Enhanced Efficiency Of Two-Stage Anaerobic Digestion By An Iron Oxide-Zeolite System

Globle challenges of energy shortage and environmental contamination companied with the fast social development and the depletion of the fossil resources,have aroused more and more concerns all over the world.Anaerobic digestion?AD?is an attractive and efficient method to generate sustainable clean energy—the biogas,from the organic wastes such as livestock manure and crop straw,and the environmental,healthy and security stress from inappropriate disposal of these wastes,can be also mitigated.Two-stage AD technology with physically seperated acidification phase and methanogenic phase have shown superior digestion efficiency,especially in treating complex and refractory organic wastes.At present,the degradation-resistant of lignocellulosic biomass?such as crop straw?in hydrolysis/acidogenesis and the weak digestion efficiency in acidification phase and methanogenic phase at lower temperature,are the two main difficulty in AD.Supplementation of the inorganic additive which are cost-effective,easy to obtain and reliable to AD system,is a valuable alternative to enhance biogas production and the conversion efficiency of substrates in AD process.This study developed and prepared a noval inorganic AD additive:iron oxide-zeolite system?IZS?,by coating iron oxide on the surface of natural zeolite.The prepared IZS had abundant pores and channels structure similar to nature zeolite,which could serve as as excellent carrier for microbes,and has more superior specific surface area and cation exchange capacity.The IZS additive was introducted into the co-AD system of cow manure and rice straw.Compared with the AD system supplemented with the mixture of iron oxide and natural zeolite,the IZS addition could facilitate the methane generation more significantly during AD process.It was of the capacity to balance and buffer solution pH.Both of the formation and utilization of VFA could be enhanced by the IZS addition.The TAN and FAN concentration was reduced.The degradation efficiency of lignocellulose and methanogen activity were promoted by the supplementation of IZS.When the IZS was added into the CSTR acidogenic reactor operated under mesophilic and ambient temperature,significantly improved hydrolysis and acidification efficiency could be achieved,eg.at room temperature?25??,with the increase of 14.43-36.43%in sCOD and 40.0-42.9%in TVFA concentration.The composition of VFA was optimized,with higher acetic and lower propionic proportion.The degradation of lignocellulose was significantly enhanced.The IZS which functioned as the carrier for microbes immobilization was coated with iron oxide on its surface,thus it can more effectively serve as the electron transfer media for the cooperative microorganisms.The interspecies electron transfer of the synergetic microbes was accordingly facilitated by the Fe?II?-based dynamic redox cycle,and acetate conversion of propionate and the homoacetogenesis were accelerated.From the analysis of the microbial community structure in the CSTR acidogenic phase,the microbial abundance has been changed notably by the IZS addition,while there was no significant influnce on population species.The predominant organisms were Clostridia and Bacteroidia in CSTR at both mosophilic and ambient temperature.In virtue of the IZS supplementation,the relative abundances of some hydrolytic,acetogenic,cellulolytic bacteria and propionate oxidizing bacteria were increased.Consequently,the hydrolysis,acidogenesis and acetogenesis has been efficiently promoted in the CSTR acidification reactor.Conventinal EGSB often encountered the challenges such as the long star-up time and the difficult cultivation of granular sludge.With the IZS addition,the EGSB in this study could achieve quickly start-up of about 8 days.The volumetic biogas productivity was improved by 59.70-87.93%,and the methane yield increased by 42.17%.The IZS addition could enhance the removal of VFAs and organic matter,and was in favor of the granule sludge formation.When the reactor was operated at 3.0 or 3.5 VS_add·m^-3·d^-1 and with HRT of 10-12 d,higher and stable methane yield could be achieved(eg.CH₄ yield of 333-382 mL·g^-1VS_add).When the temperature decreased as low as 15?,the volumetic biogas productivity could still maintain at the level of around 260 mL·L^-1_reactor·d^-1.From the analysis of the microbial community structure at the four temperatures of 30,25,20 and 15?,it can be found that Clostridia and Bacteroidia still predominated in the reactors.The dominant methanogen in each reactors was the same Methanosaeta,indicating that the acetic-utilizating pathway was the main methanogenesis pathway in all of the four reactors.When the temperature was above 20?,the abudance of hydrogenotrophic methanogen decreased with the lower temperature.The hydrogenotrophic methanogens,Methanoplanus and Methanoculleus,played extremely important role during the methanogenic process at 15?.