Study On The Mechanism Of NOB Inhibition By Anoxic/Aerobic Alternation Under Mainstream Conditions

Traditional denitrification technology cannot achieve economic sustainability or environmental balance.Nitrosation/Anammox（PN/A）process has become an emerging denitrification technology because of its energy saving and high efficiency.It has been successfully applied in wastewater treatment.However,the characteristics of low ammonia and low temperature in urban sewage make it do not have the conditions of high Free Ammonia（FA）and high Free Nitrous Acid（FNA）,and this condition is a key to the stable inhibition of Nitrite Oxidizing Bacteria（NOB）in the application of PN/A to treat industrial wastewater.Due to the lack of this condition,the nitrosation process is easily unstable,which is the biggest bottleneck problem that hinders the application of PN/A in the mainstream treatment process of urban sewage.In response to this problem,many years of research have been carried out at home and abroad,and a variety of control strategies have been proposed,but key technological breakthroughs have not yet been achieved.Low Dissolved Oxygen（DO）control is an essential control method to ensure stable nitrosation.Many studies have shown that the anoxic/aerobic alternation strategy has a good NOB inhibition effect under the mainstream conditions of urban sewage.However,the effective implementation conditions and control parameters of this regulatory strategy,as well as the main mechanism of action and other important issues have not been given clear answers in the literature,and even a consensus has not been reached.The implementation of anoxic/aerobic alternation strategy is simple and requires no additional economic investment.Therefore,further in-depth research was conducted to clarify the appropriate control parameters for the implementation of this strategy and the inhibition effect on NOB under different operating conditions,and to identify its main mechanism of action.It has important application value and theoretical significance.In this paper,the laboratory test method was used to study the conditions and effects of the low-DO coupled anoxic/aerobic alternation strategy under mainstream conditions.Artificial water distribution was used to explore the feasibility of anoxic/aerobic alternation on NOB inhibition in a laboratory PN/A reactor as a preliminary exploration;The SBR reactor was used to inoculate five different sludge sources,such as ordinary activated sludge,nitrosation sludge,and mixed sludge with different characteristics,and operated under four different parameters of anoxic/aerobic ratio.By comparing the effluent NO₃^--N concentration,nitrite accumulation rate and d（NO₃^-）/d（NH₄⁺）and other parameters,to investigate the inhibition effect of anoxic/aerobic alternation strategy on NOB under different operating conditions,and to explore its effective implementation conditions and the reasonable control range of its main control parameter anoxic/aerobic ratio;Based on the experimental research results of NOB inhibition effect,the changes of microbial community structure,real-time fluorescence quantitative PCR and enzyme activity under different reaction conditions were measured,combined with domestic and foreign literature citation analysis and demonstration,the realization conditions of anoxic/aerobic alternation inhibition of NOB and the key factors leading to NOB inhibition were deeply analyzed,and a new understanding of the mechanism of NOB inhibition by anoxic/aerobic alternation was proposed.The main conclusions of this paper were as follows:（1）The experimental results of the low-DO coupled anoxic/aerobic alternation strategy under different operating conditions showed that when the system had denitrification or anammox reaction,the average effluent NO₃^--N concentration changed from 41.64 mg·L^-1decreased to 19.82 mg·L^-1 and 11.68 mg·L^-1,respectively.Molecular microbiology and enzyme activity measurement further indicated that the system lacked NO₂^--N utilization in the anoxic stage,the NOB activity was difficult to inhibit.（2）Combined with the analysis of macroscopic and microscopic experimental results and citations from domestic and foreign literatures,it has been proved from various perspectives that the inoculation sludge source contained microorganisms such as Denitrifying Bacteria and Anaerobic Ammonium Oxidation Bacteria that can consume NO₂^--N and have high activity,which was the realization condition for the inhibition of NOB by anoxic/aerobic alternation.The existence of NO₂^--N utilization in the anoxic stage was the key factor to achieve NOB inhibition.（3）The analysis of the existing inhibition mechanism of anoxic/aerobic alternation showed that the hysteresis of NOB activity was not the main action mechanism of anoxic/aerobic alternation,and was insufficient to produce NOB inhibition.When there was no NO₂^--N utilization in the anoxic stage,NOB could not be effectively inhibited by the alternating between anoxic and aerobic environments of sludge.（4）The comprehensive experimental and analysis results showed that the inhibition mechanism of NOB by the alternation of anoxic/aerobic is mainly the substrate competition mechanism.When there are microorganisms that can consume NO₂^--N in addition to NOB in the inoculation sludge source,NO₂^--N is depleted in the anoxic stage,resulting in the lack of NO₂^--N substrate available for NOB in the next aerobic stage,forming a substrate competition.That is,during the intermittent aeration process,NOB is always in a low exposure environment of NO₂^--N,and the lack of substrate makes it difficult for NOB to grow,thereby achieving effective inhibition of NOB.