Effect Of Hydrothermal Pretreatment On Anaerobic Digestion Of Protein-rich Substrates

Development of clean energy is the unescapable choice for conquering the problems of energy shortage and environmental pollution.Among them,exploitation of biomass energy and above all that in biomass wastes is indispensable.Anaerobic digestion can convert biomass(biomass wastes)into a clean energy form-marsh gas rich in methane with high heating value.As matrix for anaerobic digestion,the compositions of biomass heavily have impacts on anaerobic digestion processes and determine the optimized anaerobic digestion technology and process regulation.For instance,various industrial fermentative residues,which usually are rich in protein and have smaller ratios of C/N,easily generate acculation of ammonia nitrogen,thus resulting in inhabitation and even complete failure of their anaerobic digestion.However,among three main nutrients in the fermentative residues protein has the largest potential for methane production,and therefore its high-efficiency anerobic digestion is crucial for energy recovery from biomass.Anaerobic digestion of antibiotic residue-one type of formentive pharmaceutical residue listed as “hazardous waste”has been previously developed by our group.The obtained results showed that antibiotics left in the antibiotic residue was decomposed effectively and the bio-degradabilty of the antibiotic residue improved.To advance the anerobic digestion technology for protein-rich antibiotic residue,the effect mechanisms of hydrothermal pretreatment of protein-rich matrix on its anaerobic digestion and the process responses must be revealed.In this study,tofu as a substitute of plant protein contained in antibiotic residue was subjected to hydrothermal pretreatment-anaerobic digestion.The biochemical methane potentials(BMP)of the product from hydrothermal treatment of tofu at temperatures of 120~180°C and treatment times of 0~90 min were conducted to investigate the effects of hydrothermal pretreatment on anaerobic biochemistry of protein-rich matrix,and meanwhile the optimal hydrothermal pretreatment conditions for such materials were determined;and then via the CSTR experiments of tofu before and after being hydrothermally treated,the performance and the microbial community structure of anaerobic digestion of protein-rich substances were compared,to reveal the responses of anaerobic digestion of protein-rich matrix to hydrothermal pretreatment.According to the experimental results,the following conclusions are obtained:(1)When the temperature was lower than 160°C,the soluble polysaccharides and soluble proteins increased with the increase in the temperature and the reaction time.But they decreased with the increase in the temperature and the reaction time at the temperatures above 160?.The max concentrations of soluble polysaccharides and soluble proteins were obtained at the moment when the temperature reach 160?.They were 13.40 mg/L and 4.177 g/L,which were 6.14 and 1.70 times those in the control group(2.18 mg/L,2.45 g/L),respectively.(2)In the hydrothermal pretreatments at 120-180 °C,the anaerobic digestibility index of protein-rich matrix showed a trend of first increasing and then decreasing,and the largest anaerobic biochemical index was 69.91% at 140 °C,which was 13% higher than that(56.78%)for the control group.Among the involved hydrothermal pretreatment conditions,the moment when temperature reach 140? has the best anaerobic performance for the protein-rich matrix and has the largest anaerobic methanogenic efficiency of 510.89 mL CH4/gVS.And as the treatment temperature is prolonged,the anaerobic digestion yield decreases.(3)The ammonia nitrogen concentration in the pretreatment experiment group was always lower than that in the control group,indicating that hydrothermal pretreatment had a certain alleviation effect on the accumulation of ammonia nitrogen in the anaerobic digestion of protein-rich substrate.(4)The colony in the anaerobic digestion of the raw samples was mainly composed of Cloacimonetes and Firmicutes with high ammonia tolerance,and the abundance was 36.23% and 23.69%,respectively.However,Bacteroidetes and Firmicutes are the main composition of the colony in the hydrothermal pretreatment group.This difference in colony structure demonstrates that hydrothermal pretreatment had certain mitigation abilities for ammonia nitrogen accumulation during anaerobic digestion of protein-rich substrates.