| In these years, water pollution of water environments is more and more severe with the rapid development of China's economy. Due to its well removal effiency, stable runing, convenient management and low energy consumption, Carrousel oxidation ditch has been wildly used all over the world. However, due to design or operation processes, it has been known that the disnitrification efficiency of Carrousel oxidation ditch is relevantly low in actural water treatment. In this study, based on a Carrousel oxidation ditch plant in Chongqing, a pilot scale system was design. The influence of hydraulic retention time(HRT), dissolved oxygen(DO), and aeration condition on the removal of pollutants, especially denitrification efficiency, was investigated. A full-scale system has been established by using the parameters obtained from pilot scale experiments. The transformation and fate of nitrogen has been explored along the modified oxidation ditch. The influence of depth in the water has also been explored. Stable nitrification under viarous conditions has been observed. The mechanism of the effect of biological micro environment on nitrogen removal performance of modified Carrousel oxidation ditch was explored by using microelectrode testing technology and molecular biology technology. The research results may provide reference for improving the efficiency of nitrogen removal in Small scale Carrousel oxidation ditch.1)In the pilot scale of modified Carrousel oxidation ditch with the water inflow of 10m3/d, DO=3 mg/L, and the internal reflux ratio and external reflux ratio remained at 200% and 50%~80%, respectively, the removal efficiency of COD reached 90%, NH3-N removal ranged from 51.9%-83.8% and TN removal ranged from 51.9%-83.8% when the HRT were 17 h and 10 h. When the HRT was maintained at 12 h and 15 h, the maximum removal efficiency of COD, NH3-N and TN reached 93.1%, 92.2% and 52.8%, respectively, probably attributed to the enhanced denitrification and ammoxidation.2)In the pilot scale of modified Carrousel oxidation ditch with HRT remained at 15 h, DO level maintained at 5mg/L, 4 mg/L and 3 mg/L, the removal efficiency of SS and TP varied from 96% to 97% and 90 to 93%, respectively. When the oxygen level was ranged from 2 to 5 mg/L, the activity of microbes were prompted hence induced enhanced nitrification and oxidation, consequently caused substantial removal of COD(93.1%-95.2%) and NH3-N(98.9%-99.1%), however, the DO level depressed the removal of TN as a result of influence of anoxic environments and accumulation of NO3-N, with the reduction of DO from 5mg/L to 2 mg/L, the denitrification process was enhanced and the TN removal efficiency dramatically increased from 28.8%o 48.1%. Besides, when properly closing off the aeration nearby the internal reflux zone and in the middle of the aerobic stage, the simultaneous nitrification and denitrification was promoted and removal of COD, NH3-N and TN reached 91.8%?77.6% and 67.0%, respectively, which were significantly higher than that under other aeration profiles.3) The full-scale experiments(1.5?104 m3/d) was conducted under the best condition(HRT=12h,DO=2~3mg/L) obtained by pilot scale experiments. Results indicated that SS?BOD5?CODcr?NH3-N?TN?TP can reach the standard fo GB18918-2002. The removal efficiency was 96.25%, 95.54%, 90.54%, 88.69%, 54.6%, and 92.17%, respectively. The removal of NH3-N and TN was reduced under lower water temperatures to a certen extent. When the water temperature was 13~15?, the concentrations was reduced to 3.95mg/L and 15.96mg/L, respectively. In the full-scale experiments, the distribution of DO and the migration and transformation of nitrogen anlong the direction of water flow and depth in oxidation ditch were studied. The experimental results showed that, the denitrification was mainly occurred when the DO is 0.2~0.5mg/L in the anoxic pool, and the removal rates of NO3--N and TN were 53.56% and 69%, respectively. The ammonification and nitrification is mainly occurred when the DO is 1.65~2.95mg/L which in the front of aerobic pool. The concentration of NH3-N decreased from 5.25 mg/L to 3.8 mg/L. At the same time, the concentration of NO3--N increased rapidly along the oxidation ditch, which increased from 5.25 mg / l up to 11.34mg/L. The DO is gradually reduced to below 1.5mg/L at the back end of the aerobic pool.the concentration of TN and NO3--N decreased simultaneously and tends to be stable because of the occurrence of nitrification,which reaches the standard of GB18918-2002.4)DO in the anoxic zone decreased from 0.51mg/L to 0.31mg/L in the water from 0.5m to 3.5m. NO3--N concentration decreased from 9.913mg/L to 9.375mg/L and NH3-N increased from 8.763 mg/L to 9.322 mg/L, along with the depth increased. DO in the aerobic zone decreased from 3.35mg/L to 3.10 mg/L in the water from 0.5m to 3.5m. NO3--N concentration decreased from 11.384mg/L to 10.883mg/L and NH3-N increased from 3.099 mg/L to 3.341 mg/L, along with the depth increased.5)The sludge flocs was detected by microelectrode technique. The structure of sludge which is loose, which radius is about 1~2 mm in the activated sludge system of modified Carrousel oxidation ditch. Along the radial direction from the outside to the inner edge of the aerobic pool, the concentration of DO decreased from 3.311 mg/L to 1.795 mg/L, the concentration of NH3-N changed from 3.8002 mg/L to 3.8003 mg/L, the concentration of NO3--N changed from 11.7475 mg/L to 11.7416 mg/L, the concentration of NO2--N remained at about 0.21 mg/L;Along the same direction in the anoxic pool, the concentration of DO decreased from 0.690 mg/L to 0.378 mg/L, the concentration of NH3-N changed from 8.4011 mg/L to 8.4012 mg/L, the concentration of NO3--N changed from 5.1578 mg/L to 5.1542 mg/L, the concentration of NO2--N was also maintained at about 0.53 mg/L. The difference of three forms of nitrogen's concentration in the micro environment which in aerobic and anoxic conditions is not large. It can be seen that, the DO is not a key factor that can produce the difference of nitrogen form in the micro environment of loose sludge.6) The microbial population was detected by high throughput sequencing technology. At the Phylum level, the major common dominant bacteria in denitrification process, whicn in the anoxic and aerobic pool of the Modified Carrousel oxidation ditch, include Proteobacteria, Planctomycetes, Bacteroidetes, Nitrospirae, Actinobacteria and so on. The bacteria's proportion of each Phylum in aerobic pool is 36.1%, 7%, 19.4%, 0.48% and 4.1%, and in anoxic pool is 36.1%, 7%, 19.4%, 0.48% and 4.1%. The production of NO2--N is inhibited by the presence of Nitrospirae. The proportion of Proteobacteria in sludge samples in the anoxic pool is higher than that in the aerobic pool, which main function isdenitrification in the anoxic pool. At the Genus level, the major common dominant bacteria in denitrification process include Caldilinea, Dokdonella, Gemmata, Hyphomicrobium, Planctomyces and so on. The bacteria's proportion of each Genus in aerobic pool is 2.33%, 0.66%, 0.94%, 1.05% and 2.50%, and in anoxic pool is 2.80%, 0.82%, 1.06%, 1.02% and 2.42%.Dokdonella, Gemmata, Hyphomicrobium and Planctomycesare the main nitrogen removal facultative bacteria in the Modified Carrousel oxidation ditch.Nitrogen removal efficiency is achieved by the nitrification and denitrification of these four generas, especially Dokdonella, which showedvaried dissolved oxygen concentration in different regions of the oxidation ditch. |
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