult to achieve in single-process systems, and multiple reactors are usually needed. Two-stage and modified oxic/anoxic processes using sequencing batch reactors can be used for the treatment of ammonia organic wastewater, but these processes have high operational costs.

In this study, MFC scale-up experiments in fed-batch modes using 100 and 250 mg L-1 ethanolamine (organic nitrogen wastewater) were effective methods to determine the principal factor of the volatilization process. Increasing the ethanolamine concentration was proportional to increasing the ion concentration, and it eventually enhanced the removal efficiency of ammonia. In addition, the low levels of maximum power density were caused by an intrinsic characteristic of the ethanolamine and the increased electrode size in the scaled-up MFC. Thereafter, the OLR conditions were responsible for the significant removal efficiencies of COD and ammonia in continuous-mode experiments conducted with changing OLR conditions but stable HRT. This relationship negligibly influenced the voltage output and maximum power density. The results from the scale-up and continuous mode experiments will be applicable when MFC systems are introduced to wastewater treatment facilities.

While previous research was recently shown that MFCs could be used to generate power and treat ethanolamine wastewater, it was also observed that COD removal was accompanied by a high level of ammonia removal. However, in accordance with the results presented in previous research, there still remains the problem of the escape of ammonia gas through the air diffusion layer due to the high pH in the vicinity of the cathode.

This volatilization of ammonia into air might be a concern for some locations and therefore, further control of the off gas could be required for the MFC in the same way it is for air stripping systems. The use of these MFC-based technologies could potentially reduce the high operational costs currently needed for aeration-based treatment systems and could create conditions where ammonia removal is accomplished while simultaneously generating electricity.

We proposed a proof-of-concept of the nitrogen recovery process by modifying MFCs using a gas diffusion chamber, and the MFC technology showed great potential for selectively recovering ammonia from synthetic wastewater. Batch experiments showed that ammonium migration through the CEM was enhanced by voltage, and that maximum ammonia removal was achieved (51.2%). In addition, ammonium recovery was made feasible using an absorption column system (54.6%). A clear improvement in ammonium migration was shown when using NaCl under the MEC mode. The advantage of using sodium chloride as a catholyte is related to a sharp pH increase in the cathode and, accordingly, ammonium stripping/absorption are favored. Effective ammonium recovery can be achieved at a high N/C ratio in wastewater. Power generation of the MFC decreased with an increase in the N/C ratio, which was also evident from the cyclic voltammogram. The energy analysis showed that this technology can be a sustainable ammonium recovery technology.

Further improvements to the ammonium transport rates are necessary in order for it to become a competitive ammonium recovery technology. Final chapter demonstrates the use of scaled-up dual anode/cathode microbial fuel cell stacks to produce electricity from actual organic nitrogen wastewater, with simultaneous wastewater treatment. The batch test showed that the MFC unit is an advanced system for nitrogen treatment and electricity generation. Stack MFCs in series connection showed a gradual increase in COD and ammonium removal. Extrapolation of the results obtained from this study show the possibility of continuous power generation from actual nitrogen wastewater/effluents using a scaled-up dual anode/cathode MFC stack.