The Effect Of N-acyl-homoserine Lactone-based Quorum Sensing On Activated Sludge Treatment System Under Nano-silver Stress

With the widespread use of Silver nanoparticle(nAg)antibacterial and disinfection products,nAg will inevitably enter sewage and eventually be discharged to urban sewage treatment plants,causing toxic stress on activated sludge microorganisms and affecting the effluent water quality of sewage treatment plants.N-acyl homeserine lactones(AHLs)signal molecules can regulate the expression of their own genes according to the changes of the cell density in the activated sludge microbial population and then regulate the population behavior.Therefore,it is of great significance to explore the relationship between the change of AHLs in activated sludge mixture and the performance of activated sludge reactor under nAg stress,thus evaluating the toxic effect of nAg and understanding the regulatory effect of AHLs on activated sludge treatment system.In this study,the Sequencing batch reactors(SBRs)were used as a simulated activated sludge sewage treatment system.10 mg/L nAg,20 mg/L nAg,3 mg/L Ag+and 6 mg/L Ag+were added to SBRs inlet water(corresponding to Ag+release of 10 mg/L nAg and 20 mg/L nAg in water,respectively).The continuous operation was conducted for 60 days to explore the effect of nAg and Ag+on SBRs effluent water quality.We added AHLs mixed solution(C6-HSL,C8-HSL,C12-HSL at concentrations of 10 nmol/L)to SBRs water inlet on the 65th day of operation,and continued to operate for 25 days and tested the effluent water quality of SBRs.The research conclusions are as follows:(1)The addition of 10 mg/L and 20 mg/L nAg to the water intake of SBRs significantly reduced the removal rates of effluent TOC and NH4+-N by 21.5%and 54.5%(TOC removal rate),19.5%and 36.9%(NH4+-N removal rate),respectively.The effect of adding 20 mg/L nAg into the water was more significant on the performance of SBRs(p<0.05).In the first 10 days of operation,TN removal rate of reactors effluent with 10 mg/L and 20 mg/L nAg were significantly reduced.After 10 days of operation,NO3--N removal rate of SBRs added with 3 mg/L Ag+and 6 mg/L Ag+were significantly reduced.Compared with Ag+released by nAg,nAg treatment has a greater stress effect on the removal performance of SBRs pollutants,which is related to the toxic effect of Ag.(2)Inductively coupled plasma mass spectrometry(ICP-MS/MS)detected that more than 90%Ag was adsorbed by activated sludge after 10 mg/L,20 mg/L nAg,3 mg/L and 6 mg/L Ag+were added into the water.Activated sludge added with 10 mg/L nAg,20 mg/L nAg and 6 mg/L Ag+were analyzed by X ray diffraction(XRD)and X-ray Photoelectron Spectroscopy(XPS),and the result shows that there exist Ag2O3.In addition,Ag2S was also detected in activated sludge with the addition of 20 mg/L nAg.nAg can generate Ag2O3 in SBRs system because the SBRs reactor is mainly in aerobic environment during the operating cycle.Higher concentrations of silver(20 mg/L nAg)combine with sulfur ions to form Ag2S.The accumulation of inorganic components in SBRs system can significantly increase the Mixed Liquid Suspended Solid(MLSS)of activated sludge.In addition,the agglomeration of activated sludge caused by nAg can increase the average particle size(volume characterization)of sludge floc.(3)Compared with CK,the relative abundance of the denitrifying bacteria Anaerolineaceae and Terrimonas increased by 0.2-0.3%when 3 mg/L Ag+and 6mg/L Ag+were added on day 5th of operation.On the 60th day of SBRs operation,the competitive bacterium with the highest relative abundance changed from the Candidatus Competibacter to Thauera.On the day 60 of SBRs operation,compared with day 5,the common number of OUT in the activated sludge of each reactor was significantly reduced and the unique number of OUT was increased.Compared with CK,the relative abundance of nitrobacteria Novosphingobium,Nitrospira could be reduced by 0.02-0.04%by adding 10 mg/L nAg and 20 mg/L nAg in the reactors.(4)Quantitative real-time PCR results showed that AOB,napA and nirS played a dominant role in the ammonia oxidation process,nitrate reduction process and nitrite reduction process of activated sludge microorganisms under nanometer silver stress.In activated sludge with 20 mg/L nAg,the toxicity of nAg and its products,Ag2S,can exert a rapid and lasting stress on the denitrification functional genes.After 10 days of SBRs operation,3 mg/L Ag+and 6 mg/L Ag+began to exert stress on the denitrification functional genes.(5)Compared with CK,the concentration of C4-HSL in nAg reactor added with 10 mg/L increased by 1.45 nmol/L,Cs-HSL,C12-HSL and C14-HSL were not detected in the addition of 10 mg/L nAg reactor.Adding 10 nmol/L mixed standard solution of C6-HSL、C8-HSL、C12-HSL to control SBRs and different treatment of adding 10 mg/L nAg、20 mg/L nAg、3 mg/L Ag+、6 mg/L Ag+SBRs,the EPS extraction and the removal rate of NO3--N increased by 20.0%and 5.8%respectively.The effect of exogenous AHLs on the TN removal rate of SBRs with 10 mg/L and 20 mg/L nAg influent is much higher than that of other reactors.AHLs can play a greater role in regulating microbial communities under nAg stress.The potential impact time of AHLs was maintained for about 5 days only,then the SBRs water quality index tended to the level of CK without adding AHLs.When nAg enters the activated sludge system,it can reduce the relative abundance of the dominant bacteria and nitrification and denitrification functional genes,thus reducing the removal efficiency of SBRs for NH4+-and other pollutants in sewage.When the activated sludge system is under nAg stress,the microbial community can be regulated by AHLs signaling molecules theoretically.However,only one kind AHLs mixed solution was added to the SBRs reactor in this paper,so the systematic analysis and comparison could not be obtained.The specific regulation mechanism and the most appropriate regulation mode of AHLs still need to be further studied.