| In recent years, in order to reduce the nitrogen pollution in water environment and protect human health, many national governments enact stricter nitrogen especially nitrate emission standards. Meanwhile, many researchers pay close attention to research of nitrogen removal technology, developing and improving a series of nitrogen removal technologies. Three-dimensional biofilm-electrode denitrifying technology has been exploited in the research background and improved with the lapse of time. However, nitrate is removed through denitrification of bacteria generally and the electrodes are not exploited sufficiently except hydrogen production in this technology up to the present, so it is difficult to degrade ammonia and COD with this technology and the application range and ability of three-dimensional biofilm-electrode denitrifying technology is limited.Therefore, this study investigated the performance of electrochemical denitrification using Ti/IrO2-TiO2-RuO2as the anode and Cu/Zn as the cathode in an undivided cell, and then selected granular activated carbon as the filling and paracoccus denitrificans as denitrification bacteria, which was fixed with embedding technology of PVA-boric acid and alginate-calcium chloride. Finally, Bipolar three-dimensional biofilm-electrode reactor was constructed, though which performance parameters influencing nitrogen removal of reactor were investigated after preliminary study on the growth characteristics and denitrification capacity of paracoccus denitrificans, determining the optimal level of each performance parameters. Furthermore, denitrification capacities of electrodes and paracoccus denitrificans of bipolar three-dimensional biofilm-electrode reactor were investigated respectively and the mutual influence between the electrodes and paracoccus denitrificans was discussed through comparative study of various types of reactor. The main conclusions of the study are listed as follows:1. The main anodic oxidation style is indirect oxidation, so the addition of sodium chloride can greatly improve the oxidation of anode so that the nitrate removal efficiency can achieve90%-95%. Direct oxidation only accounts for about10%-15%of the oxidant capacity.2. Current density and NaCl dosage have stronger effects on than initial nitrate concentration and pH on nitrate removal efficiency. The results of single-factor assay reveals that the optimal value of current density and NaCl dosage are15mA/cm2and0.5g/L, respectively.3. The reaction kinetics of nitrate reduction in cathode can be represented by pseudo first-order kinetics and the model derived from this study can simulate the reaction kinetics of nitrate to nitrogen well on the basis of analysis the results of current density and NaCl dosage.4. The corrosion of cathode is neglectable in acid/neutral condition and Zn is less active than Cu for nitrate removal. Moreover, the removal results of anode for glucose and potassium hydrogen phthalate show the glucose is much liable to be oxidized than potassium hydrogen phthalate, reflecting indirectly the oxidation capacity of anode for refractory organics is limited.5. The start-up and testing process of bipolar three-dimensional biofilm-electrode reactor demonstrated it needed to control the current intensity in order to exert the autotrophic denitrification capacity of Paracoccus denitrifwan. Moreover, the optimal value of hydraulic retention time, current intensity and sodium chloride dosage was14h,400mA and0.3g/L, respectively, through the study of performance parameters influcing nitrogen removal. The denitrification efficiency, denitrification rate and energy consumption was53.3%,0.329mg-N/cm2/d and1.81kWh/g-N, respectively under the optimal exprimental conditions。6. The results of comparative study of various types of reactors revealed three-dimensional electrode reactor was able to supply inorganic carbon of HCO3-and CO3-and possessed certain capacity of pH buffering, and its denitrification rate, ammonia oxidation rate and energy consumption are all superior to the two-dimensional electrode reactor. The comparative results between bipolar three-dimensional biofilm-electrode reactor and three-dimensional-electrode reactor demonstrated the denitrification capacity of the former was due to the denitrification of electrodes. The comparative results between bipolar three-dimensional biofilm-electrode reactor and traditional three-dimensional-electrode reactor not only shows the introduction of electrode couple with the nitrate removal capacity can enhance the denitrification capacity of traditional three-dimensional biofilm-electrode reactor, but also confirms the bipolar three-dimensional biofilm-electrode reactor possessed certain hydrogen autotrophic denitrification capacity. |
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