The Formation Of Soluble Microbial Products And The Environmental Stress Responding Mechanism In An Activated Sludge System

The activated sludge was one of the most extensive and effective applied wastewater treatment methods,the sharp change of environmental conditions will impact the bacteria metabolism in the activated sludge system,which will alter the properties of soluble microbial products(SMPs),and then threat the processes of advanced wastewater treatment.The present study explored systematacially the change of component and concentration in SMPs(especially for sized fractions)caused by the shock of heavy metals,salinity,temperature and other environmental conditions via the size-exclusion chromatography and fluorescent excitation emission matrix spectroscopy.As the environmental conditions changed,this investigation also qualitatively and quantitatively analyzed the changeable tendency of extracellular polymeric substances(EPS),which were extracted by the heated method;meanwhile,the quantitative relationship between SMPs and EPS in terms of different organic components has been set up.Besides,this study also investigated the impact of environmental conditions on the microbial metabolism and pathways via the assessment of enzymes activity and functional genomics,aim to explore the mechanism of microbial stressful response and the quantitative regulation in producing SMPs.The main conclusions were shown as follows:(1)The toxic mechanism and impact level of different kinds of heavy metals were obviously different,comparing with the shock of Ni2+,Pb2+,Co2+,Mn2+,Zn2+,and Cr3+,the appearance of Hg2+,Ag+,Cu2+,Cr6+,and Cd2+lead to much more serious impact on the expression of functional genes(e.g.,oxyR,katE,katG,emrE,rpoD,and so on);when the shock dosage of Hg2+,Ag+,Cu2+,Cr6+,and Cd2+was at 0.02 mM,the total level of TELI value of all functional genes was around 2.0;(2)The concentration of biopolymer(MW>20 kDa),humic substances(MW≈1 kDa),building blocks(0.3～0.5 kDa),and LMW substances(MW<0.35 kDa)in SMPs has been enhanced by the shock of Hg2+,Cu2+,and Ag+,and their changeable extent depends on the kinds of heavy metals:Hg2+>Ag+>Cu2+;the increase of sized fractions in SMPs were related with the dissolution of EPS,the level of ROS,and the stressful response of functional genes,while the expression of genes ykfG,recN,sulA,yeaE,yedW.flgM,clpB,and yfjG could be used to predict quantitatively the conmpoents and concentrations of SMPs;(3)Under the shock conditions of salinity,the dosage of biopolymer,humic substances,building blocks,and LMW substances in SMPs kept raising with the salinity increased.However,the relative abundance of those four fractions in SMPs didn’t behave obvious variation with the shock dosage of NaCl less than 10 g/L,they obtained 39.78±2.0%,7.48±0.9%,16.6±2.2%,and 30.7±2.1%of the total amount of SMPs,and their total content in SMPs+EPS was within±20%of that in reference test;besides,the relative abundance of sized fractions in SMPs changed a lot with the shock dosage of NaCl at 40 g/L,and those changes in SMPs were closely associated with the level of ROS,the extent of oxidative damage,and the damage extent in cell wall and membrane;(4)The content of sized fractions in SMPs and bio-utilization of SMPs increased under the shock conditions of reduced C/N ratio(1.5～2.2 times);meanwhile,the dosage of biopolymer,humic substances,and building blocks in SMPs also increased under the shock conditions of increased C/N ratio(1.1～1.6 times),but the content of LMW substances and bio-utilization of SMPs reduced with the C/N ratio increased;besides,the content of sized fractions in SMPs was negatively associated with those in EPS with the shock C/N ratio higher than 5,while the abnormal expression of DNA transcription controlled the generation of SMPs with the shock C/N ratio at 2;(5)Both the higher and lower temperature shocks could significantly change the component and concentration of SMPs.Comparing with the reference test(25℃),the generation process of SMPs has been increased significantly under the shock condition of 35℃,and biopolymer mainly contributed to the SMPs(37%～42%);The generation rate of SMPs has been inhibited significantly under the shock condition of 5 and 15℃,while LMW substance became the major components of SMPs(33%～51%);besides,the concentration of biopolymer and humic substances in SMPs was quantitatively related to their dosage in EPS under the temperature shocks,while the content of building blocks and LMW substances in SMPs+EPS was mainly controlled by the ROS level in the activated sludge system.In summary,this investigation systematically explored the sources of SMPs and the mechanism of SMPs formation,variation,and release during the water treatment process,those findings will provide valuable supporting information for the optimization of the water treatment technology.