Study On Interspecific Competition Mechanism Of PAOs And GAOs Under Salt Stress

With the development of urbanization,industry and agriculture,the discharge of saline wastewater also increases;At the same time,seawater substitution has become an important way to solve the shortage of fresh water resources in coastal areas of China,which increases the salt content of wastewater flowing into wastewater treatment plants and brings a series of burdens to wastewater treatment.The increase of salt content will have a negative impact on the removal of various substances in the wastewater,because the high salt content will change the osmotic pressure of the wastewater,and the activated sludge microorganisms will have cell wall separation,which will lead to the cracking and death of the cells in serious cases.In addition,salinity also causes changes in the population of functional microorganisms,which in turn alters the metabolic pathways of the microorganisms in the system.Generally,the processes of nitrogen and phosphorus removal occur simultaneously in wastewater treatment process,but the degree of salt tolerance of nitrogen and phosphorus removal microorganisms is not consistent.At present,the effects of salinity on denitrification performance and denitrification pathway have been widely studied.Nitrifying bacteria have strong salt tolerance and can achieve high nitrogen removal rate in the salinity range of 0-20 g/L.However,there are few studies on the effect of salinity on phosphorus removal performance and community structure,and the competition mechanism of salinity on phosphorus accumulating organisms（PAOs）and glycogen accumulating organisms（GAOs）is not clear.Therefore,the focus of this paper is to study the effect of salinity on phosphorus removal process,in order to provide a theoretical basis for regulating the survival contradiction between nitrogen and phosphorus removal bacteria in salt wastewater.Its main research contents include the following aspects:（1）Taking synthetic wastewater as the research object,the BPR system was successfully started by using anaerobic/aerobic（A/O）operation mode in sequencing batch reactor（SBR）.In order to achieve high anaerobic phosphorus release（PRA）,aerobic phosphorus uptake（PUA）and phosphorus removal rate（PRE）,the system performance was regulated from the influent substrate concentration(influent C/P,Ca²⁺,Mg²⁺concentration)and operating conditions（DO,SRT,aerobic HRT）.The results showed that:after 150 days of operation,the stable and efficient BPR system was successfully realized,PRA and PUA in the system were reached 28.67 and 33.95 mg/L,respectively.PRE was successfully stabilized at about 96.09%,and the phosphorus concentration in effluent was 0.41 mg/L,lower than 0.50 mg/L,which met the effluent standard of wasterwater.However,the relative abundance of PAOs（Hypomicrobium,3.69%;Pseudofulvimonas,1.02%;unclassified＿f＿rhodobacteraceae,2.41%）was not high,which indicated that there was no absolute correlation between the abundance of PAOs and the phosphorus removal performance of the system.As long as the influent conditions are conducive to PAOs to better absorb enough carbon to remove phosphorus in the influent（the excess carbon may be used for the reproduction of GAOs）,the system can maintain stable and good performance.The stable BPR system provided a good premise for the subsequent salinity experiment.（2）In order to investigate the effect of salt shock on phosphorus removal system,batch tests was carried out.The conical flasks were used as the test reactor,which were internally connected with the activated sludge of BPR system.Eight different salt concentration gradients were set up for batch test.In order to increase the universality and reliability of the conclusion,two groups of high influent phosphorus concentration systems were set up for salt shock test to explore the impact of salt impact on the phosphorus removal performance of the system under high influent phosphorus concentration.The test results showed that:the tolerance limit of PAOs to salt was about10 g/L.In the range of 0-10 g/L,with the increase of salt concentration,the phosphorus removal rate,specific anaerobic phosphorus release rate and specific aerobic phosphorus absorption rate of the system were also decreased.When the salt concentration was exceeded 10 g/L,the cells may break down and die,resulting in ineffective phosphorus release,and almost completely lose the ability of phosphorus uptake.Therefore,the salinity range should be set at about 0-10 g/L in the subsequent long-term test.（3）On the basis of the stable BPR system,sodium chloride was added to explore the effects of different salt concentration stress on the system from three aspects:phosphorus removal performance(COD,PO₄^3--P removal efficiency,total phosphorus change in sludge and microbial cells),sludge characteristics（particle size,extracellular polymeric substances,sludge concentration）and microbial community structure（microbial species abundance change,function prediction,competition mechanism）.At the same time,the recovery degree of salinity reduction system after high salinity acclimation was also discussed.At the same time,the recovery degree of salinity reduction system after high salinity acclimation was also discussed.The experimental results showed that:high salinity stress had little effect on COD removal efficiency,and the phosphorus removal performance of the system was inhibited immediately after adding salinity,and the removal efficiency was decreased rapidly.After a period of domestication,the performance will recover to a stable degree.Under 5 g/L salt stress,PRE was stabilized at about 73.68%,and at 10 g/L salt concentration,PRE was finally stabilized at-7.98%.Most cells was cracked,sludge particle size was decreased,and phosphorus removal ability was severely inhibited.When the salt concentration was reduced to 7.5 g/L,the system performance was recovered to about 27.39%.Long-term salt level operation led to the decline of microbial diversity,which made the relative abundance of Bacteroidota drop to 12.33%,while the relative abundance of Acidobacteriota and Planctomycetota were gradually declined,and may be eliminated by the system eventually.The relative abundance of Chloroflexi and Planctomycetota were increased gradually during salt domestication.The relative abundance of Defluviicoccus was decreased with the increase of salt concentration,and was 3.02%and 6.00%at 10 g/L and 7.5 g/L salt concentrations,respectively.