Hydrogen And Methane Production By Co-digestion Of Cornstalk And Sewage Sludge In The Two-stage Fermentation Process

Sewage sludge and cornstalk are not only pollutants and solid wastes, but alsoavailable biomass resources. Sewage sludge and cornstalk can be utilized to producebioenergy by anaerobic digestion. However, the conventional anaerobic digestiontechnology does not collect hydrogen, with methane as the sole biogas product.Hydrogen is a clean energy, producing only water upon combustion. Besides,hydrogen has an energy yield of142.35kJ/g, which is2.6times of the methane’s.However, hydrogen fermentation from organic substrates produces great amount ofbyproducts such as volatile fatty acids, which can be used to produce methane,improving the energy recovery from biomass. Therefore, the two-stage fermentationprocess will benefit the total energy recovery and the utilization of renewablebiomasses. The fermentation of single substrate has some problems, such as improperC/N and inadequate nutrients. The co-digestion of sewage sludge and cornstalk canadjust the nutrients, improve buffer capacity and improve the performance ofanaerobic fermentation.The study first compared the pretreatment methods of cornstalk. The cornstalkwas pretreated by HCl, NaOH, HCl plus enzyme and NaOH plus enzyme, respectively.Reducing sugars, change of cornstalk components and mass loss as well as hydrogenyield were investigated. Results showed the optimal pretreatment method wastoimmerge in2.0%NaOH at50℃for48h.Furthermore, batch experiments were conducted to investigate the effect ofdifferent mixing ratio of sewage sludge and cornstalk on biogas and energy yields inthe two-stage fermentation process. The maximum biogas yields were obtained attotal solid ratio of2:1for cornstalk and sewage sludge. At this ratio, hydrogen andmethane yields were13.4mL/g-VS and172.6mL/g-VS, respectively, which wererespectively around13and1.4times of those for single sewage sludge fermentation.In the study of co-digestion of sewage sludge and cornstalk in the two-stagefermentation process at different initial pH, the results showed that at initial pH of11.5, the total energy production of9.36kJ/g-VS was highest. At initial pH of5.5, theproportion of hydrogen energy was highest, and the total energy production was9.16kJ/g-VS. According to the experiment results, the optimal initial pH for hydrogenproduction is5.5. And the energy produced in the two-stage fermentation process was1.4~1.6times of that produced in the single-stage methane fermentation. Continuous experiments were further conducted to investigate the operationconditions of the two-stage fermentation process in a Continuous Stirred Tank Ractor(CSTR) using the mixture of sewage sludge and cornstalk as substrates. The hydrogenfermentation stage was operated at mesophilic (37℃) and thermophilic (55℃)temperatures, respectively, and the methane fermentation stage was operated atmesophilic (37℃) temperature. Better operation performance was observed when thehydrogen and methane fermentation temperatures were55℃and37℃, respectively.At the retention time of1d and5d for hydrogen fermentation and methanefermentation respectively, the average hydrogen and methane production rates were22.5±11.0mL/L/d and210.4±83.6mL/L/d, respectively, and the average hydrogenand methane yields were0.9±0.4mL/g-VS and42.1±16.7mL/g-VS, respectively.