Study On The Effects And Mechanisms Of Zeolite Regulation On Methane Production From Anaerobic Digestion Of Waste Activated Sludge

With the rapid development of urbanization and industrialization,the increased quality of waste activated sludge produced from sewage treatment plant has attracted more and more attention.Waste activated sludge is the main by-product of sewage treatment plant,which has the characteristics of high yield and high organic content.Organic matter in the sludge can be converted to methane after anaerobic digestion,which can be returned to the sewage treatment plant in the form of electricity,heat energy,and other energy sources.It is in line with the strategic requirement of"carbon neutralization"and"carbon peak"goal of China.However,low methane production restricts application of anaerobic digestion technology in sludge reclamation.Adding inorganic additives to sludge anaerobic digestion system can improve the performance of anaerobic digestion.In this study,natural zeolite,a green and low-cost inorganic material,was selected as the additive to study its promotion efficiency and mechanism of sludge anaerobic digestion process.In addition,the zeolite was modified to further improve the efficiency of its positive effect on methane production.Hence,it was to provide some theoretical basis and technical supports for methane production enhancement development from sludge anaerobic digestion.The sludge from the secondary sedimentation tank of a sewage treatment plant in Xi’an was used in this study.Firstly,the effects of natural zeolite dosage and particle size on sludge anaerobic digestion were investigated.From results of characterization of zeolite,it was determined that the natural zeolite used was heulandite with enriched Si-O and Al-O bonds.By designing a single factor experiment,the optimal dosage was determined to be 0.1 g/gVSS,and the optimal particle size range was 0.2～0.5 mm.Under the optimal parameters,the cumulative methane production was 186.75±7.62m L/g VSS,which was 22.08%higher than that of the blank group.The addition of natural zeolite promoted the decrease of soluble polysaccharides and proteins in the early stage,improved the utilization of organic matter by microorganisms,and thus accelerated the accumulation and transformation of short chain fatty acids.Therefore,natural zeolite could improve methane production performance by optimizing the transfer and conversion efficiencies of organic matters between microorganisms.Secondly,taking each stage of anaerobic digestion as the research object,the promotion mechanism of natural zeolite on methane production was investigated.During the dissolution stage,natural zeolite adsorbed organic substances through its own porous structure;During the hydrolysis stage,the consumption rate of proteins and polysaccharides in the zeolite group was faster than that in the blank group.The cumulative production of hydrogen was 59.23±4.29 mL,which was 27.35%higher than that in the blank group.It further indicated that natural zeolite also promoted the sludge hydrolysis stage;During the acidification stage,the short-chain fatty acids accumulation performance in the zeolite group was 375±7 mg COD/L,which was24.22%higher than that of the blank group,indicating that natural zeolite also had a promoting effect on the acidification stage;During the methanogenesis stage,the maximum daily methane production in the zeolite group was 222.26±3.95 m L,which was 9.3%higher than that of the blank group.The activity of coenzyme F₄₂₀ was also28%higher than that of the blank group,suggesting that natural zeolite could improve the activity of methanogenic enzymes,thereby increasing methane production;The microbial community structure indicated that natural zeolite enriched functional bacteria that could promote hydrolysis and acidification stages,such as Clostridium＿sensu＿stricto,Bacteroidetes＿vadin HA17,g＿＿norank＿f＿＿JG30-KF-CM45.It also improved the dominant methanogens,such as Methanosarcina and Methanosaeta.It strengthened the methanogenic pathway of hydrogenotrophic and aceticlastic methanogenesis.Finally,in order to further improve effects of natural zeolite on sludge anaerobic digestion,magnetic zeolite（MZ）was synthesized.The successful loading of magnetite on the surface of natural zeolite was confirmed by scanning electron microscopy,fourier transform infrared spectroscopy,energy dispersive X-ray spectroscopy,and X-ray diffraction.The mass fraction of Fe reached 22.5%.Compared to the blank group,the zeolite group,the magnetite group,and the mixed group,the methane production in the MZ group increased by 37.37%,15.4%,10.69%,and 22.57%,indicating that MZ promoted methane production efficiency.It was found that MZ further accelerated the consumption of soluble organic matter and promoted the transformation of LB-EPS and TB-EPS organic matter into dissolved state.MZ improved the bioavailability of organic matter by microorganisms.MZ promoted the accumulation of short-chain fatty acids during acidification stage,reaching 186±6 mg COD/g VSS.The activities of coenzyme F₄₂₀ and cytochrome C increased by 29.83%and 33.24%compared to that in the blank group,suggesting that MZ improved the activity of methanogens.Analyzing the microbial community,MZ enriched Methanosaeta and electroactive bacteria such as Clostridium＿sensu＿stricto＿13.And Methanosarcina is the dominant archaea.Electroactive bacteria might establish direct interspecies electron transfer with these two methanogens through MZ.In summary,natural zeolite has a positive effect in promoting sludge anaerobic digestion efficiency.Loading magnetite on its surface can further improve methane production efficiency from sludge.Considering the low cost and environmentally friendly of natural zeolite,it has broad application prospects in improving sludge energy conversion efficiency as an exogenous additive,and this strategy meets the development requirements of green and sustainable sludge treatment.