Study On Ultrasound Assisted Alkaline Pretreatment Enhancing Enzymatic Saccharification And Volatile Fatty Acid Production From Grass Clipping

Resources shortages and environment issues are becoming increasingly significant,and the development of green and renewable resources is imminent.Volatile fatty acids?VFAs?are one of many renewable resources that has high added value and good application prospects.It can be used as an additional carbon source for wastewater treatment,and raw material to product high-molecular polymers?bioplastics,medium and long-chain fatty acids,etc.?,multiple bioenergy and chemical products.Lignocellulose is the most abundant renewable resource on the planet.Grass biomass is the most common cellulosic biomass and has the advantages of high yield,low input,rapid growth,wide adaptability and low lignin content.Therefore,the efficient utilization of grass biomass resources is of great significance.In this study,grass clipping was pretreated by ultrasound?US?assisted alkaline pretreatment due to the advantages of ultrasonic?US?cavitation and alkali delignification.and the effect of US-alkaline pretreatment on the structure and enzymatic hydrolysis of grass clipping was discussed.Then,solid-liquid separation fermentation after US-alkaline pretreatment of grass clipping was used to improve VFA production,and VFAs were separated and recovered from hydrolytic acidification solution of grass clipping by membrane absorption method.A new model of high value application of grass biomass is created that combining US-alkaline pretreatment to promote hydrolysis of grass clipping,solid-liquid separation to promote VFA production,and membrane absorption to separate and recover VFAs.The effect of US-alkaline pretreatment on enzymatic hydrolysis of grass clipping was investigate.US-calcium hydroxide?Ca?OH?₂?pretreatment was effective to promote the enzymatic hydrolysis of grass clipping.The reducing sugar yield reached 414.0 mg/g under the optimal conditions.The components content of grass clipping before and after the different pretreatment indicated that US-Ca?OH?₂ pretreatment achieved the highest hemicellulose and lignin removal rates,reaching 13.90%and66.30%,respectively.US-Ca?OH?₂ pretreatment can synergistically destroy the cellulose-hemicellulose-lignin network structure.The analysis results of crystallinity index and FTIR spectrometer showed that US-Ca?OH?₂ pretreatment promotes the removal of lignin and hemicellulose,leading to the destruction of the amorphous part of the turfgrass,and thus improving the crystallinity of cellulose.The results of scanning electron microscope?SEM?and atomic force microscope?AFM?observations indicated that US-Ca?OH?₂ pretreatment can effectively destroy the surface structure of grass clipping,enhance the surface roughness and improve the accessibility of enzymes to grass clipping,thereby promoting the enzymatic saccharification of turfgrass.The effect of US-Ca?OH?₂ pretreatment on VFA production from grass clipping was studied,and the VFA yield of US-Ca?OH?₂ pretreated grass clipping achieved reached 3122 mg/L,which was an increase of 90.70%compared with the raw grass.US-Ca?OH?₂ pretreatment promoted effectively removal of lignin and hemicellulose,increased the accessibility contact of grass clipping to microorganisms and enzymes,thereby increasing VFA yield.Then,the optimization results of substrate concentration during fermentation process on the VFA yield showed that the highest VFA concentration reached 3028 mg/L on 1%substrate concentration.At this time,the VFA yield per unit mass grass clipping was the highest,reaching 302.8 mg/g TS,and the degradation rate of Volatile solids?VS?reached 47.30%.Maintaining a neutral p H in the VFA production process is beneficial to increase VFA production.The VFA concentration reached 4394 mg/L under maintained neutral p H,and the VS degradation rate increased to59.5%.In addition,the effect of solid-liquid separation fermentation on VFA production from grass clipping was studied.The result showed the total VFA yield and VS removal reached 515.0 mg/g TS and59.70%after solid-liquid separation,and organic matter in the pretreatment product is fully utilized.Microbial analysis showed that the dominate genera for VFA production were Halocella and Ruminiclostridium.These results show that US-Ca?OH?₂ pretreatment,optimized grass clipping load and fermentation p H,and solid-liquid separation fermentation can promote VFA production from grass clipping.The membrane absorption process was developed for separation and recovery of VFAs from hydrolytic acidification solution of grass clipping and its process was optimized.A hydrophobic hollow fiber membrane module was built to form membrane absorption device.VFAs can be separated and recovered by membrane absorption with Na OH as the absorption solution.Then the temperature,the p H of the hydrolytic acidification solution,the concentration of the absorption solution and the flow rate in membrane absorption process were optimized.The optimal conditions were 55°C,p H=3,absorption solution concentration of 0.1 mol/L and a flow rate of 100 ml/min.Under these conditions,the VFA recovery rate reached 103.8%.VFA flux and recovery rate were reduced to 76.22%and 76.11%after 5cycles membrane absorption.SEM observation shows that the main pollution of the membrane was caused by Si and Ca.The VFA flux and recovery rate reached 96.47%and 97.21%after ethanol washing.Membrane cleaning is beneficial to membrane reuse and extension of membrane life.