下水 中의 總窒素 制御를 위한 電解法의 適用性 硏究

Abstract

Recently, allowable discharge standards for the water quality improvement of discharge zones are being more and more strengthened. This suggests that sewage treatment plants to be newly established should be planned to positively employ an advanced treatment system that can remove nitrogen and phosphorus. In this study, we first prepare a mass balance for each treatment process of S sewage treatment plant and thereon examine the possibility of applying to electrolysis treatment to residue in a dehydrator that is in highest in contribution to sewage treatment. Then we select an optimum method of electrolysis treatment using the electrolysis method and, for the method, obtain optimum operation conditions, especially in terms of current density and response time. Thus the purpose of this study is to get basic information necessary for designing an electrolysis treatment system that can be actually applied to sewage treatment plants, on the basis of the above optimum operation conditions. In the study, we compare between the applications of the chemical injection and electrolysis treatment methods to the removal of total nitrogen from residue in the hydrator, ultimately determining the possibility of using the latter method for the removal. In comparison to the chemical injection method, the electrolysis treatment method was very high, or 92% in the removal rate of total nitrogen. This method was also 30% in the removal rate of COD and 40% in that of total phosphorus. These results indicate that the electrolysis treatment method is more effective in removing total nitrogen than the chemical injection method. We also suggest that electrolysis treatment is the best method of removing total nitrogen from residue in the dehydrator. The efficiency and cost-effectiveness of the treatment could be improved depending on how to deal with the method. In this study, thus, three possible methods of electrolysis treatment are selected. Out of the threes, then, the most cost-effective and optimal method is identified through experiments of total nitrogen removal for effluent. In removing total nitrogen from residue in the dehydrator, we tested three electrolysis treatment methods, or Method I(non-saline electrolysis method), Method II(saline electrolysis method) and Method III(the generation of high-concentration hypochlorous acid). We first used the Method I to electrolysis method residue in the dehydrator with the current density of 200A/㎡ or over and residence time of 20 minutes, resultantly obtaining 12% in the removal rate of total nitrogen. This is because the concentration of chlorine ion as dissolved in the reside is relatively low. Then we used the Method II to make the concentration of NaCl in residue in the dehydrator to become 9% and the electrolysis method the residue, under conditions such as source water of 760ppm, current density of 500A/㎡, initial pH of 9 and residence time of 1 minute. As a result, the removal rate of total nitrogen was found 90%. However, nevertheless, the method seems to require an excessive injection of NaCl and a high consumption of power. According to the Method III, next, we manufactured high-concentration hypochlorous acid of 8000 ppm under the current density of 500A/㎡,, and operated with 9.3 in the weight ratio of HOCl/N. As a result, the removal rate of total nitrogen was found 92%, identifying the optimal operation conditions. Thus the Method III(removing total nitrogen from residue in the hydrator through generating high-concentration hypochlorous acid) was higher in performance than the other two methods. In conclusion, findings of the study showed that electrolysis treatment is very effective in removing total nitrogen. From the findings, we could obtain basic information necessary for designing an electrolysis treatment system that can be actually used for sewage treatment plants and identify the optimum operation conditions of the system. Finally we found hopes that electrolysis treatment becomes an alternative to existing advanced wastewater treatment or these two treatments are combined.