Analysis Of Carrousel Oxidation Ditch Combined Short-Range Biological Nitrogen Removal Efficiency And Microbial Population Structure Of Municipal Wastewater With Low C/N Ratio

In China,the shortage of carbon source and low ratio of carbon to nitrogen are common in raw water.Aiming at the phenomenon of poor nitrogen removal efficiency of domestic sewage with low carbon nitrogen ratio,the Carrousel 2000 oxidation ditch reactor was used as the experimental device to study the new process of shortcut nitrification and denitrification.The feasibility of shortcut nitrification and denitrification and the treatment of domestic sewage with low carbon nitrogen ratio were analyzed.Carrousel oxidation ditch process has many advantages,such as good effluent quality,stable start-up process,easy management and low energy consumption.Therefore,Carrousel oxidation ditch is widely used in domestic and industrial wastewater treatment projects at home and abroad.But at present,Carrousel oxidation ditch is mostly used in the traditional nitrification and denitrification treatment of domestic sewage.In the process of traditional nitrification and denitrification treatment of low carbon nitrogen ratio sewage,the nitrogen removal effect is often poor,and the total nitrogen in the effluent can not meet the standard.Therefore,the short cut nitrification and denitrification process has become a new type of wastewater biological denitrification technology because of its characteristics of saving carbon source and efficient nitrogen removal.In this study,the environmental factors affecting the shortcut nitrification and denitrification process in Carrousel oxidation ditch were studied by controlling DO concentration,pH,HRT and other factors.The denitrification efficiency of the oxidation ditch in the process of successful start-up and stabilization was explored.At the same time,the application of NO₂^--N in nitrite oxidizing bacteria was effectively controlled by adjusting the reaction conditions It is further oxidized to NO₃^--N under the action of oxygen.After 127 d of operation,the activated sludge in aerobic and anoxic sections of oxidation ditch were sequenced for high-throughput sequencing to observe the distribution and population change of microbial species during the operation of shortcut nitrification and denitrification process in oxidation ditch.The main experimental conclusions are as follows:（1）A Carrousel 2000 oxidation ditch device suitable for the scale of the laboratory in this study was made.The oxidation ditch body is narrow and long,which is composed of three ditches.The depth of the tank is 250 mm,the width of the tank is 200 mm,and the length of the tank is 800 mm.The top of each ditch is equipped with a pump impeller for aeration,and the water treatment capacity is 10-15L/h.The initial operation conditions are:internal reflux ratio=100%,external reflux ratio=50%,water temperature=20±5℃,pH=7.6,DO=2.0 mg/L,HRT=12 h,and the reactor was successfully started after 21 d operation;（2）The results showed that the removal efficiency of NH₄⁺-N,TN and COD was 97.4%,94.1%and 92.4%respectively,and the average concentrations of NH₄⁺-N,TN and COD were1.2 mg/L,1.41 mg/Land 30 mg/L when HRT was 12 d and DO=1.2 mg/L;（3）Before start-up,DO and HRT curves of traditional nitrification and denitrification cycle were analyzed.Under the optimal conditions of HRT=12 h and DO concentration=1.2mg/L,the start-up method of intermittent aeration was adopted.After 27 d of operation,the accumulation rate of NO₂^--N in the reactor reached 85.86%,and the removal rate of TN was more than 97%,The removal rates of NH₄⁺-N and COD were above 96%and 90%,respectively.The denitrification performance was good and the shortcut nitrification and denitrification were successfully realized;（4）According to the optimization of shortcut nitrification and denitrification process parameters,the results showed that the NO^-₂-N accumulation rate increased with the increase of pH value.When pH value was 7.8,the NO₂^--N accumulation rate reached the maximum of91.95%;The accumulation rate of NO₂^--N decreases with the increase of DO concentration.When the concentration range of do is 0.8-1.2 mg/L,the maximum NO₂^--N accumulation rate after stable operation of the reactor can reach 99.37%.Therefore,it is helpful to control the reactor in the low DO concentration range to facilitate the stable operation of the short-range nitrification and denitrification process,When do concentration is 1.2 mg/L and pH is about7.8,TN removal rate is more than 90%,COD removal rate is more than 85%and NH₄⁺-N removal rate is more than 95%after 21 d of stable operation.（5）In the analysis of the distribution and population changes of the microbial community in the activated sludge samples,the ecological composition and distribution of the microbial community in the activated sludge in the oxidation ditch reactor were studied.The high-throughput sequencing of the activated sludge in the aerobic and anoxic sections of the oxidation ditch was carried out.From the sequencing results,it can be concluded that there are23 categories,44 classes,354 genera and departments of bacteria detected in the activated sludge in this experiment,which can be completely matched.Under the condition of influent pH 7.6 and dissolved oxygen 1.2 mg/L,shortcut nitrification occurred in the aerobic tank,and the maximum accumulation rate of nitrite nitrogen could reach 95%.The sludge in the aerobic tank was drained and the nitrite oxidizing bacteria（NOB）with long generation cycle were washed.The microbial DNA detection showed that the species abundance of ammonia oxidizing bacteria（AOB）in the aerobic tank was more than 10 times that of NOB.In this paper,the short cut nitrification and denitrification process was successfully started in Carrousel oxidation ditch by controlling the influent amount,pH and dissolved oxygen.