Review

Impact of microalgae characteristics on their conversion to biofuel. Part I: Focus on cultivation and biofuel production C. González-Fernández, Laboratoire de Biotechnologie de l’Environnement, Narbonne, France B. Sialve, Naskeo Environnement, Narbonne, France N. Bernet and J. P. Steyer, Laboratoire de Biotechnologie de l’Environnement, Narbonne, France Received August 8, 2011; revised September 8, 2011; accepted September 14, 2011 View online at Wiley Online Library (wileyonlinelibrary.com); DOI: 10.1002/bbb.338; Biofuels, Bioprod. Bioref. (2011) Abstract: Microalgae are now the focus of intensive research because of their potential as a renewable feedstock for biofuel production. This review briefly examines the effect of reactor design, nutrient, and light regimens on microalgae productivity and macromolecular composition. Downstream processing including common biofuel production as well as life cycle assessment and technoeconomical aspects are discussed. Even though algal biofuels are more environmentally friendly than fossil fuels, economical feasibility is a challenging issue. © 2011 Society of Chemical Industry and John Wiley & Sons Ltd Keywords: microalgae; cultivation; biofuel; life-cycle analysis

Introduction

T

he global energy crisis has raised the need to find alternative energy resources. Up to now, resources classified as first- and second-generation biofuels have

only overcomes all these drawbacks but also contributes to mitigating CO2 emissions, removing wastewater pollutants and generating value-added products. For all these reasons, microalgae, the third-generation biofuel, seem to be the alternative renewable resource to meet energy demands.

been generating a great controversy. The main drawbacks of these resources include the need for arable land, for large amounts of water and fertilizer, and low productivities. The use of microalgae as a renewable biological resource not

Production and harvest of microalgae In order to be able to meet the fuel demand, we must first overcome the limitations of microalgae biotechnology.

Correspondence to: C. González-Fernández, INRA, UR50, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne, F-11100, France. E-mail: [email protected]

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd

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The main general constraints of this biotechnology are the biomass production rate and harvest. One of the main advantages of using microalgae as an energy feedstock is their high growth rate. Even though microalgae grow fast, estimates by Wijffels and Barbosa1 indicate the need of increasing the production rate by three orders of magnitude for feasible biofuel production. Chisti et al.2 indicate that double microalgae growth may be commonly be reached within 24 h. Nutrients, light, temperature, mixing regimens, microalgae strain, and reactor design drastically affect microalgae growth.

Reactor design With regard to reactor design, the old, traditional type of reactors is an open-to-air system. Among these open systems, the most employed configuration is shallow big ponds, raceway ponds, or circular ponds. As Pulz3 showed, their main advantages include the easiness of scaling up and their low cost in terms of construction and maintenance. However, open systems present a high risk of contamination; they are dependent on weather conditions; and suffer high water and gas losses. As a result of these drawbacks, microalgae growth is low, attaining productivities of around 0.05–0.3 g DW L-1 day-1.4 For these reasons, the urgent need to develop new designs has resulted in the creation of closed-system photobioreactors. The basic design of these closed systems includes flat plates and tubular reactors. Closed systems not only overcome the drawbacks of open systems, they also allow microalgae productivity to reach values as high as 0.8–1.3 g DW L-1 day-1.3 To negatively counterbalance the high productivity achieved in closed photobioreactors, the high capital cost renders these cultivation systems difficult to implement on a large scale. Hallenbeck and Beneman5 estimated capital cost of US$100 and US$9.4 m-2 for closed and open systems, respectively. In the context of energetical cost, Acien et al.6 calculated that 300 and 100 W m-3 are needed to operate bubble columns and tubular photobioreactors, while open ponds decrease energy requirements to 1 Wm-3. More recently, Jorquera et al.7 confirmed those energy/ economic estimates for the production of biodiesel with Nannochloropsis sp. as a feedstock. Microalgae hype may result in confusing values. The outcome of all these appraisals would favor the performance of

Review: Impact of microalgae characteristics on their conversion to biofuel

closed photobioreactors in terms of microalgae productivity. However, open systems, because of their cost effectiveness, seem to be the most implemented growth mode for microalgae. Indeed, currently there are no large industrial plants with closed systems for biomass production. Thus, the general trend is to use closed photobioreactors when the final goal is the production of a high value-added product, which would improve economic balances while for large biomass production, open systems are still the best option.

The effect of nutrients and light regimens on microalgal composition As photoautrophic micro-organisms, microalgae require inorganic nutrients and light for their growth. Inorganic nutrients include carbon dioxide as well as nitrogen and phosphorus. Moreover, the uptake of these nutrients results in their conversion to more complex organic macromolecules within microalgae cell. Carbon, nitrogen, and phosphorus are fi xed within the cells and converted to carbohydrates, proteins, lipids, and nucleic acids. In this manner, the amount uptaken, as well as its intracellular allocation, is directly linked with the macromolecular profi le, thus playing an important role in biofuel production. To maintain microalgae activities, CO2 shortage should be avoided and partial pressure has been suggested to be higher than 0.2 KPa.8 Assuming an average carbon content of 47% in dry biomass, 1.72 g of CO2 is required stoichiometrically to obtain 1 g of biomass. Nevertheless, these requirements depend on light, temperature, and reactor design. These operational parameters directly influence the gas/liquid distribution of CO2. In this manner, CO2 microalgal biofi xation is reported to be higher when employing closed photobioreactors.9,10 In the case of open ponds, the high metabolic activities mediate a high pH in the culture broth, which results in the stripping of ammonia and carbon dioxide. Microalgae fi x nitrogen for the production of protein.11 More specifically, proteins may represent 40–55% dry weight.10,12 Ammonium is the preferred nitrogen form used by microalgae, although some other nitrogenated compounds, such as nitrate, have also been reported as possible uptake forms by Chlorella and Dunaliella.13,14 Phosphorus is employed by microalgae for the production of phospholipids, nucleotides, and nucleic acids.15 Microalgae phosphorus

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb

Review: Impact of microalgae characteristics on their conversion to biofuel

content is approximately 1%,16,17 but it can be increased to 18

3% when appropriate conditions for ‘luxury uptake’ occur. Nutrient uptake rates may vary greatly among microalgal species, reactor design, substrate source, and operational parameters, thus to make interstudies comparison would be worthless. Nevertheless, researchers have recently tried to cope with this issue. For instance, Godos et al.19 compared the performance of different microalgae species on degradation of piggery wastewater. Regarding reactor design, comparison studies on microalgae ecosystems inoculated in open and closed reactors were presented by MolinuevoSalces et al. 20 and Gonzalez-Fernandez et al.10 Microalgal ecosystem performance regarding a different feeding substrate and its effect on different mechanisms involved in nutrients removal was presented by Gonzalez-Fernandez et al.21 Furthermore, when growing a microalgae ecosystem, microbial population may shift, thus operational conditions should be carefully taken care of.22 Temperature and light have been described as the main operational factors affecting effluent quality and biomass productivity in open ponds. 23 Optimal growth of microalgae takes place in the range of 28–35 °C. 24 Lower temperature values result in low metabolic kinetics, while higher ones hamper the microbial growth due to oxidative stress (formation of damaging reactive oxygen species due to the inefficiently used excitation energy). Light penetration within the culture broth concomitantly enhances microalgae growth until the light saturation point. Beyond that point, the light receptors are damaged causing photoinhibition. 25 Moreover, light limitation cannot be entirely overcome since light penetration is inversely proportional to the cell concentration. In this manner, the self-shading effect is common on systems with high cell density or low mixing rate of culture broth. Under those conditions, the natural ability of microalgae to form aggregates and therefore formation of biofi lms contributes to hinder light penetration. The latest research developments focus on genetically and metabolically altering algal species to enhance microalgae productivity or produce a targeted macromolecule. For instance, in order to increase microalgae productivity, recent advances propose to create mutants with truncated antenna size26 while producing a target macromolecule implies the manipulation of carbon metabolism.27 Tabatabaei et al.28

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pointed out that significant progress in genetic engineering of microalgae resumed ten years ago. The low microalgae growth rate, together with the low quantity of gene expression studied, shows that this area still needs further research before the results may be implemented on biofuel productions.29 Harvest Microalgae harvesting is difficult because they are unicellular, have quite similar density to water, and can be motiles. The harvesting technique is chosen depending on microalgae characteristics and subsequent biofuel production. Indeed, this process accounts for 20–30% of the total costs of production.30 Due to the higher cell density encountered in photobioreactors, the harvesting process is cheaper under this configuration. Microalgae cultivation on closed tubular photobioreactors has been shown to retain more than 90% of the total biomass generated.20,31 Nevertheless, as already mentioned, microalgae production for biofuel purposes is usually carried out in open systems. Reviews of harvesting methods may be found elsewhere.4,32

Biofuels production strategies and life cycle assesment (LCA) Biofuel production In the constant need for a renewable energy source, microalgae are nowadays regarded as a source with more potential for producing biofuels. The advantages of microalgae compared to other sources are their fast growth, low water demand,1 and lack of dependency on soil quality (therefore no competition with arable land). In addition, nutrient requirements can be covered by waste streams (i.e. nitrogen and phosphorus may be supplied by some kind of wastewater and carbon dioxide by a power plant). The main microalgae biofuels produced by biochemical conversion include biodiesel, bioethanol, biohydrogen, and biogas. Each of them presents its own benefits and limitations and should be carefully considered when deciding which one to aim for. Green algae, as well as cyanobacteria, were reported to be able to produce hydrogen (Table 1). Microalgae hydrogen production takes place via direct or indirect water photolysis.

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb

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Review: Impact of microalgae characteristics on their conversion to biofuel

Table 1. Microalgal substrate used for different iofuel production. Algae Species

Biofuel

Production

Reference

Synechococcus

Hydrogen

0.02 μmol H2/mg chl a/h

33

Aphanocapsa montana

Hydrogen

0.4 μmol H2/mg chl a/h

33

Chlamydomonas reinhardtii

Hydrogen

22 mmol H2/(mol Chl)/s

34

Chlorococum sp

Bioethanol

3.8 g bioethanol/g DW biomass

35

Chlorella sp

Bioethanol

448 μmol ethanol/g DW biomass

36

Chlamydomonas reinhardtii

Bioethanol

0.5 g bioethanol/g DW biomass

37

Chlorococum sp

Biodiesel

0.010–0.015 g FAME/g DW biomass

38

Chaetoceros gracilis

Biodiesel

0.36 g FAME/g DW biomass

39

Tetraselmis suecica

Biodiesel

0.18 g FAME/g DW biomass

39

Chlorella sorokiniana

Biodiesel

0.18 g FAME/g DW biomass

39

Dunaliella tertiolecta

Biodiesel

0.22 g FAME/g DW biomass

40

Nannochloropsis oculata

Biodiesel

0.07–0.24 g lipids/g DW biomass

41

Chlorella vulgaris

Biodiesel

0.06–0.14 g lipids/g DW biomass

41

FAME-fatty acid methyl ester. DW-dry weight.

The direct strategy involves light energy and the photosynthetic systems of microalgae to convert water into chemical energy. Nevertheless, during photosynthesis, oxygen is produced and may inhibit hydrogenase enzyme activity. To overcome this limitation, Clamydomonas reinhardtii was addressed to be able to decrease oxygen concentration during respiration.34 The second strategy – indirect photolysis – overcomes the limitation of oxygen hydrogenase inhibition by producing photosynthetical oxygen and hydrogen in two separate stages. Hydrogen is produced in the second stage where anaerobiosis is forced by sulfur deprivation. Obviously, the indirect strategy confers an additional complexity to the system. At this point, it should be stressed that according to Benemann,42 for biophotolysis to be a competitive energy source, the solar energy conversion efficiency should be around 10%. The highest values found in literature were reported by Kruse et al.43 who achieved 2% conversion efficiency in lab-scale via direct photolysis. Thus, hydrogen production via microalgae still needs further development before this technology can play an important role in the development of sustainable biofuel production. Carbohydrates contained in microalgae cells are the feedstock for bioethanol production. Two strategies may be followed for ethanol production. The first one implies a dark fermentation by which incomplete oxidative reactions

release sub-products such as carbon dioxide, lactic, formic, and acetic acid and ethanol. The limitation of this technique is the strictly required lack of oxygen and dark conditions. The second strategy entails several steps and therefore leads to a more complex process. Intracellular starch has to be extracted with large amounts of organic solvents. After that, a gelatinization step of the extracted starch is needed prior to the subsequent saccharification and fermentation. Finally, the ethanol obtained needs to be purified and concentrated. All these steps concomitantly increase energy costs of bioethanol production. Indeed, a recent review by John et al.44 pointed out the limitations of bioethanol production which would need to be overcome before this biofuel can be affordable. Research regarding biofuel from microalgae is mainly devoted to the production of biodiesel. Biodiesel is produced by a transesterification process in which triglycerides react with a monoalcohol and a catalyst.2 Thus biodiesel production is extremely dependant on intracellular lipids content. In this context, not all algae are suitable. As a matter of fact, even though some algae such as Chlorella sp. have been claimed to possess up to 30% lipids,2 the same algae specie may present much lower percentages, depending on environmental and operational conditions applied for culturing.10,45 Additionally, not all intracellular lipids are suitable for biodiesel production.2 Therefore, biodiesel production from

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb

Review: Impact of microalgae characteristics on their conversion to biofuel

algae not only lies on the extraction of lipids but also in the finding of high lipid content algae specie.46 Biomethane is produced by anaerobic digestion of organic matter (in our particular case, microalgae). The main products of this process are a semi-solid stream (digestate) and biogas (methane and carbon dioxide). Methane is a renewable form of energy while the other two products (mineralized effluent and carbon dioxide) may be recycled to the microalgae culture system. Opposite to the biofuels production mentioned previously in which only one macromolecule was used as a substrate for each case, biomethane production may use proteins, carbohydrates, and lipids. Furthermore, anaerobic digestion may be applied to microalgae spent once the desired components are used for some other fuel production.47,48

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of water was needed per liter of biodiesel. This is an example that clearly illustrates the confusing numbers that may rise when reading different studies. Additionally, nutrient requirement was also investigated by Yang et al.50 0.33 kg nitrogen and 0.71 kg phosphorus were required to generate 1 kg microalgae biodiesel. Those calculated values could be diminished by 84% and 55%, respectively, when using recycled harvested water. Lardon et al.51 conducted a comparative LCA of different hypothetical strategies that may be followed for biodiesel production. Their LCA revealed that low nitrogen culture condition not only decreases the input of nutrients but also the energy requirement. Most of the impacts relate to energy consumption, fuel combustion, and nutrient input. Nevertheless they demonstrated that oil extraction from the wet algal paste would reduce to 70% the energy

Environmental, technical, and economic aspects of biofuels production Life cycle analysis (LCA) is often referred to as a ‘cradle to grave’ analysis. This type of assessment attempts to quantify the environmental impact of a selected process. Thus, LCA is an essential tool for determining the environmental impacts of each stage involved, from microalgae cultivation to the final biofuel utilization. At this point, it should be stressed that LCA relies on extrapolation of laboratory- or pilot-scale data. LCA reported by different authors may use different assumptions which can result in confusing analysis. Such an assessment is therefore not an exact science and it should be read with care since some authors may overestimate some parameters while others may use a more conservative approach. Even though four main biofuel productions have been mentioned before, literature regarding techno-economic and energy analysis with regard to biohydrogen production is scarce.49 In fact, all recent studies on this topic focus on biodiesel and biomethane production. Therefore, this section will point out some of the later advantageous or disadvantageous findings on biodiesel and biomethane production by using microalgae as a substrate. Concerning water needed for biodiesel production, Yang 50

required (compared with 90% for dry extraction), as well as the impacts. Low impacts for eutrophication and land use were calculated for the best-case scenario, which was the low nitrogen cultivation system and wet oil extraction. Noteworthy to mention is the high negative impact on global warming potential relative to the combustion of microalgae fuel in a diesel engine. The authors suggested that this LCA of hypothetical strategies for biodiesel production should be taken into consideration to identify drawbacks of this technology. In this manner, figures given should be orientative and may help to diagnose which milestone of the whole process needs more research efforts. The overall conclusions of this LCA pointed out nutrient input decrease, wet oil extraction, and anaerobic digestion of the oilcakes as potential valuable options for the sustainable production of biodiesel. An LCA of extensive microalgae culture coupled to biogas production revealed that the main energy consumption was due to the electrical input needed.52 From an energy point of view, methane production would require less energy than biodiesel production, since high biomass concentration and oil extraction may be avoided. Collet et al.52 divided the process into four categories: impacts related to energy required in the facility, to build the infrastructure, for biogas combustion and finally, those related to nutrients. The

et al. indicated that 3726 kg water was required to produce

impacts related to energy corresponded mostly to electric-

one kg microalgae biodiesel. On the other hand, gross estima-

ity consumption of the pond paddlewheel, followed by the

1

tions reported by Wijffels and Barbosa showed that 1.5 liters

pumping between pond and settler, mixing and pumping of

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb

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Review: Impact of microalgae characteristics on their conversion to biofuel

the digester, and centrifugation of the algae. Heating of the digesters did not account for electricity consumption since the heating needed was provided by biogas combustion. Similarly to findings of Lardon et al.,51 biogas combustion

mentioning that these numbers were calculated only for

contributed slightly to eutrophication and acidification; however, global warming contribution was high. Nevertheless, this contribution could be diminished by recycling CO2 to the algal pond. With regard to the infrastructure, Collet et al.52 described the use of concrete for building the ponds as a negative impact by the high emission of CO2 at the cement work level as well as by the fuel consumption for transporting the blocks. In accordance with Lardon et al.,51 nutrient input was also described as an important impact for the sustainable production of biogas. However, the impacts related to nutrients may be drastically reduced by the mineralisation of nitrogen and phosphorus taking place during anaerobic digestion.52 The release and recycling of inorganic compounds may help to offset the nutrients required for microalgal growth.

applied to the anaerobic digestion of microalgae. As a first

Razon and Tan54 compared biodiesel and biogas produc-

microalgae cultivation while leaving lipids extraction and their conversion to biodiesel out of the study. Zamalloa et al.57 presented a technoeconomic study remark, these authors suggested the use of a high-rate anaerobic reactor instead of the completely mixed reactors commonly found in literature. The reason for such a choice was the decrease on hydraulic retention time needed while applying high organic loading rates. While the electrical requirement was 50 KWh ha-1 d-1, the net electric energy produced was 130, 181, and 233 KWh ha-1 d-1 for biomass productivities of 20, 25, and 30 g DW m-2d-1, respectively. Among the different scenarios evaluated, they showed that for this technique to be profitable, productivities of around 25 g DW m-2d-1 are required. Additionally, these authors claimed the need for commitment on behalf of governments to stimulate investment in this technology by introducing subsidies and feed-in tariffs. The outcome of this

tion by net energy analysis. The outcome of their study deter-

study showed the need for feed-in tariffs of 0.133€ KWh-1.

mined that the energy required to culture, dry, and disrupt

Assumptions used in Zamalloa et al.57 were quite optimistic.

the microalgae cell were the main hurdles to attaining a ben-

For instance, some of these overestimations include a really

eficial net energy. Instead of nutrients recyling, these authors

low sedimentation time (4–6 h), quite high fermentation effi-

propose the use of waste-water nutrients as a possible strategy

ciency (75% VS removed), and the absence of pre-treatment

to decrease its energy demand.

or possible inhibitions at high organic loading rates.

Energy burdens given by an LCA calculated for biofuel production nowadays should be used to identify potential hurdles that a given technology may present, thereby indentifying where research should be focused. A less controversial issue is the techno-economic aspects of this renewable energy form which should be economically competitive to replace or cohabit with fossil fuels. Thurmond55 estimated that the production cost of one gallon of biodiesel ranged from $9 to $25 with ponds as cultivation systems; the cost rose to $15–$40 when microalgae were cultivated in photobioreactor. For biodiesel production purposes, Wijffels et al.56 presented the costs for producing microalgae. The initial calculated cost was €4/kg-1, but this was markedly reduced to one-tenth by using wastewater nutrients, exhaust CO2, and by improving location and technology (i.e. enhancing photosynthetic efficiency). Even at this lower price, Wijffels et al.56 indicated that the biodiesel production was not economically feasible. It is worth

Harun et al.58 proposed an integrated approach which includes biodiesel and methane production in order to improve the economics and sustainability of the overall biodiesel productive chain. The theoretical study evaluates a different integrated approach, which includes the production of methane as a sole biofuel, methane-ethanol, methanebiodiesel, and biodiesel-ethanol by using the microalgae Tetraselmis suecica. The underlying idea in this biofuel combination is the use of the different macromolecules separately for each individual biofuel production and their comparison with methane production obtained by digesting the whole cells. Indeed, this later approach provided the highest methane production (0.48 m3 Kg DW biomass-1). With regard to the economic analysis, the cost of 1 m3 of biodiesel could be reduced by 35% when biodiesel is produced with the integration of methane production compared to biodiesel production itself. Additionally, the impact on greenhouse gas emissions was also reduced by combining methane and biodiesel

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb

Review: Impact of microalgae characteristics on their conversion to biofuel

production. More specifically, 75% higher carbon emission was calculated when electricity was supplied by the grid instead of using electricity produced from methane generation. It is noteworthy to mention that this combination may be not be energetically feasible if the lipid cell concentration is lower than 40%.47 The aforementioned studies highlight the great potential of microalgae as a renewable feedstock for biofuel production. Nowadays, in order to decrease costs and improve the sustainability of processes, the tendency is to combine different biofuel production processes (biorefinery approach). Nevertheless, according to Wijffels and Barbosa,1 a feasible production of biodiesel from microalgae on a large scale will have to wait for another 10–15 years.

Conclusions Microalgae are regarded nowadays as a promising sustainable energy resource. Large quantities of biomass are required for successful biofuel production. Current efforts are focused on cultivation systems. Nutrients, light, temperature, mixing regimens, microalgae strain, and reactor design drastically affect microalgae growth. Modifying any of those parameters causes an effect on microalgae growth, composition, or structure. Therefore, microalgae cultivation is directly linked to biofuel production. Different biofuel production methods, such as biodiesel, bioethanol, biohydrogen and biogas, present advantages and limitations. Based on LCA and technoeconomic studies regarding biofuel production, the results indicate that even though microalgae are more environmentally friendly than fossil fuels, microalgae production and subsequent biofuel production are still unaffordable. At the current stage of biofuel development, it is still too early to point out which one would be the most beneficial route for biofuels production from algal biomass.

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Acknowledgement This research was financially supported by the French National Research Agency for the Symbiose project (ANR-08-BIO-E11).

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Dr Nicolas Bernet is a research director at INRA-LBE. He joined the LBE in 1992 where he works on biological wastewater treatment processes, including nutrient removal and anaerobic digestion. His research interests focus on bioprocesses for bioenergy production in a context of environmental biorefinery. He received his PhD in Food Science at Montpellier SupAgro.

(2011).

Cristina González-Fernández

Jean-Philippe Steyer

Dr Cristina González-Fernández is a PostDoc at INRA-LBE in Narbonne (France). Her main

Dr Jean-Philippe Steyer is Director of the Laboratoire de Biotechnology de l’Environnement

research topics focus on microalgae environmental technology and bioconversion processes. She first undertook a masters degree in Cincinnati (USA) and later obtained her PhD degree from the University of Valladolid (Spain). After that, she worked at a public institution on wastewater treatment.

of INRA in Narbonne France where he has been working since 1993. An international leading expert on the instrumentation, modeling and control of anaerobic digestion processes, he has participated in many international projects including six European projects. In 2005, he was visiting professor within an EU Marie Curie fellowship at the Technical University of Denmark studying modeling and control of anaerobic digestion for bioenergy (hydrogen) production. For the last three years, he has focused on the combination of anaerobic digestion processes with microalgae cultivation ponds.

Bruno Sialve Bruno Sialve is a research engineer at the Naskeo Environment and project manager of the Symbiose research program focusing on the combination of anaerobic digestion process with microalgae cultivation. His field of research also includes anaerobic digestion, algae production for bioenergy, and wastewater treatment. He has been working for several private companies and research institutes in the field of waste and wastewater treatment since 1996.

© 2011 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. DOI: 10.1002/bbb