Livestock industry generates a large amount of manure and wastewater. Minnesota, the third largest hog producing state in the US, produces 7 million pigs per year and meanwhile generates 11 million tons of dry manure. Wastewater from swine farms contains high concentrations of organic matters, nitrogen and phosphorus. A poor management and treatment of the wastewater would cause severe environmental issues to soil, water, and air, such as eutrophication, impairment in drinking water quality, and the odor issue. So an appropriate treatment of the swine wastewater is an urgent and crucial issue to sustain the industry. Microbial fuel cell (MFC) is an emerging technology that shows a potential use in swine wastewater treatment. The reactor realizes biological oxidation at anode for organic matters, and electrochemical reduction at cathode. It is sustainable because it converts waste to electricity, recovers nutrients, and reduces the cost for wastewater treatment. The overall goal of this study is to develop effective MFCs for treating synthetic and swine wastewater to generate electrical energy and, at the same time, to achieve efficient removal of COD and total ammonium nitrogen. The first step was to choose suitable bacterial consortia to inoculate single-chamber air-cathode MFCs. Activated (AC) and anaerobic (AN) sludge showed faster enrichment of MFC anodic biofilm by 2 to 3 d than river sediment (RS), while AN-MFC presented highest VFA degradation rate, indicating that the bacteria in AN sludge were better adapted to MFC anodes due to the similar anaerobic environment and volatile fatty acid concentrations in a swine manure anaerobic digester. However, RS-MFC anode surface was covered with well-developed layers of biomass (bacterial cells and extracellular polymeric substances) and had a much larger power output (195 μW or 98 mW m-2) than AC- and AN-MFC after one month operation. For mature MFCs that were under long-time operation, a transient application of negative voltages (-3 V) improved the cathode activity and maximum power output by 37%, due to the bactericidal effect of the electrode potential higher than +1.5 V vs. standard hydrogen electrode (SHE). The second step was to model the single-chamber MFCs based on the assumption that the anode attached bacterial monolayer serves as biocatalysts for MFC exoelectrogenesis. By modifying the Freter model and combining it with Butler-Volmer equation, this model adequately describes the processes of electricity generation, substrate utilization, and suspended and attached biomass growth, in both batch and continuous operational mode. The results showed that the activation overpotential of the anode substantial reduced during the anode enrichment process, which was a result of increased exchange current density due to the increased biocatalyst. It was also found that electricity generation reduced sludge generation. Smaller external resistors were suggested to use to improve the organic matter removal and to reduce sludge generation, while an external resistor close to the internal resistor should be used to obtain the maximum power generation.
The third step of this study modeled the kinetic data of swine wastewater characteristics in MFCs, including conductivity, COD, volatile fatty acids (VFAs), total ammoniacal nitrogen (TAN), nitrite, nitrate, and phosphate concentrations. The removals of VFA and TAN had the half-life times of 4.99 and 7.84 d, respectively. Among the removed TAN, 13.6% was recovered from the evaporated air outside of MFC cathode, indicating its potential use for ammonium recovery from animal wastewater. The mechanism for phosphate removal was principally the salt precipitation from cathode, and needed improvement as the removal was far from completion. MFC with an external resistor of 2.2 kΩ and fed with raw swine wastewater generated relatively small power (28.2 μW), energy efficiency (0.37%) and Coulombic efficiency (0.15%). The main reason for the impaired performance was the inhibitory effects associated with TAN on Pt activity and VFA on anodic biofilm activity. Diluted swine wastewater, with a dilution factor of 2 or higher, dramatically improved the power generation as the inhibitory effect was reduced. Smaller external resistor in the circuit promoted the organic matter degradation and shortened the required reaction time in batch mode.
The fourth step was to reduce the inhibitory effect of swine wastewater in electricity generation by selective removal of ammonium and VFAs. This study showed that sorption using natural zeolite was an effective way for ammonium mitigation in swine wastewater. The kinetic process of the ammonium sorption on zeolite was best described by the pseudo-second-order model, and the resulting TAN sorption capacity at equilibrium was 11.6 mg/g. The isotherm data were best fitted by the Langmuir model, and the maximum TAN sorption capacity was 34.2 mg/g. The thermodynamic parameters indicated the spontaneity (ΔG° = -6.65 kJ/mol by the Langmuir model) and exothermic nature (ΔH° = -22.3 kJ/mol) of ammonium sorption on zeolite. Addition of GAC in zeolite decreased ammonium diffusion to zeolite particles, but it enhanced the maximum zeolite sorption capacity and COD (mainly VFAs) removal. Zeolite and GAC were effective in the selective adsorption of ammonia and VFAs in swine wastewater and consequently improved the power generation by over 80%, energy efficiency by up to 78%, and Coulombic efficiency by up to 37% of microbial fuel cells.
The final step was to optimize air-cathode and MFC configuration for ammonium removal. The 5% PTFE-treated cathode had a leaking problem, while the other cathodes, including 20% PTFE+GDLs, 5% PTFE+GDLs, and 20% PTFE, did not have the problem of leaking, and the last one performed best both in power generation and ammonia removal. Tests in MFCs A and C revealed that the half-life time of the total ammonium was proportional to electrical current, which was a strong evident demonstrating that the oxygen reduction reaction at cathode promoted ammonia volatilization by elevating pH nearby. On average, an increase of 1 mA in electrical current would reduce the half-life time by 2.8 d and 0.85 d for MFC A and C, respectively. Modifying regular MFCs to membrane contactor mode improved ammonia removal, because the surface area of hydrophobic membrane was increased. This improvement was indicated by the substantially reduced half-life time from the best case of 2.54 d of the best performed regular MFCs to only 0.67 d. The modification also allowed ammonia recovery from wastewater, and 78% of the removed ammonia was captured in sulfuric acid solution. This study demonstrated a novel way of ammonium recovery from wastewater by MFCs based on membrane contactor mode, and better performance is still expected through optimizing the gas-diffusion materials and reactor configuration.
University of Minnesota Ph.D. dissertation. November 2013. Major: Bioproducts/Biosystems Science Engineering and Management. Advisor: Jun Zhu. 1 computer file (PDF); x, 198 pages.
Electricity generation with organic matter and ammonium removal from swine wastewater via microbial fuel cells.
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