Bioenergy from dairy manure: technologies, challenges and opportunities

Dairy manure (DM) is a kind of cheap cellulosic biomass resource which includes lignocellulose and mineral nutrients. Random stacks not only leads damage to the environment, but also results in waste of natural resources. The traditional ways to use DM include returning it to the soil or acting as a fertilizer, which could reduce environmental pollution to some extent. However, the resource utilization rate is not high and socio-economic performance is not utilized. To expand the application of DM, more and more attention has been paid to explore its potential as bioenergy or bio-chemicals production. This article presented a comprehensive review of different types of bioenergy production from DM and provided a general overview for bioenergy production. Importantly, this paper discussed potentials of DM as candidate feedstocks not only for biogas, bioethanol, biohydrogen, microbial fuel cell, lactic acid, and fumaric acid production by microbial technology, but also for bio-oil and biochar production through apyrolysis process. Additionally, the use of manure for replacing freshwater or nutrients for algae cultivation and cellulase production were also discussed. Overall, DM could be a novel suitable material for future biorefinery. Importantly, considerable efforts and further extensive research on overcoming technical bottlenecks like pretreatment, the effective release of fermentable sugars, the absence of robust organisms for fermentation, energy balance, and life cycle assessment should be needed to develop a comprehensive biorefinery model.

Construction of a structural enzyme adsorption/kinetics model to elucidate additives associated lignin-cellulase interactions in complex bioconversion system

Enzymatic hydrolysis is a rate-limiting process in lignocellulose biorefinery. The reaction involves complex enzyme-substrate and enzyme-lignin interactions in both liquid and solid phases, and has not been well characterized numerically. In this study, a kinetic model was developed to incorporate dynamic enzyme adsorption and product inhibition parameters into hydrolysis simulation. The enzyme adsorption coefficients obtained from Langmuir isotherm were fed dynamically into first-order kinetics for simulating the equilibrium enzyme adsorption in hydrolysis. A fractal and product inhibition kinetics was introduced and successfully applied to improve the simulation accuracy on adsorbed enzyme and glucose concentrations at different enzyme loadings, lignin contents, and in the presence of bovine serum albumin (BSA) and lysozyme. The model provided numerical proof quantifying the beneficial effects of both additives, which improved the hydrolysis rate by reducing the nonproductive adsorption of enzyme on lignin. The hydrolysis rate coefficient and fractal exponent both increased with increasing enzyme loadings, and lignin inhibition exhibited with increasing fractal exponent. Compared with BSA, the addition of lysozyme exhibited higher hydrolysis rates, which was reflected in the larger hydrolysis rate coefficients and smaller fractal exponents in the simulation. The model provides new insights to support process development, control, and optimization.

Modelling of enzyme kinetics: cellulose enzymatic hydrolysis case

Enzymes as industrial biocatalysts offer numerous advantages over traditional chemical processes resulting on improvements in process economy and environmental sustainability. Because enzymes are extensively used in different industrial areas, the enzyme kinetics is an important factor for industry as it is able to estimate the extent of substrate conversion under known conditions and evaluate reactor performance. Furthermore, kinetic modelling is useful in the analysis, prediction, and optimization of an enzymatic process. Thus, kinetic modelling is a powerful tool for biochemical reaction engineering. In addition to the aforementioned, in the industrial technology, modelling together with simulation play a key role because they help to understand how a system behaves under specific conditions, and thus they allow saving on costs and lead times. Enzymatic conversion of renewable cellulosic biomass into biofuels is at the heart of advanced bioethanol production. In the production of bioethanol from cellulosic biomass, enzymatic hydrolysis of cellulose to fermentable sugars accounts for a large portion (∼30%) of the total production costs. Therefore, a thorough understanding of enzymatic hydrolysis is necessary to create a robust model which helps designing optimal conditions and economical system. Nevertheless, it is a challenging task because cellulose is a highly complex substrate and its enzymatic hydrolysis is heterogeneous in nature, and thus the whole process of cellulose conversion to glucose involves more steps than classical enzyme kinetics. This chapter describes the bases of enzyme kinetic modelling, focussing on Michaelis-Menten kinetics, and presents the models classification based on the fundamental approach and methodology used. Furthermore, the modelling of cellulose enzymatic hydrolysis is described, also reviewing some model examples developed for cellulose hydrolysis over the years. Finally, the application of enzyme kinetics modelling in food, pharmaceutical and bioethanol industry is presented.

Development of modified HCH-1 kinetic model for long-term enzymatic cellulose hydrolysis and comparison with literature models

BACKGROUND

Enzymatic hydrolysis is a major step for cellulosic ethanol production. A thorough understanding of enzymatic hydrolysis is necessary to help design optimal conditions and economical systems. The original HCH-1 (Holtzapple-Caram-Humphrey-1) model is a generalized mechanistic model for enzymatic cellulose hydrolysis, but was previously applied only to the initial rates. In this study, the original HCH-1 model was modified to describe integrated enzymatic cellulose hydrolysis. The relationships between parameters in the HCH-1 model and substrate conversion were investigated. Literature models for long-term (> 48 h) enzymatic hydrolysis were summarized and compared to the modified HCH-1 model.

RESULTS

A modified HCH-1 model was developed for long-term (> 48 h) enzymatic cellulose hydrolysis. This modified HCH-1 model includes the following additional considerations: (1) relationships between coefficients and substrate conversion, and (2) enzyme stability. Parameter estimation was performed with 10-day experimental data using α-cellulose as substrate. The developed model satisfactorily describes integrated cellulose hydrolysis data taken with various reaction conditions (initial substrate concentration, initial product concentration, enzyme loading, time). Mechanistic (and semi-mechanistic) literature models for long-term enzymatic hydrolysis were compared with the modified HCH-1 model and evaluated by the corrected version of the Akaike information criterion. Comparison results show that the modified HCH-1 model provides the best fit for enzymatic cellulose hydrolysis.

CONCLUSIONS

The HCH-1 model was modified to extend its application to integrated enzymatic hydrolysis; it performed well when predicting 10-day cellulose hydrolysis at various experimental conditions. Comparison with the literature models showed that the modified HCH-1 model provided the best fit.

Eco-friendly process combining acid-catalyst and thermomechanical pretreatment for improving enzymatic hydrolysis of hemp hurds

The aim of this study was to investigate a pretreatment by combined H₂SO₄ acid-catalyst and thermomechanical process to improve hemicelluloses solubilization of hemp hurds and subsequently enzymatic hydrolysis extent of potentially fermentable sugars. It was found that the sugars released were gradually increased with treatment severity. Soluble sugars generated before enzymatic hydrolysis (R₁) increased up to 2.23 g/L indicating that autohydrolysis reaction occurred during pretreatment. Consequently, the solubilization of hemicelluloses was correlated with combined severity factor (CS). As a result, increase of overall reducing sugars (ORS) from 23.4% (untreated) to 81.4% was observed at optimized conditions of steaming temperature of 165 °C for 30 min and acid loading of 62.9 g/kg DM (dry material) corresponding to CS = 1.2, with limited production of identified by-products: 0.035 g/L and 0.46 g/L (per 100 g DM) for furfural and HMF, respectively. Structural and physicochemical modifications of biomass were observed by FTIR, A_BET and SEM.

Charcoal from anaerobically digested dairy fiber for removal of hydrogen sulfide within biogas

Anaerobically digested fibrous solid (AD fiber) is an abundant material that offers potential to produce value-added products such as biochar. The objective of this paper is to better understand how thermochemical processing conditions affect the capacity of biochars derived from AD fiber to adsorb H₂S from biogas. AD fiber was pyrolyzed in an electric tube reactor at temperatures up to 600 °C and 60 min. The chars were employed for H₂S scrubbing tests from a synthetic biogas. Results showed that the chars' capacity for H₂S removal is comparable to that of activated carbon. An additional step consisting of impregnation of the chars with Na₂CO₃ resulted in an improved capacity for H₂S removal. To study the effect of ash, the AD fiber was also subjected to an alternative thermal treatment, hot water extraction (HWE), at 200 °C for 60 min. The resulting HWE material showed no removal of H₂S from biogas, indicating that the ash and the environment employed for the thermal treatment of AD fiber play an important role in the char's performance for H₂S removal. Results also suggest that a portion of the S in the charcoal after the H₂S sorption process exists as free or adsorbed S (i.e., not chemically bonded to the charcoal).

A discretized model for enzymatic hydrolysis of cellulose in a fed-batch process

In the enzymatic hydrolysis of cellulose, several phenomena have been proposed to cause a decrease in the reaction rate with increasing conversion. The importance of each phenomenon is difficult to distinguish from batch hydrolysis data. Thus, kinetic models for the enzymatic hydrolysis of cellulose often suffer from poor parameter identifiability. This work presents a model that is applicable to fed-batch hydrolysis by discretizing the substrate based on the feeding time. Different scenarios are tested to explain the observed decrease in reaction rate with increasing conversion, and comprehensive assessment of the parameter sensitivities is carried out. The proposed model performed well in the broad range of experimental conditions used in this study and when compared to literature data. Furthermore, the use of data from fed-batch experiments and discretization of the model substrate to populations was found to be very informative when assessing the importance of the rate-decreasing phenomena in the model.

Potential use of feedlot cattle manure for bioethanol production

This paper reports on processing options for the conversion of feedlot cattle manures into composite sugars for ethanol fermentation. Small-scale anaerobic digestion trials revealed that the process significantly reduces the content of glucan and xylan (ca. 70%) without effecting lignin. Moreover, anaerobic digestate (AD) fibres were poor substrates for cellulase (Cellic® CTec 2) saccharification, generating a maximum combined sugar yield of ca. 12% per original dry weight. Dilute acid pretreatment and enzyme saccharification of raw manures significantly improved total sugar recoveries, totalling 264 mg/g (79% theoretical). This was attained when manures were pretreated with 2.5% H2SO4 for 90 min at 121°C and saccharified with 50 FPU CTec 2/g glucan. Saccharomyces cerevisiae efficiently fermented crude hydrolysates within 6 h, yielding 7.3 g/L ethanol, representing glucose to ethanol conversion rate of 70%. With further developments (i.e., fermentation of xylose), this process could deliver greater yields, reinforcing its potential as a biofuel feedstock.

Kinetic modeling of multi-feed simultaneous saccharification and co-fermentation of pretreated birch to ethanol

Fed-batch simultaneous saccharification and fermentation (SSF) is a feasible option for bioethanol production from lignocellulosic raw materials at high substrate concentrations. In this work, a segregated kinetic model was developed for simulation of fed-batch simultaneous saccharification and co-fermentation (SSCF) of steam-pretreated birch, using substrate, enzymes and cell feeds. The model takes into account the dynamics of the cellulase-cellulose system and the cell population during SSCF, and the effects of pre-cultivation of yeast cells on fermentation performance. The model was cross-validated against experiments using different feed schemes. It could predict fermentation performance and explain observed differences between measured total yeast cells and dividing cells very well. The reproducibility of the experiments and the cell viability were significantly better in fed-batch than in batch SSCF at 15% and 20% total WIS contents. The model can be used for simulation of fed-batch SSCF and optimization of feed profiles.

Epidemic based modeling of enzymatic hydrolysis of lignocellulosic biomass

An epidemic based model was developed to describe the enzymatic hydrolysis of a lignocellulosic biomass, dilute sulfuric acid pretreated corn stover. The process of substrate getting adsorbed and digested by enzyme was simulated as susceptibles getting infected by viruses and becoming removed and recovered. This model simplified the dynamic enzyme "infection" process and the catalysis of cellulose into a two-parameter controlled, enzyme behavior guided mechanism. Furthermore, the model incorporates the adsorption block by lignin and inhibition effects on cellulose catalysis. The model satisfactorily predicted the enzyme adsorption and hydrolysis, negative role of lignin, and inhibition effects over hydrolysis for a broad range of substrate and enzyme loadings. Sensitivity analysis was performed to evaluate the incorporation of lignin and other inhibition effects. Our model will be a useful tool for evaluating the effects of parameters during hydrolysis and guide a design strategy for continuous hydrolysis and the associated process control.

Numerical analysis of the impact of structural changes in cellulosic substrates on enzymatic saccharification

Here, a simple cellulose conversion model that considers the cellulose surface area and surface density of adsorbed cellulase as substrate-derived and cellulase-derived factors controlling reaction rates is provided. Microcrystalline cellulose (Avicel) and delignifed softwood were used as controls, and structure-modified samples were prepared. It was shown that the initial cellulose conversion rate is largely controlled by the cellulose surface area. Moreover, the proposed model demonstrates that increases in cellulose surface area reduce retardation of the cellulase reaction. The proposed model was used to estimate the impact of structural changes in a substrate (i.e., cellulose surface area) by pre-treatment on enzymatic saccharification. It was found that increasing the cellulose surface area is the most effective way to optimize enzymatic saccharification of cellulose substrates.

Kinetic modeling of rapid enzymatic hydrolysis of crystalline cellulose after pretreatment by NMMO

Pretreatment of cellulose with an industrial cellulosic solvent, N-methylmorpholine-N-oxide, showed promising results in increasing the rate of subsequent enzymatic hydrolysis. Cotton linter was used as high crystalline cellulose. After the pretreatment, the cellulose was almost completely hydrolyzed in less than 12 h, using low enzyme loading (15 FPU/g cellulose). The pretreatment significantly decreased the total crystallinity of cellulose from 7.1 to 3.3, and drastically increased the enzyme adsorption capacity of cellulose by approximately 42 times. A semi-mechanistic model was used to describe the relationship between the cellulose concentration and the enzyme loading. In this model, two reactions for heterogeneous reaction of cellulose to glucose and cellobiose, and a homogenous reaction for cellobiose conversion to glucose was incorporated. The Langmuir model was applied to model the adsorption of cellulase onto the treated cellulose. The competitive inhibition was also considered for the effects of sugar inhibition on the rate of enzymatic hydrolysis. The kinetic parameters of the model were estimated by experimental results and evaluated.

Bioconversion of dairy manure by black soldier fly (Diptera: Stratiomyidae) for biodiesel and sugar production

Modern dairies cause the accumulation of considerable quantity of dairy manure which is a potential hazard to the environment. Dairy manure can also act as a principal larval resource for many insects such as the black soldier fly, Hermetia illucens. The black soldier fly larvae (BSFL) are considered as a new biotechnology to convert dairy manure into biodiesel and sugar. BSFL are a common colonizer of large variety of decomposing organic material in temperate and tropical areas. Adults do not need to be fed, except to take water, and acquired enough nutrition during larval development for reproduction. Dairy manure treated by BSFL is an economical way in animal facilities. Grease could be extracted from BSFL by petroleum ether, and then be treated with a two-step method to produce biodiesel. The digested dairy manure was hydrolyzed into sugar. In this study, approximately 1248.6g fresh dairy manure was converted into 273.4 g dry residue by 1200 BSFL in 21 days. Approximately 15.8 g of biodiesel was gained from 70.8 g dry BSFL, and 96.2g sugar was obtained from the digested dairy manure. The residual dry BSFL after grease extraction can be used as protein feedstuff.

Comparison of mechanistic models in the initial rate enzymatic hydrolysis of AFEX-treated wheat straw

BACKGROUND

Different mechanistic models have been used in the literature to describe the enzymatic hydrolysis of pretreated biomass. Although these different models have been applied to different substrates, most of these mechanistic models fit into two- and three-parameter mechanistic models. The purpose of this study is to compare the models and determine the activation energy and the enthalpy of adsorption of Trichoderma reesei enzymes on ammonia fibre explosion (AFEX)-treated wheat straw. Experimental enzymatic hydrolysis data from AFEX-treated wheat straw were modelled with two- and three-parameter mechanistic models from the literature. In order to discriminate between the models, initial rate data at 49 degrees C were subjected to statistical analysis (analysis of variance and scatter plots).

RESULTS

For three-parameter models, the HCH-1 model best fitted the experimental data; for two-parameter models Michaelis-Menten (M-M) best fitted the experimental data. All the three-parameter models fitted the data better than the two-parameter models. The best three models at 49 degrees C (HCH-1, Huang and M-M) were compared using initial rate data at three temperatures (35 degrees , 42 degrees and 49 degrees C). The HCH-1 model provided the best fit based on the F values, the scatter plot and the residual sum of squares. Also, its kinetic parameters were linear in Arrhenius/van't Hoff's plots, unlike the other models. The activation energy (Ea) is 47.6 kJ/mol and the enthalpy change of adsorption (DeltaH) is -118 kJ/mol for T. reesei enzymes on AFEX-treated wheat straw.

CONCLUSION

Among the two-parameter models, Michaelis-Menten model provided the best fit compared to models proposed by Humphrey and Wald. For the three-parameter models, HCH-1 provided the best fit because the model includes a fractional coverage parameter (varphi) which accounts for the number of reactive sites covered by the enzymes.

Co-production of fumaric acid and chitin from a nitrogen-rich lignocellulosic material - dairy manure - using a pelletized filamentous fungus Rhizopus oryzae ATCC 20344

Fumaric acid is widely used as a food additive for flavor and preservation. Rhizopus oryzae ATCC 20344 is a fungus known for good fumaric acid production. It also has been reported that the fungal biomass has high chitin content. This study investigated the possibility of producing both fumaric acid and chitin via R. oryzae fermentation of dairy manure. Co-production of valuable bio-based chemicals such as fumaric acid and chitin could make the utilization of manure more efficient and more profitable. A three step fermentation process was developed which effectively utilized the nitrogen as well as the carbohydrate sources within the manure. These steps were: the culturing of pellet seed; biomass cultivation on liquid manure to produce both biomass and chitin; and fumaric acid production on the hydrolysate from the manure fiber. Under the identified optimal conditions, the fermentation system had a fumaric acid yield of 31%, and a biomass concentration of 11.5 g/L that contained 0.21 g chitin/g biomass.