Investigation Of Enhanced Microbiological Removal Of Ammonium Based On AHL Dependent Quorum Sensing From Slightly Polluted Water And Mechanisms

Biological removal of ammonium from polluted source water has been widely applied.However,biofilm,the main body of drinking water treatment system has several problems,including reducing the biomass of carrier surface,decreasing the microbial activity,severely inhibiting of growth of the autotrophic nitrifying bacteria and sharply decreasing of ammonium removal efficiency.Thus,the technology of biological removal of ammonium based on biofilm is facing great challenges.In order to enhance ammonium removal efficiency at low temperature in drinking water treatment system,this study primarily investigated the enhancement of ammonium removal efficiency by quorum sensing(QS)activation technology based on N-acyl homoserine lactone(AHLs)signaling molecules as the regulators,and combined enhanced cell adhesion effect to further improve the ammonium removal efficiency as coupling effect at low temperature.Suspended carrier biological fluidized reactor was applied for pilot-scale study.System performed good removal efficiency for common contaminants such as ammonium,iron ion and manganese ion at normal temperature.Autotrophic nitrifying bacteria(AOB and NOB)were the dominant bacterial communities of the system.Pseudomonas had Mn oxidation function,Rhodoferax had Fe oxidation function,and Sphingomonas could help the extracellular polymeric substance(EPS)synthesize extracellular polysaccharides.However,low temperature seriously inhibited the growth of autotrophic nitr ifying bacteria in SCBFR system,decreased the content of EPS components in the biofilm,reduced the EPS fluorescent substances and reduced the ammoni um removal efficiency and the nitrogen conversion efficiency.At low temperature,the average removal rate of ammonia nitrogen was 24.1%,less than 1/2 of ammonium removal rate at normal temperature(56.2%),the nitrite oxidation capacity was weakened,and the transformational concentration of nitrate was about 1/2 of the normal temperature.The dominant bacteria in SCBFR system at low temperature were transformed into Flavobacterium,and the relative abundance was 20.3% detected by highthroughput sequencing analysis.The performances of SCBFR systems were modulated by different kinds of AHLs signal molecules at normal and low temperature to enhance ammonium removal efficiency.At normal temperature,the average ammonium removal rates modulated by C6-HSL and C8-HSL were 66.3% and 63.7%,respectively,and their control was 54.5% during the whole start-up period;the average ammonium removal rates modulated by C6-HSL and C8-HSL were 94.9% and 93.0%,respectively,and their control was 80.6% during the stable operational period;consumptions of operational days for reaching removal rate to 80% modulated by C6-HSL and C8-HSL shortened by 5 d and 4 d,respectively,comparing to their control.Analysis of the mechanism of modulation showed that at normal temperature C6-HSL and C8-HSL modulated the morphologies of biofilm to form filamentous connective structures and increased the exchange of information between cells and cells;improved the relative abundance of autotrophic nitrifying bacteria,therein,the relative abundances of Nitrosomonas were 5.9% and 5.2%,respectively,and their control was 2.7%,relative abundances of Nitrospira were 33.4% and 29.9%,respectively,and their control was 25.7%.C8-HSL could also promote EPS formation to enhance ammonium oxidation at normal temperature.At low temperature,only C12-HSL played a role in promoting the removal of ammonia and the highest ammonium removal efficiency reached to 86%,the average value was 48.2%,and its control was 35.1% during the whole start-up period;the rate was 75.6%,and its control(16 d to 24 d)was 53.6% during the stable operational period.Modulation of C12-HSL not only improved the operational efficiency of ammonium,but also shortened the start-up period in low temperature system.C12-HSL also modulated filamentous connective structures and varieties of nitrifying ability microorganisms,including Rhodoferax,Zoogloea,Dechloromonas to participate the process of ammonium removal.However,C12-HSL had no regulatory ability of Flavobacterium at low temperature.The surfaces of suspended carriers were introduced active groups,including carbonyl(C=O),carboxyl(O=C-O)and hydroxyl(C-OH)by PLA nanofiber modification.Modification increased the adhesion amounts of bovine serum albumin(BSA)and fibrinogen(FIB)by 17% and 27%,respectively.The micronano structures of modified carriers surfaces helped microorganisms adhere to carrier,increased the biomass,improved ammonium removal efficiency.The coupling technique,combining with enhanced cell adhesion process and C12-HSL modulation process,further improved ammonium removal.The average removal rate was 82.1% during the stable operational period,the ammonium removal efficiency further improved by coupling effect.Flavobacterium was the dominant genus in the coupling process,and its relative abundance was 45.4%.Simultaneously,coupling technique was also modulated by C12-HSL and Rhodoferax emerged in the system with relative abundance of 2.2%.The Shannon index value of the coupling process was 4.04,and the diversity of the bacterial community was higher than others,so that the relative abundance of the dominant bacteria in the system was reduced,which resulted in fluctuation period of efficiency for 7-9 d.The contents of exogenous addition of AHLs signal molecules were analyzed and the analysis showed that the concentrations of signal molecules were lower than the detection limit when they played a role in modulating the bacterial community structure in the system.C6-HSL(normal temperature),C12-HSL(low temperature)and C12-HSL(initial operational period of coupling process)were all lower than the detection limit.It was presumed that the signal molecules entered into the cells by diffusion.C8-HSL was always detectable in SCBFR system.The concentrations of C8-HSL were 5.67 and 6.98 ng/L at normal temperature,and 5.08 and 5.42 ng/L at low temperature,respectively.The exogenous addition of three kinds of signal molecules could not be a threat to water quality safety.In this study,the enhancement of efficiency and mechanism of the SCBFR system at normal and low temperature were deeply investigated.The modulation modes of three AHLs signal molecules were analyzed and provided a new idea to enhance ammonium removal efficiency at low temperature and improved the situation of bottleneck of the existing biological ammonium removal technology,and broadened the application of biological methods.