Wastewater Treatment Using Microbial Fuel Cells with Peroxide Production
Dr. César Torres | Arizona State University
The overarching goal of this study was to develop a novel energy-neutral wastewater treatment technology that could aid in water reuse within a forward operating base, thereby reducing water transport. To achieve this, a microbial fuel cell capable of generating hydrogen peroxide as primary product was developed. This technology took advantage of the high-energy content of blackwater; the microbial fuel cell consumed and converted it into an electrical current that was used to generate significant amounts of hydrogen peroxide. The hydrogen peroxide can have several uses: (1) direct treatment of graywater towards reuse, (2) tertiary treatment of graywater and/or blackwater, (3) odor control for blackwater, and (4) treatment of blackwater itself for better effluent quality and solids destruction.
More specifically, the objectives were to (1) show the feasibility of hydrogen peroxide production from blackwater, (2) achieve hydrogen peroxide production at high efficiency, and (3) demonstrate the effective treatment of blackwater at near energy-neutral conditions using a microbial fuel cell.
The approach was divided into 8 tasks aimed at achieving the described objectives. In Task 1, the kinetics of microbial hydrolysis of organic solids and anode respiration in blackwater was studied. This knowledge allowed for the defining of hydrolysis kinetic parameters, assessment of the efficiency of the microbial fuel cell treating blackwater, and development of mathematical models to predict treatment in Task 2. A non-steady-state mathematical model, MYAnode, was developed which integrated the chemical and biological processes in the bulk liquid of the microbial fuel cell with substrate utilization and current production in the anode biofilm.
The production of hydrogen peroxide at the cathode required an optimization of materials that were suitable for this oxidant. In Task 3, the research team selected the best carbon-based materials for hydrogen peroxide at the cathode, anode materials with high surface area for current generation and blackwater treatment, and different membranes or separators between anode and cathode for their suitability when hydrogen peroxide was produced. In Task 4, control strategies were developed that would be useful for process optimization. Specifically, the focus was on the development of anode or cathode potential control and the optimization of pH in the system. These strategies were implemented in microbial fuel cell prototypes in Tasks 5 and 6. Through design optimization, potential losses in microbial fuel cells were systematically characterized and reduced and each part of the system was optimized by studying cathodic hydrogen peroxide production and blackwater treatment separately.
In Task 7, the microbial fuel cell prototypes were optimized to maximize effluent quality and hydrogen peroxide yields and rates of production. Two designs were tested, one in which hydrogen peroxide was collected for other uses and one in which the hydrogen peroxide was directly utilized to treat the blackwater. Task 8 compiled the experimental results to develop an engineering design of a pilot-scale microbial fuel cell that produced hydrogen peroxide. The scope was expanded to include a life cycle assessment conducted in collaboration with SERDP project ER-2216.
Comparisons on the operational conditions of microbial fuel cells fed with blackwater show that the hydraulic retention time (HRT) was an important parameter that controled its performance. Semi-continuous operation achieved over 60% total chemical oxygen demand (TCOD) removal. The conversion of these solids to electrical current increased with shorter HRTs: 28%, 34%, and 32% for 12-, 9-, and 6-day HRTs, respectively, and methane fractions declined proportionally. A newly developed mathematical model, MYAnode, was able to predict similar behavior between methanogens and anode respiration, stressing the importance of pH and influent methanogens into the expected performance of the reactor. Based on these results, other studies focused largely on a 6-9 day HRT.
The study of cathode materials identified Vulcan carbon with a Nafion binder as the best catalyst for hydrogen peroxide production. Lower loadings of Vulcan carbon at ~0.45 mg/cm2 were optimal to achieve the highest peroxide production over a wide range of cathode potentials. Membrane-stability tests identified AMI-7001as the most stable membrane in hydrogen peroxide conditions, making it ideal for further studies. Hydrogen peroxide stability in different buffers and pH values showed that high pH led to a faster degradation rate, suggesting that operation at near neutral pH conditions at the cathode are desired. Based on this information, an adaptive pH controller was developed to control pH at the anode and cathode, taking into account the unpredictable variation of sludge alkalinity. Experimental results with the direct adaptive pH controller adapt reasonably well to the gain changes in the plant with the operating points and with the change in buffering capacity.
The development of an efficient microbial fuel cell prototype began with a characterization of potential losses and approaches to alleviate them. Cathodic pH was again responsible for 0.344± 0.019 V loss in the system, and the addition of CO2 in the cathodic air removed most of this loss. Hydrogen peroxide production was tested as a function of air flow rate, cathodic HRT, buffer concentration, and the addition of EDTA as stabilizer using acetate as electron donor. The maximum concentration achieved was 3.1 ± 0.37 g H2O2 L−1 at a 4-h HRT and a 37% cathodic efficiency, while the highest rate of production was 57 g Lcathode−1 d−1 at a 1-h HRT and a 78% cathodic efficiency. These efficiencies and concentrations are much higher than what is required for graywater tertiary treatment and other uses. However, upon using primary sludge as surrogate for blackwater, current densities were significantly lower and the same reactor design yielded a maximum of 0.23 g H2O2 L−1. On the other hand, the use of hydrogen peroxide for the treatment of the sludge itself led to an effluent quality similar to that of class B biosolids, with a 55% volatile suspended solids removal and 1.2x105 ± 1.2x104 most probable number (MPN) fecal coliforms.
Through the kinetic and design parameters developed, an engineering design was developed specifically focusing on a pilot-scale system that can be demonstrated under ESTCP and takes into account the materials selected and an adequate size for blackwater treatment.
This research demonstrates the feasibility of producing hydrogen peroxide from blackwater using a microbial fuel cell and shows that high rates and production efficiencies are possible. The microbial fuel cell efficiently treats the blackwater with or without the aid of the hydrogen peroxide produced. However, higher quality and rates are achieved through the aid of hydrogen peroxide. The development of this technology can change approaches at forward operating bases and at municipal wastewater treatment plants by having an in situ chemical production of a strong oxidant for treatment.