The Response Of Aerobic Granulation And Microbial Community Structure Shifts To Exogenous Nitrate

Aerobic granular sludge（AGS）is a promising technology in treating wastewater.It consumes less footprint area,energy with lower waste sludge production compared with conventional activated sludge.The granulation of activated sludge（AS）,as start-up stage of AGS,is crucial in its application.And aerobic granulation is a complicated process which could be influenced by various environmental parameters like wastewater quality,temperature,types of carbon source and heavy metal ions,making it a hot issue in wastewater treatment.Nitrate,which exists wildly in waterbody,might have effects on aerobic granulation,but related research is limited.This paper aimed at exploring the influence of exogenous nitrate on the cultivation of AGS and studying the response of aerobic granulation and shifts of microbial community to exogenous nitrate by operating four AGS reactors with different dose of exogenous nitrate（One was cultivated without nitrate as control group）.The influence of exogenous nitrate on the granulation and pollutants removal ability was investigated by monitoring the effluent quality and sludge index.And the mechanism of influence of exogenous nitrate on AGS properties was investigated by assessing the concentration of EPS,c-di-GMP,elements distribution as well as strength of AGS.Besides,the microbial community of AGS at different time point was analyzed to study the effects of exogenous nitrate on microbial shifts.The monitoring of long-term operation of AGS reactors implied that exogenous nitrate had two-sided effects on aerobic granulation.Initial granulation was stimulated by exogenous nitrate with an average size of 460±10μm,100μm larger than control group.But long-term feeding brought about instabilities,which resulted in disintegration of AGS finally.In the end of operation,the average size of control group reached 809±10μm while that of experimental group was below 500μm.Simultaneously,exogenous nitrate accelerated nitrogen removal.The maximum TN removal efficiency of experimental group was 89.9%,85.29%,83.41%respectively while it was 81.35%of control group.And the simultaneous nitrification and denitrification efficiency of AGS was enhanced from 0.924 to 0.984 with exogenous nitrate,which facilitated nitrogen removal.The assessment of AGS properties showed that in initial operation,extracellular protein in experimental group was higher than that of control group.At this stage,the correlation between protein production and operation time was 0.982,0.911,0.936respectively while that of control group was 0.879.But long-term feeding of exogenous nitrate inhibited EPS production.In the end of operation,EPS production of control group increased to 120.45 mg/g VSS,much higher than that of experimental group with55.34,102,100.9 mg/g VSS.Exogenous nitrate promoted the retention of calcium,which was believed to stimulated initial granulation in experimental group.Exogenous nitrate inhibited the increase rate of c-di-GMP in experimental group and the enzyme activity of phosphodiesterases（PDEs）in control group was always higher.The mechanism of impact of nitrate in aerobic granulation was concluded:In initial granulation,more calcium was retained in reactors due to alkaline aquatic environment attributing to enhanced denitrification to form nucleus,which induced the attachment of microorganisms to accelerate granulation with nitrate feeding.In this time,the interaction of nucleus was more important than the regulation of c-di-GMP.Until the granules became bigger,the induction of nucleus became weaker,the regulation of c-di-GMP started to play a significant role in granulation.More intermediate products nitric oxide（NO）was produced in enhanced denitrification,which inhibited the increase rate of c-di-GMP.AGS in control group got continuous increase with higher level while stagnate and disintegration of AGS appeared in experimental group with low level of c-di-GMP.The results of analysis of microbial structure demonstrated exogenous nitrate feeding and operation time both played important roles in structuring microbial community.And exogenous nitrate feeding accelerated microbial shifts in AGS to varying degrees due to different dose.In the 1^st month,exogenous nitrate accelerated the enrichment of Micropruina（p<0.01）while it was vacant in control group.In the 3^rdmonth,the average size of control group was higher than experimental group.Micropruina was observed to increase to 10.64%（p<0.01）in control group.Therefore,denitrifying glycogen accumulation microorganism Micropruina was thought to be close correlated with aerobic granulation.At the same time,the growth of microorganisms owning denitrifying ability like Thauera,Flavobacterium was enhanced by exogenous nitrate,improving the nitrogen removal ability.On the flipside,long-term feeding inhibited the growth of EPS function bacterium like Zoogloea,Acidovorax,leading to the disintegration of AGS.Besides,nitrate feeding accelerated amino acid and carbohydrate metabolism.