Influence And Mechanism Of Operating Conditions On The Microbial Social Behavior In A UASB

During the context of“carbon peaking and carbon neutrality”,anaerobic digestion(AD)has the advantage of recovering methane,which promotes the wide application of AD in organic wastewater treatment.It is important for the efficient and stable operation of AD process to maintain the metabolic balance of acidogenic fermentation bacteria,acetogenic bacteria and methanogens.Guided by quorum sensing(QS)theory and based on the start-up and operation of up-flow anaerobic sludge bed(UASB),this paper systematically studied the effects of engineering control factors such as organic loading rate(OLR),reflux ratio,alkalinity,C/N ratio and activated carbon on the efficiency of AD system,characteristics of anaerobic granular sludge(An GS),microbial community structure and secretion of N-acyl-homoserine lactones(AHLs).The influencing mechanism of regulatory factors on microbial social behaviors was further analyzed from the perspective of QS.This paper aims to provide theoretical guidance and technical support for the precise control and efficient operation of AD system.The study showed that regulating OLR by reducing and then gradually increasing chemical oxygen demand(COD)could quickly recover the treatment efficiency of UASB under loading shock.Methanosaeta played a key role in the recovery process.When OLR was controlled at 11.0 kg COD/(m~3·d),the average COD removal rate,biogas production and methane content(volume percentage)reached 80%,14.1 L/d and 63.4%,respectively.The results showed that C4-HSL and C14-HSL could mediate and increase the polysaccharide content of extracellular polymeric substances(EPS),which further enhanced the aggregation of microorganisms.C4-HSL,C8-HSL and 3-oxo C14-HSL could promote the growth and metabolism of acidogenic fermentation bacteria.Correlation analysis of functional genes showed that AHLs-mediated QS system was more likely to regulate the aggregation and growth metabolism of acidogenic fermentation bacteria.It was found that an appropriate reflux ratio(200%)was favorable to forming more compact granular sludge and enriching methanogens.During the regulation period of reflux ratio(50%～500%),C4-HSL was always the most abundant signal molecule,and its content was in the range of 1.33～5.33 ng/g MLVSS.C4-HSL and C8-HSL jointly promoted protein synthesis of EPS,which could maintain the excellent characters of granular sludge.Both 3-oxo C6-HSL and 3-oxo C14-HSL enhanced the growth and metabolism of methanogens.When the influent alkalinity of UASB decreased from 2800 mg/L to 700 mg/L in stages,the COD removal rate remained stable at about 80.9%.When the influent alkalinity was 1800 mg/L,the protein content of EPS and the abundance of Methanosaeta were both high.Biogas production and methane content reached 14.6L/d and 73.4%,respectively,which were also the highest.Based on the analysis of the co-occurrence network,Mesotoga,Sulfurospirillum and Methanoregula were the key microbes to maintain matter transformation and the balance of metabolism in the community.AHLs-mediated QS coordinated the growth and metabolic balance of the microbial community by regulating EPS secretion and the expression of key genes in metabolic pathways.When OLR was kept at 11.0 kg COD/(m~3·d),reducing influent C/N stage by stage(25～1)showed that the average COD removal rate was always maintained above 79.0%under the condition of C/N not less than 5.C10-HSL played an outstanding role in enhancing the growth and metabolism of aceticlastic methanogens.The lower influent C/N had a significant impact on the treatment efficiency of UASB.When C/N was only 1,the COD removal rate decreased to 71.0%significantly.At this time,the microbes in the system secreted more loosely bound EPS,resulting in the disintegration of a large amount of granular sludge.The relative abundance of Methanosaeta also decreased significantly and the microbial community structure was seriously disturbed.During the operation period of influent C/N,C4-HSL and C14-HSL played a role in accelerating microbial aggregation and slowing down the disintegration process of An GS.The start-up process of the UASB could be accelerated and the treatment efficiency could also be strengthened by using powdered activated carbon as the medium(5 g/L).With the sludge inoculation of 6.40 g MLVSS/L and OLR of 2.0 kg COD/(m~3·d),UASB could reach a stable state after 20 d and remained stable when OLR increased to 5.0 kg COD/(m~3·d).The average COD removal rate and methane content were still kept at 80.7%and 72.0%,respectively,when OLR was 5.0 kg COD/(m~3·d).The addition of activated carbon significantly stimulated the secretion of protein in EPS and promoted the formation of An GS.After 80 days of continuous operation,the average particle size of An GS increased from 57μm to 356μm.At the initial stage of OLR of 2.0 kg COD/(m~3·d),the growth and enrichment of Methanosaeta were greatly enhanced by activated carbon.In the stage of higher OLR,though the relative abundance of acidogenic fermentation bacteria and hydrogenotrophic methanogens kept increasing,the abundance of aceticlastic methanogens remained at a high level.It was found that C6-HSL and C14-HSL inhibited the content of protein in tightly bound EPS and further had negative effect on the formation of An GS.However,C8-HSL,3-oxo C8-HSL and 3-oxo C14-HSL played a role in enhancing microbial aggregation.During the increase of OLR,the growth and metabolism of Methanosaeta and Methanobacterium mediated by C4-HSL,C6-HSL and C14-HSL maintained the methane fermentation efficiency of the UASB.These results provided theoretical guidance for regulating AD processes by quorum sensing.