Pretreatment of lignocelluloses for enhanced biogas production: A review on influencing mechanisms and the importance of microbial diversity

Received 13 August 2019; Received in revised form 10 July 2020; Accepted 28 July 2020, Available online 11 August 2020.As one of the most efficient methods for waste management and sustainable energy production, anaerobic digestion (AD) countenances difficulties in the hydrolysis of lignocelluloses biomass. Different pretreatment methods have been applied to make lignocelluloses readily biodegradable by microorganisms. These pretreatments can affect biogas yield by different mechanisms at molecular scale, including changes in chemical composition, cellulose crystallinity, degree of polymerization, enzyme adsorption/desorption, nutrient accessibility, deacetylation, and through the formation of inhibitors. The present article aims at critically reviewing the reported molecular mechanisms affecting biogas yield from lignocelluloses via different types of pretreatments. Then, a new hypothesis concerning the impact of pretreatment on the microbial community developed (throughout the AD process from an identical inoculum) was also put forth and was experimentally examined through a case study. Four different leading pretreatments, including sulfuric acid, sodium hydroxide, aqueous ammonia, and sodium carbonate, were performed on rice straw as model lignocellulosic feedstock. The results obtained revealed that the choice of pretreatment method also plays a pivotally positive or negative role on biogas yield obtained from lignocelluloses through alteration of the microbial community involved in the AD. Considerable changes were observed in the archaeal and bacterial communities developed in response to the pretreatment used. Sodium hydroxide, with the highest methane yield (338 mL/g volatile solid), led to a partial switch from acetoclastic to the hydrogenotrophic methane production pathway. The findings reported herein undermine the default hypothesis accepted by thousands of previously published papers, which is changes in substrate characteristics by pretreatments are the only mechanisms affecting biogas yield. Moreover, the results obtained could assist with the development of more efficient biogas production systems at industrial scale by offering more in-depth understanding of the interactions between microbial community structure, and process parameters and performance

High efficient ethanol production from corn stover by modified mild alkaline pretreatment

Sodium carbonate pretreatment, an environmentally-friendly and efficient pretreatment, was used to improve solid-state ethanol production from corn stover (CS). To further enhance ethanol yield from the pretreated CS, especially at low enzyme loadings, a combined pretreatment, including steam and sodium carbonate pretreatments, was developed. The removal/modification of lignin by sodium carbonate pretreatment (at 100 °C for 3 h) helped to obtain high ethanol titer at high solids and low enzymes loadings. The maximum ethanol concentration was about 44 g/L, obtained after 72 h simultaneous saccharification and fermentation (SSF) of CS pretreated with sodium carbonate without mixing and prehydrolysis at 30% solid and 15 FPU/g enzyme loadings. Applying prehydrolysis and increasing the SSF time to 120 h improved ethanol concentration to 59 and 67 g/L, respectively. Steam pretreatment at 190 °C for 10 min before sodium carbonate pretreatment led to 76% increase in ethanol concentration. At the enzyme loading of only 5 FPU/g substrate, the modified pretreatment increased the ethanol concentration from 24 g/L to 41 g/L, compared with that of sodium carbonate pretreatment. The results showed that hemicellulose removal and delignification by steam and sodium carbonate pretreatment, respectively, considerably improved ethanol concentration at high solids and low enzyme loadings

High titer ethanol production from rice straw via solid-state simultaneous saccharification and fermentation by Mucor indicus at low enzyme loading

High titer ethanol production from rice straw using Mucor indicus fungus was investigated through the solid-state simultaneous saccharification and fermentation (SSSF) process. The straw was pretreated with 0.5 M sodium carbonate solution for 3, 5, and 10 h to improve the efficiency of the process. Effects of the pretreatment on the composition, structural morphology, cellulose crystallinity, swelling capacity, and buffering capacity of the straw were studied. Moreover, the effects of SSSF reaction time, enzyme loading, and solid loading on glucose and ethanol production were investigated. Additionally, the nutritional value of the residue from the SSSF process, as an animal feedstock, was determined in terms of lipid and protein contents. The highest total sugar concentration was 89.2 g/L, obtained from the straw pretreated for 10 h after hydrolysis with 10 FPU/g straw at 15% (w/w) solid loading. Total sugars concentration was not significantly improved by increasing the pretreatment time at low enzyme loadings, whereas it was significantly improved at high enzyme loadings. The highest ethanol concentration and SSSF yield were 99.4 g/L and 89.5% (71.8% based on the raw material), achieved from the straw pretreated for 10 h through the 72-h SSSF at 30% and 15% solid loading, respectively, using 20 FPU/g straw. The low enzyme loadings of 2.5 and 5 FPU/g straw yielded ethanol with concentrations as high as 66.3 and 90.9 g/L, respectively, after 120-h fermentation at 30% solid loading from the straw pretreated for 5 h

Enhanced Solid-State Biogas Production from Lignocellulosic Biomass by Organosolv Pretreatment

Organosolv pretreatment was used to improve solid-state anaerobic digestion (SSAD) for methane production from three different lignocellulosic substrates (hardwood elm, softwood pine, and agricultural waste rice straw). Pretreatments were conducted at 150 and 180°C for 30 and 60 min using 75% ethanol solution as an organic solvent with addition of sulfuric acid as a catalyst. The statistical analyses showed that pretreatment temperature was the significant factor affecting methane production. Optimum temperature was 180°C for elmwood while it was 150°C for both pinewood and rice straw. Maximum methane production was 152.7, 93.7, and 71.4 liter per kg carbohydrates (CH), which showed up to 32, 73, and 84% enhancement for rice straw, elmwood, and pinewood, respectively, compared to those from the untreated substrates. An inverse relationship between the total methane yield and the lignin content of the substrates was observed. Kinetic analysis of the methane production showed that the process followed a first-order model for all untreated and pretreated lignocelluloses