생물전기화학의 원리를 이용한 하폐수의 고도처리

Abstract

Biological processes for advanced wastewater treatment are an integral part of managing the water environment. However, the conventional biological processes have some technical limitations based not only on thermodynamics but also on the physiological properties of microorganisms. Interestingly, it has been revealed that electroactive microorganisms that catalyze the polarized electrode can enrich the bioelectrochemical reaction, and the redox potential of the bioelectrochemical reaction can be shifted by the polarized potential on the electrode surface. The shift of the redox potential can alter the thermodynamic equilibrium of the bioelectrochemical reaction. This indicates that the bioelectrochemical reaction has the great potential to enable advanced wastewater treatment beyond the limits of conventional biological treatment. In this thesis, an upflow bioelectrochemical reactor (UBER) for advanced wastewater treatment was devised, and the optimal design and operational conditions of UBER were investigated. In details, the conductive materials and applied voltage that affect the advanced wastewater treatment were studied in UBER using the synthetic wastewater with the standard discharge quality for the wastewater treatment plant. The advanced treatment performance of UBER was examined using the effluent discharged from a sewage treatment plant and the raw sewage to be treated in a sewage treatment plant. In the UBER, electroactive microorganisms were enriched by the polarized electrodes, and the organic matter, as well as nitrogen compounds, contained in the low strength synthetic wastewater with the standard discharge quality, were removed by the direct interspecies electron transfer (DIET). The effluent concentrations in COD and ammonia nitrogen were less than 3.5 mg/L and 7.46 mg/L at the 1 hour of HRT (Hydraulic Retention Time), respectively. This suggests that the polarized potential of the electrodes can improve the substrate affinity of bacteria. However, when conductive materials, including activated carbon particle and graphite fiber sheet, were added into the UBER, the effluent concentrations in COD and ammonia nitrogen were improved up to 1.98 mg/L and 2.65 mg/L, respectively, by the conductive sheets. It seems that the conductive materials between the electrodes in UBER not only increased the biomass retention but also further improved DIET by altering the abundance of dominant bacterial groups. This suggests that conductive materials between the electrodes are an essential part of UBER for the advanced wastewater treatment process. Interestingly, the effluent quality in UBER was affected by the physicochemical properties of conductive materials. In control without the activated carbon, the effluent COD and T-N from UBER were 2.72±0.08 mg/L and 11.62±0.05 mg/L, respectively. However, the effluent COD and T-N were improved to less than 1.66±0.06 mg/L and 4.45±0.03 mg/L, respectively, by the addition of conductive particles, especially activated carbons pretreated with Fenton oxidation, to UBER. Fenton oxidation improved the surface area and electric conductivity of activated carbon. Based on the decision tree for the effluent quality and EIS data, it has appeared that the effluent quality (COD and T-N) in UBER is highly dependent on the charge transfer resistance and the biomass amount. Another parameter affecting the removals of nitrogen compounds in UBER was the intensity of the electrostatic field created to the bulk solution by the polarized electrodes. The effluent T-N gradually decreased as the electrostatic field increased in the range of 0.2 to 0.83 V/cm. It seems that AOE and DNE enriched more under the higher electrostatic field to promote the DIET between them for nitrogen removal. However, the organic matter was easily degraded in UBER with a low intensity of the electrostatic field, and the effluent COD was not significantly affected by the electrostatic field. The continuous UBER with high porous conductive particles and 0.83 V/cm of the electrostatic field was designed and the advanced treatment performance for the effluent discharged from a sewage treatment plant was examined. The effluent COD of the UBER was at 1.61±0.03 mg/L at the steady state, which was significantly less than 4.90±0.40 mg/L in the control without the electrostatic field. In the case of T-N, the effluent concentration in the UBER was as low as 2.74±0.12mg/L. It suggests that when the electric field is exposed to the bulk solution of the UBER, electroactive microorganisms, including AOE and DNE, are enriched and the removals of organic matter and nitrogen compounds are promoted by the DIET between them. The treatment performance in UBER for a sewage to be treated in a sewage treatment plant was also examined. The effluent COD was 1.89±0.04 mg/L at HRT longer than 3 hours. However, the effluent T-N was 6.67±2.17 mg/L at the 3 hours of HRT, which was decreased to 4.31±0.14 mg/L at the 5 hours of HRT. In addition, it has found that the effluent T-N can be further decreased to 3.26±0.45 mg/L by recycling of the effluent at 0.5Q. It can be concluded that UBER can be applied not only as a tertiary treatment process for the effluent discharged from the sewage treatment plant but also as an advanced treatment process for raw sewage. Furthermore, UBER is also expected to be applicable to the advanced treatment of various wastewater, such as industrial wastewater, agricultural wastewater, and fishery wastewater if the design and operational conditions are obtained from the pilot test.