Enhancing Co-generation Of Fermentative Biohvdrogen And Biomethane From High-Moisture Biomass Through Hydrothermal Degradation And Carbon/Nitrogen Adjustment

The accumulation of high-moisture biomass wastes exerts considerable pressure on the environment.Biological hydrogen and methane co-production through anaerobic fermentation can serve as a promising strategy for energy utilization of these high-moisture biomass wastes.In this study,the competing reactions during the fermentation of typical amino acids derived from proteins in biomass were investigated.Hydrothermal treatment was employed to degrade algal biomass and food wastes.Carbon/nitrogen(C/N)adjustment through mixing different biomass feedstocks was used to maintain the balance of microbial metabolism.The fermentative hydrogen and methane co-production from biomass was significantly improved through hydrothermal treatment and carbon/nitrogen adjustment.The mechanisms of fermentative hydrogen production from typical amino acids obtained by protein degradation were investigated.The competing reactions during the dark fermentation of amino acids were revealed.The amino acids were degraded into abundant soluble metabolic products during dark fermentation,resulting in the high carbon conversion efficiencies of 85.3%and 94.1%for alanine and serine,respectively.Through combined dark and photo fermentation,the hydrogen yields of alanine and serine increased to 418.6 mL/g and 270.2 mL/g,corresponding to the energy conversion efficiencies(ECEs)of 25.1%and 21.2%,respectively.The biomass components of cyanobacterium Arthrospira platensis were regulated through nutrient adjustments.Relative nitrogen starvation with continuous 15%(v/v)CO2 bubbling and increased osmotic pressure by NaCl addition led to the accumulation of intracellular carbohydrates.Hydrothermal degradation of the harvested A.platensis was then employed to facilitate the co-production of fermentative hydrogen and methane.Microstructural characterization analyses revealed that the A.platensis cells were fractured section by section after hydrothermal treatment,and the released intracellular carbohydrates were degraded into reducing sugars with the assistance of dilute sulfuric acid.Through combined dark and photo fermentation,the hydrogen yield of A.platensis significantly increased to 429 mL/gVS.The overall energy yield was boosted to 10.51 kJ/gVS after dark methane fermentation.The effects of various treatments(e.g.,hydrothermal treatment,hydrothermal treatment with dilute acid,enzymolysis,etc.)on the degradation of macroalgae biomass and the subsequent co-production of fermentative hydrogen and methane were investigated.Scanning electron micrographs demonstrated that macroalgae was depolymerized through hydrothermal treatment.Lots of pores and debris were generated,whilst the undegradable components remained as filamentous skeletons.The presence of dilute acid during hydrothermal treatment catalyzed the degradation of carbohydrates derived from macroalgae,resulting in a 2.48-fold higher yield of carbohydrate monomers than the untreated macroalgae biomass.The hydrogen yield of macroalgae degraded by the dilute acid assisted hydrothermal treatment was 60.8%higher than that of the untreated one,whilst the byproducts such as 5-hydroxymethylfurfural exhibited strong inhibitory effects on the subsequent methane fermentation.The hydrothermal degradation of macroalgae facilitated the co-generation of hydrogen and methane through anaerobic fermentation,resulting in an overall ECE of 66.0%.The protein-rich microalgae and carbohydrate-rich macroalgae were mixed to adjust the carbon/nitrogen ratio to be 20:1 for a two-stage process combining hydrogen and methane fermentation.Co-fermentation of microalgae and macroalgae facilitated hydrolysis and acidogenesis,resulting in the hydrogen yields 21.6-27.1%higher than the weighted average ones calculated from mono-fermentation.Through the second stage of methane co-fermentation,the overall ECEs of mixed algal biomass remarkedly increased to 57.1-76.6%.A 32-week long two-stage continuous fermentative hydrogen and methane co-production using mixed algal biomass at a C/N ratio of 20 was also established.When the overall hydraulic retention time(HRT)was fixed at a short time of]6 days,the highest specific hydrogen yield of 55.3 mL/gVS was obtained at an organic loading rate(OLR)of 6.0 gVS/L/d in the H2 reactor,whilst a specific methane yield of 245.0 mL/gVS was achieved at a corresponding OLR of 2.0 gVS/L/d in the second-stage CH4 reactor.At these loading rates,the two-stage continuous system secured an overall energy yield of 9.4 kJ/gVS.The two-stage system outperformed the one-stage system in both energy return and process stability.Hydrothermal treatment was employed to degrade food waste for improving the fermentative hydrogen and methane co-production.The variation trends of degradation products under various hydrothermal conditions were investigated.When the hydrothermal temperature increased,the large-molecular-weight proteins were gradually degraded into soluble proteins,polypeptides,and amino acids,whilst the soluble carbohydrate yield increased to a peak at 140 ? and then decreased due to the enhanced degradation of carbohydrate monomers and Maillard reactions between reducing sugars and amino acids.The hydrogen and methane yields from hydrothermally degraded food waste under the optimum condition(140 ?,20 min)through two-stage fermentation were 43.0 mL/gVS and 511.6 mL/gVS,respectively,resulting in an overall ECE of 78.6%which was higher by 31.7%than that of untreated one.The physiochemical properties of food waste and sewage sludge were identified to investigate the effects on hydrogen and methane production through co-fermentation.The mixture of food waste(rich in carbohydrates and lipids)and sewage sludge(rich in proteins and minerals)balanced the C/N ratio and promoted the contact between substrates and microbes,thus contributing to the enhanced degradation of organics and resulting in a hydrogen yield that was 49.9%higher than the weighted average calculated from mono-fermentation of food waste and sewage sludge.The energy yield of the mixed food waste and sewage sludge significantly increased to 11.3 kJ/gVS in the two-stage fermentative hydrogen and methane co-production.A 21-week long continuous fermentation using food waste was performed.As compared with one-stage methane production,the two-stage hydrogen and methane co-production offered better buffering capacity and stability when increasing the loading by shortening the HRT.When the hydrothermally degraded food waste was used as feedstock,the steady second-stage methane yields over 400 mL/gVS were still guaranteed at a short overall HRT of 18 day,thus exhibiting superiorities in both energy conversion and process stability.Baded on the continuous experimental results,an industrial demonstration project on two-stage fermentative hydrogen and methane co-production using food waste was designed.A hydrogen fermenter with an effective volume of 480 m3 was proposed to facilitate food waste degradation and improve the second-stage methane production,hence reducing the overall HRT from 30 days to 20 days.A strategy of digestate recirculation was proposed to stabilize the system.Part of the digestate from the methane digester,slightly alkaline and rich in anaerobic microbes,could be pumped back to the hydrogen fermenter to maintain the pH around 5.5 and supplement hydrogenogens.