| With the high requirements for sewage discharge in the country,the discharge standards have become stricter,especially in key areas for water environment protection.The discharge quality needs to meet the standard of four class of surface water or even higher,causing a higher requirement for nitrogen removal in the wastewater treatment plant.This study firstly optimized the structure of the air-lift loop reactor and then established the integrated internal circulation aeration bioreactor for efficient denitrification with a low dissolved oxygen level and a high sludge concentration.CFD simulation software was applied to optimize the structural parameters and the denitrification mechanism was proposed by investigating the reaction rate,denitrification path,nitrogen balance analysis,and microbial community analysis.The main results are summarized as follows:(1)FLUENT simulation software was used to evaluate the influence of different inner cylinder widths,aeration head height,backflow gap size,and other parameters on the flow field in the internal circulation aerated bioreactor.The simulation results showed the best conditions for flow filed with the inner cylinder width of 40 mm,the reflux slot of 20 mm,and the aeration head height of 1.7 m.High mass transfer efficiency of the reactor could result in a more uniform gas-liquid two-phase stirring,and a complete mixing.(2)The experimental reactor was established according to the optimal structure parameters.The effects of various operating conditions,such as sludge concentration,influent flow rate,and aeration rate,on the wastewater treatment effect,were investigated.The results showed that the optimal conditions of the reactor could be operated with the sludge concentration of 5000 mg/L,an influent flow rate of 3 L/h,and an aeration rate of 0.6 L/min.This could lead to the average effluent COD concentration of 20.0 ± 8.8 mg/L with the removal rate of 93.6%,the average effluent concentration of ammonia nitrogen of 0.4 ± 0.2 mg N/L with the removal rate of up to99.2%,and the average effluent concentration of total nitrogen of 10.8±1.4 mg/L with the removal rate of 73.5%.(3)Based on the results of reaction rate,nitrogen removal path,and nitrogen balance,the main nitrogen removal pathway in the bioreactor consisted of shortcut nitrification and denitrification,traditional nitrification and denitrification,and simultaneous nitrification and denitrification,with the contribution rates of 77%,17%,and 6%,respectively.The denitrification rate of activated sludge was 6.0 mg·(g·h)-1,and the denitrification rate was 15.6 mg·(g·h)-1.The 16 S r RNA high-throughput sequencing was applied to analyze the microbial community diversity at different heights in the reactor.The results showed that there was no significant difference in the microbial community diversity at different sludge concentration areas in the reactor.At genus level,compared with the inoculated sludge,the abundances of anaerobic bacteria and aerobic bacteria in the reactor were much lower,and the abundance of facultative bacteria increased significantly.Heterotrophic bacteria Subgroup＿6 accounting for more than 15.0% was identified as the dominant group for degrading organic matters.The relative abundance of ammonia-oxidizing bacteria(AOB)increased from 2.3% to6.9%;the relative abundance of nitrite-oxidizing bacteria(NOB)decreased from 2.3%to 0.5%;Nitrosomonas with the relative abundance of 3.6% was a typical genus of dominant shortcut nitrifying bacteria.The microbial community analysis suggested that the shortcut nitrification and denitrification pathways were one of the major nitrogen conversion process in the reactor. |
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