Study Of Reactor Performance And Microbial Community In Anaerobic Co-digester Of Sludge And Organic Waste

With the rapid development of city construction in China over last decades,the amount of sewage production also increased.More and more wastewater treatment plants(WWTPs)has been come into service to treat sewage.Traditionally,WWTPs adopt activated sludge method to treat domestic sewage,resulting in the dramatic increasing of sewage sludge as the by-productions.About 5 tones sewage sludge(80%moisture contents)will be generated when treating 10 thousand tones sewage.Accordingly,the production of sewage sludge can be increased with the increasing of sewage treatment.Based on the data from China's ministry of environmental protection,a total of 6910 WWTPs have been setup in 2015.The capacity of daily treatment reached 190 million tones and more than 53.2 billion tones sewage has been treated in 2015.That means the production of sewage sludge reached more than 26 million tones this year.Due to the complexity of components,sludge concentrated various toxic and harmful substance,including pathogenic bacteria,parasite,heavy metal,antibiotic,etc.Moreover,the great bulk of sludge posed a significant challenge in the following transportation and treatment due to their high moisture content.If these amount of sludge were not properly handled,it will create direct damage to water body,soil and atmosphere.There were many reports showed that the people living and ecosystem have been influenced by the improper treatment of sewage sludge.Thus,it is urgent to study how to treat sludge effectively.It is reported that using anaerobic digestion(AD)is a promising technology to treatment sewage sludge,due to the reduction of volume and production of renewable energy(methane).Anaerobic digestion process is conducted by various microorganism to convert complex organic materials(including polysaccharide,protein and lipid)into biogas(including carbon dioxygen,methane,hydrogen and water)under anaerobic condition.During this process,the interaction of bacteria and archaea shaped a balanced ecosystem in anaerobic digestion reactor.However,mono digestion of sewage sludge is low efficiency because the low concentration of organic materials in Chinese sewage sludge.Accordingly,this study try to add other organic waste to improve the nutrition composition of feeding substrates,finally enhancing the anaerobic digestion performance.Besides,this study compared the performance of mesophilic digestion(35?)and thermophilic digestion(55 ?),in order to improve anaerobic digestion efficiency.Last,this study used next generation high-throughput sequencing technology to reveal the composition and change of microbial community in anaerobic digestion reactors,a further explanation of potential digestion mechanism was discussed.Details contents are as followed:(1)Due to the characteristic of high sands concertation and low organic material contents,the carbon to nitrogen ratio in Chinese sludge is relative low,which could not meet the requirement for effective anaerobic digestion.Carbon to nitrogen ratio influenced methane production by the growth of microorganisms and metabolic process.A suitable carbon to nitrogen ratio can improve the degradation of feeding substrates during methane production.Thus,in order to adjust the imbalance of sludge in mono-digestion reactor,this study selected 3 kinds of organic waste(pig manure,algae and fruit vegetable waste)to codigestion with sewage sludge.All experiments were conducted by one-phase sequencing digestion.The performance of anaerobic digestion was evaluated by methane daily production,methane accumulative production,COD removal rate,pH,VS concentration,VFA concentration and ammonia concentration.Results showed methane accumulative production in pig manure and sludge codigestion group reached 270 L ·(kg VS)-1,following algae(230 L ·(kg VS)-1),fruit vegetable waste(180 L ·(kg VS)-1)and sludge mono digestion(140 L ·(kg VS)-1).As for the methane production,the order of recommended codigestion substrates is pig manure>algae>fruit vegetable waste.Moreover,microbial community has a suitable pH condition during digestion process,which was sensitive to the change of pH value.Generally,the optimal pH range in one-phase digestion reactor is 6.8-7.2.Over acid condition(pH<6.0)or over alkalization condition(pH>8.0)can both inhibit microbial activity,finally digestion performance is restricted.This study found that pH in fruit vegetable waste codigestion group dropped to ca.5.0 after digestion process,which means a significant accumulatio of organic acid was observed in the reactor.While the pH maintained ca.7.0 in pig manure codigestion group,which means no obvious acid/ammonia inhibition was observed in the group.Moreover,COD removal rates in pig manure codigestion reactor reached the highest 58%,when compared with sludge mono-digestion of 32%.These results indicated that the introduction of pig manure into sludge anaerobic digestion can improve reactor efficiency and keep process stable in some degree.(2)As indicated above,the optimal mixture components are pig manure and sludge.Based on the result,this study set 5 different mixture ratios(pig manure:sludge = 0:1,2:1,1:1,1:2,1:0,based on VS concentration)to further compare the influence of mix ratio on the digestion performance.The nutrition balance can be largely effected by different mixture ratio,a relative high/low concertation of feeding substrate may inhibit microbial activity.The total VS concertation was set at 40 g VS·L"1 in this study,each group received 200 mL inoculums.Results showed that of methane daily production reached the peak value on day 23(30.6 L ·(kg VS)-1),day 15(26.0 L ·(kg VS)-1)and day 11(25.0 L ·(kg VS)-1),in 3 codigestion groups(2:1,1:1 and 1:2).When compared with pig manure mono-digestion group,the peak value day delayed 15 days,7 days and 3 days.As for the sludge mono-digestion group,the methane daily production was relative low and only 1 peak value was observed on day 10(13.3 L ·(kg VS)-1).After 50 days digestion process,the accumulative methane production in 3 codigestion groups(2:1,1:1 and 1:2)reached 684,515 and 494 L ·(kg VS)-1,which were improved by 50-120%when compared with sludge mono-digestion.Moreover,when the mixture ratio of pig manure and sludge is 2:1,reactor showed the highest VS removal rate(ca.63.1%).Accordingly,the mixture ratio of pig manure and sludge of 2:1 is highly recommended for engineering application.(3)Traditional anaerobic digestion technology was performed at mesophilic condition(35?)or thermophilic condition(55 ?).Each function microorganism has a suitable temperature range for their growth in an anaerobic digestion reactor.Generally,thermophilic digestion showed a higher digestion efficiency than mesophilic condition,but also showed an unstable state along with higher cost.This study constructed two anaerobic digestion reactors by water-bath to investigate the influence of different temperature on digestion process.Results showed that methane daily production reached 18.1 L ·(kg VS)-1 in mesophilic condition,while thermophilic condition reached 37.4 L ·(kg VS)-1.The accumulative methane production reached 142-271 L ·(kg VS)-1 and 175-363 L ·(kg VS)-1 in mesophilic and thermophilic condition,respectively.Results indicated thermophilic digestion improve methane production by 28-37%,together with COD removal rates by 8-10%.Meanwhile,VS concentration was higher in thermophilic condition.VS concentration in thermophilic condition reached 38-65%when compared with mesophilic condition of 32-58%.Results suggested that anaerobic digestion under thermophilic condition improved the degradation of feeding substrates and a higher organic material remove and methane production were obtained.Moreover,ammonia concentration reduced by 33%in thermophilic condition and never exceed 800 mg/L,which means thermophilic digestion can overcome the ammonia inhibition.Furthermore,digestion temperature has no influence on the reactor pH values.After 50 days digestion process,pH in mesophilic condition and thermophilic condition showed limit change.(4)Anaerobic digestion is a microbial-driven process.The effective and stable performance of digestion reactors rely on the interaction of various function microbes.However,current study on the environmental function of microbial community in anaerobic digestion reactor is still considered as a "black box",due to the limited research method.Traditional method,such as PCR-DGGE cannot reveal the composition and dynamic change of microbial community in depth.This study used the next generation high-throughput sequencing technology,comprehensively revealed the microbial composition and dynamic change during anaerobic digestion process.Also,mutilative analysis were combined to explain the potential environmental function of dominant/key species.Results showed that microbial diversity in 3 codigestion groups(2:1,1:1 and 1:2)was much higher that the mono-digestion groups(1:0 or 0:1),which means adding pig manure into digestion systems can improve microbial diversity.This can be explained by the better synergy existed in the codigestion reactors,resulting a higher microbial activity and finally organic material degradation and methane production were improved.Based on the results of operational taxonomic units(OTUs)annotation,the identified bacteria/archaea only accounted for 30-60%,which means a large number of unclassified species existed in sludge samples.The codigestion groups shared a similar core microbial community but with significant dynamic change.These core members including Bacteroides?Clostridium?Lactobacillus?Ruminococcus and Eubacterium.Among them,Bacteroides was the most abundant member with relative abundance of 6.99-12.30%,which suggested the critical role in maintain the stability of anaerobic digestion.In the archaea kingdom,the relative abundance of Methanosaeta and Methanosarcina was higher in pig manure mono-digestion reactor(29.23%and 8.21%).With the addition of sludge,the relative abundance of Methanosaeta and Methanosarcina was decreased.On the contrary,Methanobacterium and Methanobrevibacter maintained stable during whole process,which means these 2 members were not sensitive to the change of feeding substrates.Redundancy analysis showed that pH,VFA concentration,total alkalinity and accumulative methane production can explain 50.0%environmental variation.pH and VFA concentration showed the most significant impacts on digestion process.A positive correlation between pH and accumulative methane production was observed.pH and accumulative methane production were also found to be correlated with the Methanosaeta,Methanosarcina and Methanobacterium.This study indicated that adding pig manure to codigest with sewage sludge can improve reactor efficiency.The treatment of 2 or more organic waste by anaerobic digestion showed significant social benefit and economic benefit.The results of this study can bring additional information into engineering application of anaerobic digestion.