| Anaerobic digestion(AD)technology has been widely used as a sustainable alternative to recycle,reuse and reduce agricultural residues to produce biomethane.However,due to the complex physicochemical properties of lignocellulose,bioconversion of straw by anaerobic microorganism is limited in AD process.Therefore,modification of the lignocellulose structures of the straw is considered as the key process for the enhancement of AD performance.Fungal pretreatment is a simple process for lignin degradation,which has become considerable interests to modify lignocellulose structure for biofuels and biochemical production.However,delignification and digestibility by fungal modification depend on the substrate,fungal strain,and modification condition.The prolonged modification would lead to either extensive consumption of carbon sources or decreases the pretreatment efficiency that could affect the anaerobic digestion(AD)performance of the modified substrate.However,cultivation of mushroom could be considered as an economically feasible way for converting the organic matter in biomass into protein-rich food.Additionally,the spent mushroom substrate(SMS)can provide an alternative substrate for microorganisms in the anaerobic environment,because mushroom cultivation improved the degradation of the crystallinity and hydrolysis of polymers in lignocellulosic compounds,and increased the content of nitrogen in strawThis study proposed a feasible way to combine three independent processes(Pleurotus ostreatus cultivation,biogas production,and nutrition of digestion residue analysis)and introduce material-energy cycling from straw.The purpose of this research was to:(1)investigate the effect of P.ostreatus cultivation on physical and chemical properties of straw,(2)analyze the efficiency of modified straw on AD performance and explored the mechanism of modification by P.ostreatus(3)evaluate the nutritional value of digestion residue,(4)assessed the feasibility of full utilization of nutrients in the straw by mass and elements conversion.Based on the perspective of full utilization of nutrients in straw,this study aimed not only to provide energy but also to circulate biomass in an integrated straw-mushroom-biogas-fertilizer process.Modification of corn stover for biomethane production by P.ostreatus and Ceriporiopsis subvermispora was compared to evaluate the feasibility of P.ostreatus cultivation coupled with anaerobic digestion for improving utilization of nutrients in corn stover.The results showed that both P.ostreatus and C.subvermispora had efficient degradation of lignocellulosic materials in corn stover.However,the deligninfication efficiency of P.ostreatus was less than that of C.subvermispora.The consumption rate of cellulose and hemicellulose by P.ostreatus was higher than that of C.subvermispora.The biomethane yield of modified corn stover by P.ostreatus and C.subvermispora treatment was decreased by 15.4%and 34.3%,respectively.However,the bioconversion of lignocellulose composition of corn stover was improved by P.ostreatus cultivation combined anaerobic digestion process for both mushroom and biomethane generation.P.ostreatus was cultivated on rice straw,corn stover,and wheat straw to investigated the correlations between enzymes activities and changes of the characteristic of modified straw during P.ostreatus cultivation.Results showed that the lignolytic enzymes(LiP,MnP,and Lac)fluctuated and the peak value was achieved in the three stages during P.ostreatus cultivation.Cellulase and xylanase remained at a relatively high level throughout the cultivation process.The degradation rate of lignocellulosic components in straw was constantly changing with the changes in cultivation stages.The physical properties and chemical structure of 25 days' modified straw have the highest enzyme accessibility and availability.However,the enzyme accessibility and the content of cellulose and hemicellulose of 45 days'modified straw were decreased.Cellulase and xylanase adsorption capacity and hydrolysis on modified straw were analyzed to study the effect of modified straw properties on AD performance.Results showed that the enzymes adsorption rate of 25-days modified straw for cellulase and xylanase increased by 12.3%-19.3%and 21.0%-37.9%than that of the raw straw.However,the xylanase adsorption rate of straw modified for 45 days decreased by 36.8%-47.2%than that of raw straw.The initial reaction rate of cellulase and xylanase on modified straw decreased with the modification time.The straw modified for 25 days had the highest enzyme accessibility,lowest desorption rate and longest effective enzymatic hydrolysis time.The biomethane yield,biomethane conversion rate,and the biodegradability of 45-days modified straw were decreased by 9.6%-13.7%,4.8%-21.2%,and 2.2%-6.8%than those of raw straw,respectively.The amount of C remained in the modified straw limited its biomethane conversion.The ammonia-potassium hydroxide composite pretreatment was used to improve the AD performance of spent mushroom substrate(SMS).The results showed that 2%NH3+4%KOH increased the biomethane yield,biomethane conversion rate,and biodegradability of SMS by 16.7%-22.4%,67.5%-145%,and 27.4-39.1 percentage points,respectively.The cellulose crystallinity of 2%NH3+4%KOH treated SMS decreased by 10.0%-15.3%.Hemicellulose solving and crystalline cellulose degradation increased the extraction contents.The conversion of cellulose during AD was increased by 5.9%-9.7%than that of the untreated one,indicating that the nutrients in SMS were more utilized during AD process.The nutritional value and heavy metals pollution assessment were used to evaluate the feasibility for reusing the digestion residue as organic fertilizer.The nutrient content of the digestion residue complied with the Organic Fertilizer Agricultural Industry Standard"(NY 525-2012),implied that it has the potential to be reused as organic fertilizer.The ammonia-potassium composite pretreatment reduced the organic matter content of the digestion residue but increased the content of potassium,which improved the fertilizer efficiency.The heavy metal content of the digestion residues was fulfilled the "Organic Fertilizer Agricultural Industry Standard"(NY 525-2012).There is no heavy metal pollution and potential ecological hazard.The mass balance and economic benefit were analyzed to evaluate the feasibility of full utilization of nutrients in the straw by integrating P.ostreatus cultivation and biomethane production.The results showed that the integrated process of P.ostreatus cultivation and AD could increase the conversion of biodegradable components in straw to the high-value products of mushroom and biomethane.Meanwhile,the quality of C,H,and O in straw was also improved.The ammonia-potassium composite pretreatment could further improve the utilization rate of nutrients during anaerobic digestion.The integrated process had higher ecological benefits than the individual mushroom cultivation or anaerobic digestion,suggesting that it has feasibility in practical application.In summary,the full utilization of nutrients in the straw by integrating P.ostreatus cultivation and biomethane production could improve the bioavailability of nutrients in straw,produce diversified products and achieve the recycling of biomass.These indicate that the full utilization of nutrients in the straw by integrating P.ostreatus cultivation and biomethane production process had high ecological benefits and application. |
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