Abstract
BackgroundEnzymatic treatment of lignocellulosic material for increased biogas production has so far focused on pretreatment methods. However, often combinations of enzymes and different physicochemical treatments are necessary to achieve a desired effect. This need for additional energy and chemicals compromises the rationale of using enzymes for low energy treatment to promote biogas production. Therefore, simpler and less energy intensive in situ anaerobic digester treatment with enzymes is desirable. However, investigations in which exogenous enzymes are added to treat the material in situ have shown mixed success, possibly because the enzymes used originated from organisms not evolutionarily adapted to the environment of anaerobic digesters. In this study, to examine the effect of enzymes endogenous to methanogenic microbial communities, cellulolytic enzymes were instead overproduced and collected from a dedicated methanogenic microbial community. By this approach, a solution with very high endogenous microbial cellulolytic activity was produced and tested for the effect on biogas production from lignocellulose by in situ anaerobic digester treatment.ResultsAddition of enzymes, endogenous to the environment of a mixed methanogenic microbial community, to the anaerobic digestion of ensiled forage ley resulted in significantly increased rate and yield of biomethane production. The enzyme solution had an instant effect on more readily available cellulosic material. More importantly, the induced enzyme solution also affected the biogas production rate from less accessible cellulosic material in a second slower phase of lignocellulose digestion. Notably, this effect was maintained throughout the experiment to completely digested lignocellulosic substrate.ConclusionsThe induced enzyme solution collected from a microbial methanogenic community contained enzymes that were apparently active and stable in the environment of anaerobic digestion. The enzymatic activity had a profound effect on the biogas production rate and yield, comparable with the results of many pretreatment methods. Thus, application of such enzymes could enable efficient low energy in situ anaerobic digester treatment for increased biomethane production from lignocellulosic material.
Highlights
Enzymatic treatment of lignocellulosic material for increased biogas production has so far focused on pretreatment methods
Induction and harvesting of cellulolytic enzymes The Biochemical methane potential (BMP) test of ensiled forage ley was used to analyze the effect that the induced endogenous cellulolytic enzymes had on the anaerobic digestion of a lignocellulosic material
By subtracting the small calculated value of the fitted data for the slow second phase from each time point in the first phase, the kinetics for degradation of the digested material in the first phase could be estimated (Fig. 6a; Table 4) and the two simultaneous processes separated (Fig. 6b). These results suggest that for treatment of the accessible fraction of the material, enzyme augmentation is unnecessary if the residence time is more than approx. 9 days because after this time, the net difference between the two samples was minimal (Fig. 6a, b)
Summary
Enzymatic treatment of lignocellulosic material for increased biogas production has so far focused on pretreatment methods. These include substrates such as excess waste activated sludge, micro algae and lignocellulosic plant material, either derived from agricultural waste or grown purposely as biogenic energy crops All these bioresources are of interest as substrates for increased biogas production but all are associated with low biogas product yields due to slow hydrolysis in the first step of the biological methanogenesis process. A common opinion is that for an effective process, pretreatments that eliminate structural barriers to enzymatic hydrolysis are necessary in preparation of lignocellulosic biomass for biogas production These methods are either physical or chemical, including, but not limited to, e.g., comminution, acid hydrolysis, ammonia fiber expansion, steam explosion, etc. The use of enzymes for enhancing biogas production from various substrates is of general interest [7, 8]
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