| The traditional biological nitrogen removal process for wastewater treatment has common shortages such as long flow chart, large space occupy, low oxygen utilization efficiency and low automatic level. It is an urgent task to study and develop new biological nitrogen removal technologies for wastewater treatment along with the increasingly severe water eutrophication and continuously raising of nitrogen discharge standard. Bio-Doubling Process for wastewater treatment which attracting widespread attention at home and abroad, could be operated on condition of low oxygen concentration, and has the characteristics of simultaneous nitrification and denitrification(SND). It has been commonly applied in industrial wastewater treatment field. However, the application in municipal wastewater treatment is still rare now. Therefore, it has great significance to study nitrogen removal efficiency and affecting factors of Bio-Doubling Process for municipal wastewater treatment systematically. Aiming at characteristics of our municipal wastewater, this subject systematically studied the nitrogen removal efficiency of Bio-Doubling Process for treating municipal wastewater by carrying out lab-scale and on-site experiment.In the lab-scale experiment, this study systematically studied the start-up characteristic of Bio-Doubling Process, and analyzed the effect of dissolved oxygen(DO) concentration and wastewater temperature on the start-up stability of the process. In addition, the nitrogen removal efficiency of Bio-Doubling Process in stable phase was intensively studied, and effect of different technological condition on nitrogen removal efficiency was analyzed. Effects of carbon source species, COD/TN ratios and influent mode on nitrogen removal efficiency were inquired. Then the characteristic of SND bacteria was analyzed initially.Lab-scale experiment results showed that DO concentration and temperature were very important for start-up of Bio-Doubling Process for wastewater treatment. DO was the important factor affecting start-up stability when started above 12℃. Excessive growth of Microthrix Parvicella could be induced with the operation condition of sustained low temperature, DO concentration and sludge loading when temperature was below 12℃, which leading to simultaneous filamentous sludge bulking and biological foaming. Sludge bulking caused by oxygen-lacking could be controlled by regulating DO concentration. Filamentous sludge bulking and biological foaming caused by low temperature could be controlled effectively by raising temperature and regulating sludge retention time(SRT).Removal efficiency of organic pollutants was favorable because of peculiar high volume loading of Bio-Doubling Process. The effluent COD concentration could be steadily met the demand of(GB18918-2002) standard or more superior. Long SRT was propitious to enrich nitrifying bacteria and the ammonia nitrogen removal rate could be steadily achieved 100%. The Bio-Doubling process showed strong capacity of counteracting low temperature and quality and quantity shock loading from influent wastewater, and good sludge settleability. When hydraulic retention time(HRT) was between 12 and 21 hours, it had little effect on nitration and nitrogen removal efficiency. The average TN removal rate was 58.24% during stable operation phase. At the same HRT and COD loading, the lower ammonia nitrogen loading, the better TN removal efficiency.External carbon source species, sodium acetate, glucose and sucrose had little effect on nitrogen removal efficiency, while influenced phosphorus removal obviously. Sodium acetate showed the best performance among those three external carbon source species. On the condition of little variation of influent COD loading, the TN removal efficiency was mostly restricted by organic substrate in denitrification process. The removal rate of TN and efficiency of SND increased along with the increasing of influent COD/TN. The average TN removal rate was 64.89% when the influent COD/TN ratio was 2.9. In order to ensure good nitrogen removal efficiency in different wastewater temperature periods, the step-feed influent regulation method based on three wastewater temperature periods was studied. The optimal influent flow distribution ratios between aerobic zone and anaerobic zone were determined for enhancing nitrogen removal at low COD/TN ratio, which were 1:1 at high temperature period(25.3~26.1℃), 1:2 at normal temperature period(15.7~25.3℃) and 1:3 at low temperature period(9.7~15.7℃).The mechanism of SND occurred in the lab-scale Bio-Doubling Process adopted step-feed was analyzed initially. Static experiment was carried out to determine the relative contribution of novel microorganisms capable of autotrofic denitrification and heterotrophic nitrification-aerobic denitrification to this process system. The results indicated that existence of micro-anoxic environment, which made significant expression of heterotrofic denitrification based on conventional metabolism was the major cause for nitrogen loss in aerobic zone.In on-site test of Bio-Doubling Process, the change feature of mixed liquor suspended solids(MLSS) along with time and space, characteristics of DO concentration and oxygen transfer in aerobic zone, and wastewater indexes along the process were investigated systematically. Then efficiency of SND mode for nitrogen removal in aerobic zone and the influence factors were also studied.Bio-Doubling Process showed good hydraulic mixing characteristic. Mixed liquor suspended solids(MLSS) was well-distributed both along vertical or horizontal direction in aerobic zone, and the fluctuation by time was also insignificant. MLSS variation was almost influenced by influent characteristic and operational condition. By real-time control, the DO concentration distribution along the tank width varied slightly, compared with the significant differences along the depth and length. Along the length direction of aerobic tank, the DO concentration declined significantly at first, then slightly reached zero, with the least at the end of the aerobic tank. DO concentration decreased gradually along the depth direction of the aerobic tank. The declining distribution of oxygen transfer efficiency and oxygen uptake rate along tank length indicated Bio-Doubling Process both had characteristics of completely-mixed reactor and plug-flow reactor. Data showed that there was no clear anoxic macro-environment in aerobic zone. However, larger flocs size and low DO concentration control were conducive to form anoxic micro-environment to support simultaneous nitrification and denitrification.Finally, the carbon source utilization of step-feed mode was applied in wastewater treatment plant. Practical applications had proved that this kind of optimal regulation scheme could observably enhance nitrogen removal efficiency. The effluent total nitrogen concentration could always meet the discharge standard in three typical wastewater periods. |
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