Improvement Of Fungal Pretreatment On Production Of Fermentative Volatile Fatty Acids From Lignocellulosic Biomass And Fermentative Products Regulation

With the rapid growth of population and the increasing energy demand for economic development,the capacity of fossil fuels would not meet the energy consumption in future.The future energy consumption and production of chemicals will show an increasing shift from petroleum-based to renewable biomass-based resources.As an efficient and relatively simple biotechnology,anaerobic digestion has been widely applied throughout the world for the stabilization of,and energy reco very from organic wastes.Recently,production of volatile fatty acids?VFA?instead of biogas from organic residues as a product during anaerobic digestion has been regarded promising,because VFA is considered as a precursor to produce high-value-added such as gas production,chemical organic materials and additional carbon source in biological nutrient removal.There are a number of lignocellulosic waste materials available for production of valuable resources as feedstocks,which may alleviate many problems for environment and energy to a certain extent.However,most of these waste materials contain a relatively high content of lignin,preventing the valuable carbohydrates to be degraded.Therefore,large quantities of so-called low value lignocellulosic waste are not fully utilized and dumped in open environment,which is not a recommended approach in term of environmental and ecological aspects of sustainable development.The feasibilities of improving VFA production from champost and solid digestate by fungal pretreatment were carried out and systematically investigated,and the mechanism of fungal pretreatment was analyzed in details in this paper.Besides,this study also investigated the feasibility and effect of co-fermentation on the improvement of VFA production of champost and sewage sludge.Furthermore,an insight of fermentative products regulation was investigated in this study.Firstly,this study investigated the feasibility of fermentative volatile fatty acids production from mushroom champost,and then some important process parameters such as total solid?TS?content?6%to 18%?and pH?4.0 to 12.0?were optimized.The ammonia and phosphorus release,and VFA production were analyzed.The results showed that within the TS range from 6%to 18%,a TS of 15%was optimal for VFA production.Besides,the results also indicated that within the different pH values,the higher VFA concentration was produced under alkaline condition compared to acid and blank test,and the maximum VFA concentration of 3480 mg/L was obtained at pH 10.0.The total percentages of acetate and propionate accounted for about 70%of total VFA in all conditions,which was beneficial to the application of VFA in biological nutrient removal.To further enhance the fermentative VFA production from champost,two types of mushroom champost were pretreated with selected white rot fungi.The results demonstrated that C.subvermispor was not able to grow on champost,while both P.sajor-caju and T.versicolor fungal strains grew and disrupted the structure of champost and changed their chemical compositions in terms of variety of functional groups.For Oyster champost,the cellulose/lignin ratio decreased after fungal pretreatment,indicating fungi degraded cellulose rapidly than the rate of lignin degradationduringfungalpretreatment.ForRawchampost,theP.sajor-cajuselectively degraded lignin and P.sajor-caju pretreatment resulted in high cellulose/lignin of 0.92.On the other hands,T.versicolor had limited effect on lignin degradation.Consequently,anaerobic fermentation tests showed that after pretreatment of with P.sajor-cajua,the maximum VFA yield from Raw champost was 203 mg COD/gVS_added,which was higher by 60%and 74%than that of the control?autoclaved?and untreated champost.The increase in VFA yield was maybe attributed to the increased surface area and porosity and the preference for lignin degradation during fungal pretreatment.Hence,application of P.sajor-caju provides an effective method to improve biodegradation of Raw champost.Conversely,a maximum VFA yield of437 mgCOD/gVS_addeddded from Oyster champost was obtained in the control group.Oyster champost was proven to be a potential biomass source for VFA production,but fungal pretreatment using P.sajor-cajuand T.versicolor decreased VFA yield from Oyster champost probably due to many cellulose degradations during fungal pretreatment.Anaerobic digestion has been widely applied throughout the world for lignocellulosic biomass treatment and energy recovery.However,the solid digestate from anaerobic digestion still contains a rather large fraction of poorly anaerobic degradable lignocellulosic fibers due to inhibition of lignin,which deeply limits the bioenergy production from lignocellulosic biomass.A novel fungal pretreatment method using P.sajor-caju and T.versicolor was investigated to improve solid-state fermentative VFAs production from solid digestate.The results showed that a maximum VFA yield of 240 mgCOD/gVS_addeddded was obtained from solid digestate pretreated by P.sajor-caju in 6 weeks,which was 1.17-fold and 1.24-fold higher than that of the autoclaved group and raw substrate,respectively.Acetate and propionate were dominant,accounting for more than 70%of the total VFA at the end of fermentation in all tests.The mechanism analysis indicated that these fungal strains could grow on the solid digestate and secrete ligninolytic enzymes such as laccase and manganese peroxidase to degrade lignin in different extents.The chemical compositi on analysis indicated that fungal pretreatment substantially changed the solid digestate characteristics such as cellulose/lignin ratio and the presence of specific functional groups.Moreover,fungal pretreatment using P.sajor-caju selectively degraded lignin and increased cellulose/lignin ratio,which was one of main reasons for improvement of VFA yield.However,T.versicolor showed the weak effect on lignin degradation but degraded more cellulose during fungal pretreatment,which was a negative effect on the anaerobic fermentation process.The effect of the mixed ratio of champost and sewage sludge on VFA production was investigated.The results indicated that a single fermentative system of sewage sludge?R1?produced higher fermentative VFA production than a single fermentative system of Oyster champost?R5?.Besides,the co-fermentation system increased VFA yield.The maximal VFA yield of 596 mgCOD/gVS_addeddded was obtained from the co-fermentation system with 50%content of sewage sludge.The maximal VFA yield was higher 14.3%and 36.7%than that of R1 and R5 respectively.Besides,the co-fermentation system had an influence on degree of hydrolysis.The maximal soluble chemical oxygen demand?SCOD?concentration in fermentation broth obtained from R3,which was higher 13.2%and 26.6%than that of R1 and R5,respectively.The more soluble organic matter in the co-fermentation system could be utilized for anaerobic bacteria in acidification process.Furthermore,the co-fermentation system increased buffer capacity of fermentative liquid probably due to higher ammonium release,making sure the pH was in the suitable range of anaerobic fermentation?pH>6.0?.After anaerobic fermentation,in all reactors was in the sequence of acetate>propionate>butyrate>valerate>hexanate.In the mixed fermentation system,when the content of mushroom residue exceeds 50%in substrate,the content of propionic acid in the fermentation broth accounts for about 30%of the total VFA content.At the same time valeric acid content decreased by more than 30%.Furthermore,the Logistic-based model could well explain the VFA production in this study,which indicated that co-fermentation system increased hydrolysis rate and improved VFA yield.Therefore,the co-fermentation of champost and sewage sludge is promising for effective fermentative VFA production from champost.An efficient product chain in anaerobic fermentation process requires,amongst others,a high performance at high VFA yield and fermentative products regulation.This study achieved a high-rate mainly odd-numbered VFA?propionate and valerate?fermentative system,and investigated the effect of pH from 6.0 to 6.5 on the performance of reactor operation.The results indicated that the total contents of propionate and valerate accounted for 68%to 69%of VFA concentration when the pH was 6.0 and 6.5.Besides,the increase in pH led to an increase in glucose conversion rate,but did not have any effect on the lactate conversion rate that is the limited rate of the anaerobic fermentation process.Butyrate and valerate were produced via chain length elongation of respectively acetate and propionate with lactate.The propionate was produced via three fermentation pathways:polyglucose,acetate-succinate and lactate conversions.The acetate-succinate conversion was the most important way to produce propionate when the pH was 6.0.When the pH increased from 6.0 to 6.5,the direct conversion of polyglucose stored in cells was the most important way to produce propionate.