Study On Lignin Wastewater Treatment By Direct Current Enhanced Activated Sludge Approach

Lignin and its degradation products,which are one of the important components in papermaking wastewater,mainly consist of complex aromatic rings.It is difficult to be treated by traditional biological technology:long time for degradation and low removal rate.As a promising technology,electro-microbial system(EMS)has been widely used in the field of refractory organic wastewater treatment(papermaking wastewater,aromatic substances,phenolic substances,etc.).However,few studies have been reported on the degradation of lignin and relevant microbial community in EMS.In this study,effect of direct current(DC)(current intensity,DC application modes,electrode materials,etc.)on the removal efficiency of lignin,electrochemical characteristics of electrode biofilms,sludge characteristics,phospholipid fatty acids(PLFA)compositions and microbial community were systematically studied.Results are as follows:(1)Effect of current intensity on the efficiency,sludge characteristics,PLFA compositions and microbial community in lignin wastewater treatment by activated sludge were studied.The highest lignin removal(30.19%±0.47%)in Bio-electrode Activated Sludge Reactor(R3,activated sludge,I=10-60 mA)was achieved at 30 mA and was 2.01 and 1.46 times that of Activated Sludge Reactor(R1,activated sludge only,no current)and Bio-electrode Reactor(R2,I=10-60 mA,no activated sludge),respectively.The viability of suspended sludge was not affected by the increase of the currents(p>0.05)and LiP enzyme activity appeared to be irrelevant to lignin removal.PLFA analysis revealed that high currents(40?60 mA)led to the decrease of C15:0 anteiso content and increase of long chain fatty acids(C19,C20)in comparison with low currents(10?30 mA).High-throughput 16S rRNA sequencing verified that the relative abundance of lignin-degrading bacteria like Flavobacterium,Pseudomonas and Janthinobacterium were enriched in R3 under applied current.For microorganisms on electrodes,a denitrifier-Methyloversatilis dominated the cathodic biofilm of R2.Besides,more genera possessing the electron transfer ability were detected in the anodic biofilm of R2(Pseudomonas,Ralstonia).RDA analysis indicated that the relative abundance of Mycobacterium(one kind of lignin-degrading aerobic bacteria)in activated sludge were significantly positively correlated with current I(p<0.05)and negatively correlated with UFA(p<0.05).(2)Effect of DC application modes on the efficiency,electrochemical characteristics of electrode biofilms,sludge characteristics,PLFA compositions and microbial community in lignin wastewater treatment by activated sludge were studied.The Bio-electrode Activated Sludge Reactors with continuous DC application mode(continuous ON)and intermittent DC application mode(12 h-ON/12 h-OFF,2 h-ON/2 h-OFF)all had higher lignin removals,which were almost two times that of the Activated Sludge Reactor(control,no current).Furthermore,2 h-ON/2 h-OFF mode exhibited the smallest molecule size in effluent(76 Da,67.63%)and the lowest ohmic resistance(Rs)of electrode biofilms(97 ?,94 ?).The concentration of dehydrogenase activity(DHA),electron transfer system(ETS),and adenosine triphosphate(ATP)in IR2h were the highest,which were 1.48,1.28 and 1.47 times that of the control reactor.The viability of activated sludge was not affected by different DC application modes.2 h-ON/2 h-OFF mode had highest content of C16:0 and the relative abundance of genera Pseudoxanthomonas(lignin-degrading bacteria)and Micropruina was the highest in electrode biofilms.Continuous ON mode increased cell membrane fluidity by increasing the synthesis of C15:0 anteiso and enriched lignin-degrading bacteria Mycobacterium.RDA analysis indicated that the relative abundance of Micropruina showed high positive correlation with COD removal(p<0.05)and the relative abundance of Gp6 showed high negative correlation with lignin removal(p<0.05).(3)Effect of electrode materials on the efficiency,electrochemical characteristics of electrode biofilms,sludge characteristics,PLFA compositions and microbial community in lignin wastewater treatment by activated sludge were studied.Average lignin removals in the Bio-electrode Activated Sludge Reactors with different electrode materials were:nickel foam electrodes reactor(72.44%±2.58%)>graphite electrodes reactor(41.46%±1.40%)>titanium electrodes reactor(27.34%±1.85%)>stainless steel electrodes reactor(26.63%±1.29%).However,the viability of activated sludge in the nickel foam electrodes reactor was only 43.43%±2.70%,which was 55.73%of the graphite electrodes reactor.The concentration of nickel ion in the effluent of nickel foam electrodes reactor increased continuously(9.26?50.73 mg·L-1)during the operation time.Adsorption by nickel hydroxide in sludge was one of the main removal ways of lignin.The graphite electrodes reactor had the smallest electrode resistance(163 ?,130?)and the hightest concentration of ETS,ATP and LiP.Titanium and stainless steel electrodes reactors had higher SOUR and DHA concentration.Microbial community structure was affected by electrode materials.Lignin-degrading bacteria,Microbacterium,Microlunatus and Mycobacterium were enriched in the graphite,titanium and stainless steel electrodes reactors.The dominant genera were Dechloromonas and Sediminibacterium in the nickel foam electrodes reactor.For microorganisms on electrodes,electrode biofilms of graphite had more bacteria capable of degrading lignin,including Microlunatus,Hydrogenophaga,Microbacterium and Mycobacterium.The genera of Tessaracoccus,Methylobacterium and Cupriavidus enriched on the electrode biofilms of nickel foam not only had the ability to degrade lignin but also tolerated high concentration of nickel ions.RDA analysis showed that the nickel foam electrodes reactor was positively correlated with COD and lignin removals,but negatively correlated with activity index.(4)The enhanced effect of DC field,Pearson correlation of physiological characteristics and PLFA composition were studied and microbial network based on the results of high-throughput sequencing analysis of activated sludge and electrode microorganism was constructed.Considering the efficiency of lignin wastewater treatment,electrochemical characteristics(resistance)of electrodes and electron transfer rate of microorganisms,the Bio-electrode Activated Sludge Reactor with applied current of 30 mA,2 h-ON/2 h-OFF mode,and graphite electrodes was preferred.Although the reactor with nickel foam electrodes had the highest COD and lignin removal rates,the fast corrosion rate of nickel foam electrodes and high nickel ion concentration in the effluent(50 mg·L-1)posed a hazard to the environment.It need further study to be optimized.Under applied DC current,the viability and SOUR showed high negative correlation with C12-SFA,C12-AFA and C16-UFA(p<0.05).DHA showed high positive correlation with C20-UFA,and C18-SFA showed high positive correlation with ATP(p<0.05).Network analysis showed that Zoogloea was the central node in the microbial network of activated sludge and played an important role in the biodegradation of benzene.Bradyrhizobium,Thauera,and Brevundimonas were identified as key species in the microbial network of cathodic biofilm,and had the ability to degrade aromatic compounds.Zoogloea,Ignavibacterium,Pseudonocardia,Porphyrobacter,Novosphingobium,Thauera,Sphingobium were identified as key species in the microbial network of anodic biofilm,in which both Ignavibacterium and Sphingobium were reported as electroactive bacteria.There was a more complex and close relationship in the microbial populations of the anodic biofilm.Microbes of the cathodic biofilm interacted more closely than that of activated sludge.The microbial community in the cathodic biofilm had the lowest niche differentiation.DC field had the greatest impact on the microbial community of activated sludge and the smallest impact on the microbial community of anodic biofilm.