Abstract
BackgroundThe recalcitrant cell walls of microalgae may limit their digestibility for bioenergy production. Considering that cellulose contributes to the cell wall recalcitrance of the microalgae Chlorella vulgaris, this study investigated bioaugmentation with a cellulolytic and hydrogenogenic bacterium, Clostridium thermocellum, at different inoculum ratios as a possible method to improve CH4 and H2 production of microalgae.ResultsMethane production was found to increase by 17?~?24% with the addition of C. thermocellum, as a result of enhanced cell disruption and excess hydrogen production. Furthermore, addition of C. thermocellum enhanced the bacterial diversity and quantities, leading to higher fermentation efficiency. A two-step process of addition of C. thermocellum first and methanogenic sludge subsequently could recover both hydrogen and methane, with a 9.4% increase in bioenergy yield, when compared with the one-step process of simultaneous addition of C. thermocellum and methanogenic sludge. The fluorescence peaks of excitation-emission matrix spectra associated with chlorophyll can serve as biomarkers for algal cell degradation.ConclusionsBioaugmentation with C. thermocellum improved the degradation of C. vulgaris biomass, producing higher levels of methane and hydrogen. The two-step process, with methanogenic inoculum added after the hydrogen production reached saturation, was found to be an energy-efficiency method for hydrogen and methane production.
Highlights
The recalcitrant cell walls of microalgae may limit their digestibility for bioenergy production
Considering that cellulose contributes to the cell wall recalcitrance in the microalgae C. vulgaris, this study investigated bioaugmentation with a thermophilic, anaerobic, cellulolytic, and hydrogenogenic bacterium, Clostridium thermocellum, which is available from cellulose-fed anaerobic digester [19], as a possible method to improve the degradation of C. vulgaris biomass to enhance the efficiency of methane and hydrogen production
When the concentration of algal biomass was increased to 3 g volatile solid (VS)/L in Series 2, the highest methane yield and maximum methane production rate were much lower than those noted for the corresponding treatment in Series 1 with the same inoculum ratio of 5% of C. thermocellum (Figure 1b)
Summary
The recalcitrant cell walls of microalgae may limit their digestibility for bioenergy production. Anaerobic digestion could be carried out on microalgal residues after lipid extraction [3,4,5,6] or directly on freshly collected algae. With regard to the latter, the resistance of Besides, the use of thermochemical pretreatment may lead to a possible formation of inhibitory substances (e.g. furfurals) [17]. Ehimen et al [13] reported a pretreatment process of addition of a combined enzyme mixture and individual enzymes to the Rhizoclonium biomass prior to anaerobic digestion. Appropriate bacterial species should be carefully selected to be effective for microalgae hydrolysis and be compatible with subsequent or synchronous anaerobic digestion
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