Regulation And Enhancement Mechanisms Of High-frequency Micro-aeration Process For Partial Nitrification And Anammox

In recent years,the standards for municipal wastewater treatment have become increasingly stringent in China,together with the situation that carbon source in sewage usually inefficient for biological nitrogen removal,partial nitrification-anammox（PN-A）has been extensively studied for municipal wastewater treatment for its advantage in energy and carbon source demands.However,in municipal wastewater treatment processes,it is difficults to maintain stable enrichment of anaerobic ammonia oxidation（anammox）babcteria,and it usually takes a long time to inhibite activity of nitrite oxidation bacteria（NOB）,and the complex interaction among functional microorganisms associated to nitrogen removal restrain application of partial nitrification and anammox.To solve the aboved-metioned problems,the current study proposed a high-frequency micro-aeration（HFMA）process for PN-A and its control and strengthening strategy,evaluated the characteristics of HFMA mode on the efficiency of partial nitrification and anammox as well as the treatment efficiency of municipal wastewater treatment,analyzed the metabolism interactions of functional microorganisms and nitrogen removal pathways,and explored the metabolic regulation mechanism of HFMA process.This study provided theoretical support for the development of anammox technology of municipal wastewater treatment.To achieve stable operation of PN-A process,the HFMA process was proposed,the key to the process is to utilize the high-frequent low dissolved oxygen（DO）aeration and anaerobic alternating conditions（aeration frequency of 12 times/h and DO concentration lower than 0.5 mg/L）inhibit activity of NOB and promote growth of anammox bacteria.The results demonstrated that both intermittent and continuous influent conditions were suitable for HFMA system,which has a stable treatment effect on municipal wastewater treatment（nitrogen effluent concentration and removal rate were 8.59±0.33 mg/L and83.87±1.58%,respectively）.The amount of nitrogen removal was higher than that of the conventional anammox process in the form of activated sludge（the total nitrogen removal amount and the nitrogen removal by anammox were 125.36±2.47 and 43.82±1.62g N/m³/d,respectively）.Therefore,the current study built a PN-A process with HFMA mode and achieved stable treatment efficiency for municipal wastewater.Due to the low concentration of NH₄⁺-N in municipal wastewater,the HFMA process of PN-A usually has a long start-up period before achieving stable treatment of low C/N ratio municipal wastewater.Therefore,the control and strengthening strategy of HFMA process was proposed to solve the aboved metioned problems.The core of the strategy is to increase the content of organic matter and NO₂^--N in the sludge acclimation stage,and to strengthen the competition of DNB and AOB on various NOB substrates（O₂ and NO₂^--N）.After directional strengthening,the time for start-up of PN-A was reduced to 19 and 21 days,respectively.During the rapid start-up of partial nirification,the substrate competition between functional bacteria was strengened and the nitrogen removal efficiency was improved.Meanwhile,the TN removal amount during micro-aeration increased by 1.3 times and the nitrite accumulation rate（NAR）increased by20.47%.After rapid start-up,TN removal and anammox nitrogen removal increases by31.39±2.55 and 20.52±1.69 g N/m³/d,respectively.Therefore,improving the regulation of biological substrate competition in the micro-aeration periods can effectively improve the treatment efficiency of HFMA process for PN-A.To evaluate the nitrogen removal performance of a HFMA process for PN-A,the characteristics of sludge,nitrogen conversion pathway and activity were analyzed under HFMA and low-frequency aeration modes（lower than 3 times/h）.The results showed that,the accumulation rate of NO₂^-N（NAR）and the contribution rate of nitrogen removal by anammox under HFMA mode were higher than that in low-frequency aeration mode（20.88%and 19.34%,respectively）,and the difference of oxidation activity between NH₄⁺-N and NO₂^--N increased by 1.06 times.Thus,the anammox activity was increased by 32.11%,and the anammox activity with NH₂OH as intermediate metabolite was significantly increased.Furthermore,the results of sludge characteristic analysis showed that the microbial communities had better aggregation and nitrogen conversion activity under HFMA mode（the protein/polysaccharide ratio of microbial extracellular polymer was as low as 2.34,and the protein content was as high as 36.89 mg/g MLSS）.Therefore,microbial nitrogen conversion performance and sludge characteristics under HFMA mode significantly contributed to improve the efficiency of municipal wastewater treatment.The regulation of functional bacteria community and metabolic activity were the key points for the efficiency of HFMA.During the directional reinforcement period,the utilization capacity of NO₂^--N by DNB was improved（the relative abundance of DNB increased by 1.63%）,and it inhibited the activity of NOB which led to the relative abundance of NOB decreased by 2.31%.After the reinforcement period,AOB has more advantages in the use of O₂,and AOB and DNB continued to inhibit the growth of NOB,which accelerated the stabilization of partial nitrification.Metatranscriptome analysis showed that,gene expression levels of transformation from NH₄⁺-N to NO₂^-N and NO₂^-N to N₂increased significantly after directional reinforcement.There were sufficient NH₄⁺-N,NO₂^--N and intermediate metabolites in the effluent of the partial nitrification reactor,which enhanced the expression activity of genes involved in nitrogen metabolism,energy metabolism and carbon fixation of functional bacteria in the subsequent anammox reactor.Therefore,the change of metabolic mechanism of functional bacteria after synergistic enhancement accelerated the adjustment of microbial community structures and enabled the HFMA system to achieve a more efficient and stable effect for municipal wastewater treatment.