Elsevier

Energy Conversion and Management

Volume 119, 1 July 2016, Pages 257-265
Energy Conversion and Management

A biorefinery concept using the green macroalgae Chaetomorpha linum for the coproduction of bioethanol and biogas

https://doi.org/10.1016/j.enconman.2016.04.046Get rights and content

Highlights

  • Chaetomorpha linum was used as sustainable feedstock for co-production of bioethanol and biomethane.

  • An eco-friendly process was developed, only generating 0.3 ± 0.01 g/g of waste.

  • Ethanol yield obtained was 0.41 g/g reducing sugar.

  • Methane yield obtained was 0.26 ± 0.045 L/gVS.

Abstract

An innovative integrated biorefinery approach using the green macroalgae Chaetomorpha linum was investigated in the present study for the co-production of bioethanol and biogas. Among three pretreatments of C. linum biomass, consisting of acidic, neutral and alkali ones, 3% NaOH pretreatment gave the best result in terms of thallus disintegration, biomass recovery and enzymatic digestibility as demonstrated by scanning electron microscopy and saccharification tests. The hydrolysis of C. linum feedstock with a crude specific enzyme preparation, locally produced from fermentation of Aspergillus awamori, at 45 °C, pH 5 for 30 h gave the maximum yield of fermentable sugar of 0.22 ± 0.02 g/g dry substrate. An ethanol yield of 0.41 g/g reducing sugar corresponding to about 0.093 g/g pretreated algae was obtained after alcoholic fermentation by Saccharomyces cerevisiae. In the integrated proposed process, mycelium issued from the fungal fermentation, liquid issued from alkali pretreatment, residual from the non-hydrolysable biomass and all effluents and co-products represent a heterogeneous substrate that feed an anaerobic digester for biogas production. GC-analysis of this later showed that the biomethane yield reached 0.26 ± 0.045 L/gVS. This study presents therefore an eco-friendly biorefining process, which efficiently coproduce bioethanol and biomethane and generate only a single waste (0.3 ± 0.01 g/g) allowing an almost complete conversion of the algal biomass.

Introduction

The decrease of oil resources combined to the increase of the world population and therefore the energy consumption are the main requirements for using renewable energies. Among these, biomass constitutes a renewable source of biofuel, namely bioethanol, biogas and biodiesel [1]. It represents a promising alternative for the substitution, at least in part, of fossil fuels. Indeed, the development of reliable, cost-effective and ecological processes from biomass becomes a global priority despite the two known main limits for this energetic bioconversion. In fact, the culture of lignocellulosic plants is done in detriment of cultivable land used for human consumption which is not a long term solution to the increase of population [2]. Besides, lignocellulosic biomass which is consisted of cellulose, hemicelluloses and lignin, requires mechanical, thermal and/or chemical pretreatment steps to make the cellulose accessible to enzymes during the enzymatic hydrolysis [3], [4]. These pretreatment steps usually affected the cost of energetic conversion. Thus, all the research on the biological transformation of lignocellulose were interested in several issues namely finding suitable pretreatments which do not generate harmful products to the environment and fermentation inhibitors [5] and producing specific and stable enzymes with reasonable cost [6], [7]. Some research were also interested in developing strains of yeasts or bacteria able to ferment simultaneously hexoses and pentoses resulting from the enzymatic saccharification as well as resistant to the various inhibitors which may be generated [8], [9].

Recently, several studies are interested in finding an alternative to the use of lignocellulosic biomass. In fact, beside the use of microalgae as a source of sustainable biodiesel production [10], marine macroalgae have received considerable attention as source of third-generation biofuels [11] such as bioethanol [12], [13], [14], [15], [16] and biogas [17], [18], [19]. Compared to microalgae, macroalgae are multicellular plants that possess plant-like characteristics with thallus-type morphology, composed mainly of carbohydrates. They can be therefore considered as a good candidates for biofuel production like biogas, bioethanol and bio-oils [11]. Additionally, their harvesting were also easier, they represent a renewable abundant biomass that could be easily cultivated with low cost of collection and null environmental damage [20]. Obviously, they do not compete with land use (avoiding arable land) and water consumption, necessary for food crops [2]. Furthermore, macroalgae are characterized by a higher biomass production due to its fast growing rate in the open aquatic media [20] and does not require agricultural additives such as fertilizer and pesticides [21], [22]. Moreover, they have higher photosynthetic activity than terrestrial plants [20] and they contain little or no lignin-like molecules [23].

Using macroalgae in biorefinery concepts would reduce petroleum dependence while assuring a positive environmental impact [20]. The bioethanol has been the most biofuel type produced from the macroalgae [24]. Nevertheless, the cost-effectiveness of biorefinery concept which is based on the production of bioethanol is debatable for the low cellulose content (15–25%) and the seasonal and environmental variation of macroalgae which influences its biochemical composition including the content of cellulose principal source of fermentable sugars [25].

Thus, the objective of our study is to develop a novel integrated biorefinery concept based on the co-production of both bioethanol and biogas from the green macroalgae Chaetomorpha linum with one coproduct. C. linum is very abundant in the coasts of Tunisia but it is not very valued. The feasibility of different stages of the process such as pretreatment of macroalgae, alcohol fermentation and anaerobic digestion was demonstrated. In this work environmental friendly cell-wall degrading enzymes, locally produced, were used for the saccharification of C. linum.

Section snippets

Biological materials

The green macroalgae C. linum was collected in September 2013 from the shores of Tunis lagoon (GPS: 36.813095, 10.192673, salinity: 33.8 psu) suffering from eutrophication problem. A bioremediation of this ecosystem could be attempted using these stranded algae as feedstock of a biorefinery process. Samples were washed, dried, finely ground and stored until they were used.

Aspergillus awamori (NBRC 4033, Osaka, Japan) was maintained at 4 °C in potato dextrose agar plates. The spores were collected

A. awamori culture and specific enzyme production

Filamentous fungi are well known microorganism of decomposition of organic matter in general and of cellulosic substrates in particular as reported in many studies [39]. A. awamori was used in this study as a source of hydrolytic enzymes capable of hydrolyzing the green macroalgae C. linum and liberating fermentable sugars. Our strategy consisted of culturing A. awamori in submerged minimum medium supplemented by the ground C. linum algae as the sole carbon source. Table 1 showed that

Conclusion

This study presents a proof of concept demonstrating the feasibility of the co-production of both biogas and bioethanol from the entire green macroalgae C. linum using an eco-friendly process with minimum waste. The proposed process was composed of three main stages of pretreatment, saccharification and fermentation aimed at the bioethanol production, one annexed stage for locally cell-wall degrading enzymes production, and one main stage of anaerobic digestion for biogas production including

Acknowledgements

The authors gratefully acknowledge the Tunisian Ministry of Higher Education and Scientific Research – University of Carthage (Tunisia) for financial support (project LR11ES24) and for concession of a research grant for Nesrine Ben Yahmed’s Ph.D. We express our gratitude to Caroline Rémond and Nathalie Aubry from the FARE unit (INRA Reims, France) that helped to the determination of fine carbohydrate chemical composition of the algae C. linum.

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