Study On Influencing Factors Of Partial Nitrification In Sequencing Batch Activated Sludge Reactor

Partial nitrification（PN）is the key node inmany novel efficient nitrogen removal processes,but the fact is that ammonia oxidizing bacteria（AOB）often cannot successfully compete with nitrite oxidizing bacteria（NOB）,and it is difficult to achieve efficient accumulation of nitrous nitrogen.The focus of relevant research nowadays is to achieve long-term stable operation of partial nitrification,propose strategies to enhance PN performance,effectively inhibit NOB and overcome the environmental adaptation mechanism of NOB,in order to fundamentally solve the problems of PN process,and promote the engineering and scale of new denitrification processes.Taking the sequencing batch activated sludge reactor SBR as the research carrier,this study aims to identify the scientific and reasonable operating conditions of these reactors,through which to realize rapid AOB enrichment and quick start of the nitrification system.Meanwhile,through investigation into the gradual reduction of the influent ammonia nitrogen mass concentration and the reaction temperature and the impact of different C/N ratios on the performance of nitrification system,the present study explores the optimal conditions under which long-term stable operation of nitrification system can be achieved and the suitable operating conditions for partial nitrification.The long-term continuous accumulation of nitrite nitrogen and long-term good partial nitrosation performance of the system can be achieved through the inhibition of internal conditions of the system,the competitive inhibition of organic matter,and enhancement of nitrification performance by addition of Fe³⁺.On the basis of batch and long-term operation data and with high-throughput sequencing technology,this study explores the intrinsic link between the effect of denitrification process and the microbial community from both macroscopic and microscopic perspectives.The main research results obtained in this study are as follows:（1）The quick start of the PN system was achieved by adopting the following operatingstrategy:the HRT was set to 24 h,the concentrations of FA and FNA in the whole process were kept at 8.19-19.04 mg/L and 0.49-0.74 mg/L,respectively and the system temperature was32°C.The system realized the accumulation of nitrite nitrogen within 10 days when the aforementioned strategies were adopted together with intermittent aeration（aeration time was 12 h,aeration stop time was 12 h）with various conditions of real-time monitoring of dissolved oxygen being controlled.It was observed that real-time control of various conditions such as high FA and FNA effectively inhibited the metabolic activity of NOB.At the end of the start-up stage,the system could detect no more NOB bacteria,which indicated that AOB enrichment was achieved.（2）In order to investigate the effect of gradually reducing the influent ammonia nitrogen concentration and the reaction temperature on the operating performance of the PN system,the operating conditions were set as follows:with the same aeration rate,AOB degraded ammonia nitrogen with the same mass concentration,and compared with higher ammonia nitrogen influent condition（200 mg/L）,in the ammonia nitrogen influent condition of 50-100 mg/L,the degradation time lasted for a longer period than that in low ammonia nitrogen concentration influent condition,and influent with high ammonia nitrogen concentration speeded up the formation rate of nitrite nitrogen.Under the influent condition of low ammonia nitrogen concentration,the average degradation rate of ammonia nitrogen decreased by 8.83 mg/（g VSS.d）,and the average generation rate of nitrite nitrogen decreased by 9.29 mg/（g VSS.d）,with reaction rate losses of 42.78%and46.43%,respectively.Apart from this,shortening the HRT and reducing the aeration rate also resulted in degradation rate reduction of ammonia nitrogen through the metabolic activity of microorganisms.Compared with a high temperature of 32°C,a low temperature environment of17±1°Ccauseda loss of 57.48%in the ammonia nitrogen specific degradation rate.Ammonia oxidation rate,nitrosation rate and AOB metabolic activity were observed positively correlated with temperature in the temperature range（16-32℃）controlled in this study.Meanwhile,under high temperature conditions,the sludge settling performance of the PN system was good,which nonetheless deteriorated accompanied by turbid effluent with the decrease of temperature.（3）Toachieve partialnitrification through further control of the ammonia nitrogen degradation efficiency and nitrification efficiency,the HRT was optimized and was maintained at 9±1 h,the reaction temperature was set at 23.5±1.5°C,the p H of the influent water was greater than 8.0,the concentration of dissolved oxygen in the system was kept lower than 0.5 mg/L,and the ammonia nitrogen conversion rate was real-time monitored throughout the whole operation process of the PN system.It was found that the quality of partial nitrification effluent of the PN system lasted for up to 120 d.（4）To investigatehow the long-term autotrophic nitrite sludge performance of PN system was affected by organic matter,the C/N fluctuation range of the system influent was set at 0.95-2.61.It was found that there was still continuous accumulation of nitrite nitrogen after 180-day operation.In addition,under different influent conditions of C/N（<1.5,2.0 and 2.5）,the conversion rate and nitrous accumulation rate were 51.23%,58.69%and 43.68%,96.48%,75.36%and 70.25%,respectively.The NO₂^--N/NH₄⁺-N ratio of effluent mainly distributed in the range of 0.80-1.30,0.75-1.45 and 0.46-1.15,respectively.Under the condition of C/N<2.0,the system performed well on partial nitrification.（5）Batch experiments were conducted to investigate the effect of organic matter on the activity of Nitrosomonas and Nitrospira.In the process of increasing the C/N ratio from 1.0 to 2.5,the nitrate nitrogen ratio and nitrous ratio production rate exhibited a lossof69.88%and 25.82%,respectively.The abundance ratio of Nitrosomonas decreased from 32.75%to 18.53%whilethat of Nitrospiradecreased from 3.91%to 0.33%.As higher concentration of organic matter is not conducive to the growth of autotrophic nitrifying bacteria,within the range of organic matter concentration controlled in this study,the growth and activity inhibition of nitrifying bacteria exerted by organic matter was positively correlated with the concentration of organic matter,and the growth inhibition by organic matter on Nitrospirawas more obvious.And with the concentration of organic matter increases,the degree difference of its inhibition on AOB and NOB became increasingly obvious,which indicated that appropriate concentration of organic matter helps to achievesmooth accumulation of nitrous state.（6）Fe³⁺at an appropriate concentration can promote the growth of Nitrosomonas and reduce the adverse effects of organic matter on the Nitrosomonasmetabolism.It was found in this study that the growth and metabolic activity of AOB were promoted when the Fe³⁺concentration was greater than 1.75 mg/L,and that the abundance ratio of Nitrosomonas in the PN system increased from18.53%to 25.20%,compared with that before the Fe³⁺coupling.The findings of this studyprovide insightsto deal with the instability issue of PN process in low C/N sewage treatment.The competitive inhibition through organic matter and the enhancement of Fe³⁺are proved to be useful in alleviating nitrate accumulation and dealing with the strong adaptability of NOB in unfavorable environments.Additionally,the present study also offers enlightenment for the development of new energy-efficient denitrification process.