Anaerobic digestate as substrate for microalgae culture: the role of ammonium concentration on the microalgae productivity

In spite of the increasing interest received by microalgae as potential alternatives for biofuel production, the technology is still not industrially viable. The utilization of digestate as carbon and nutrients source can enhance microalgal growth reducing costs and environmental impacts. This work assesses microalgal growth utilizing the liquid phase of anaerobic digestate effluent as substrate. The effect of inoculum/substrate ratio on microalgal growth was studied in a laboratory batch experiment conduced in 0.5 L flasks. Results suggested that digestate may be an effective substrate for microalgal growth promoting biomass production up to 2.6 gTSS/L. Microalgal growth rate was negatively affected by a self-shading phenomenon, while biomass production was positively correlated with the inoculum and substrate concentrations. Thus, the increasing of both digestate and microalgal initial concentration may reduce the initial growth rate (µ from 0.9 to 0.04 d-1) but significantly enhances biomass production (from 0.1 to 2.6 gTSS/L).Peer ReviewedPostprint (published version

CO2 addition to increase biomass production and control microalgae species in high rate algal ponds treating wastewater

Challenges regarding microalgal cultivation need to be solved in order to enhance microalgae potential as a feedstock for biofuel, bioenergy, and bioproducts. The optimization of the operating strategy in high rate algal ponds treating wastewater still requires research on microalgal ecosystem response to variations in nutrients availability. For this reason, the aim of this study was to determine the effect of CO2 addition on microalgal population diversity and wastewater treatment performance. To this end, batch and continuous experiments were carried out in an experimental plant constituted by four high rate algal ponds (500 L each) treating urban wastewater with and without pH regulation. As expected, CO2 addition induced a significant increase in biomass concentration (between 66 and 100%). Moreover, a positive effect on microalgal biomass concentration was observed, reducing the effect of the variation in influent wastewater characteristics. Concerning the microalgal populations, the variation of inorganic carbon availability induced a shift in the dominant microalgae species. In spite of this, no variations were observed in terms of wastewater treatment efficiency. Taking together, this study highlighted the positive effect of CO2 addition to increase biomass production and control microalgae species in high rate algal ponds treating wastewater.Peer ReviewedPostprint (author's final draft

Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable.

International audienceThe potential of microalgae as a source of biofuels and as a technological solution for CO2 fixation is subject to intense academic and industrial research. In the perspective of setting up massive cultures, the management of large quantities of residual biomass and the high amounts of fertilizers must be considered. Anaerobic digestion is a key process that can solve this waste issue as well as the economical and energetic balance of such a promising technology. Indeed, the conversion of algal biomass after lipid extraction into methane is a process that can recover more energy than the energy from the cell lipids. Three main bottlenecks are identified to digest microalgae. First, the biodegradability of microalgae can be low depending on both the biochemical composition and the nature of the cell wall. Then, the high cellular protein content results in ammonia release which can lead to potential toxicity. Finally, the presence of sodium for marine species can also affect the digester performance. Physico-chemical pretreatment, co-digestion, or control of gross composition are strategies that can significantly and efficiently increase the conversion yield of the algal organic matter into methane. When the cell lipid content does not exceed 40%, anaerobic digestion of the whole biomass appears to be the optimal strategy on an energy balance basis, for the energetic recovery of cell biomass. Lastly, the ability of these CO2 consuming microalgae to purify biogas and concentrate methane is discussed

Hydrodynamics-Biology Coupling for Algae Culture and Biofuel Production

International audienceBiofuel production from microalgae represents an acute optimization problem for industry. There is a wide range of parameters that must be taken into account in the development of this technology. Here, mathematical modelling has a vital role to play. The potential of microalgae as a source of biofuel and as a technological solution for CO2 fixation is the subject of intense academic and industrial research. Large-scale production of microalgae has potential for biofuel applications owing to the high productivity that can be attained in high-rate raceway ponds. We show, through 3D numerical simulations, that our approach is capable of discriminating between situations where the paddle wheel is rapidly moving water or slowly agitating the process. Moreover, the simulated velocity fields can provide lagrangian trajectories of the algae. The resulting light pattern to which each cell is submitted when travelling from light (surface) to dark (bottom) can then be derived. It will then be reproduced in lab experiments to study photosynthesis under realistic light patterns

New mechanistic model to simulate microalgae growth

The prospect of treating wastewater and at the same time producing microalgae biomass is receiving increasing attention. Mechanistic models for microalgae growth in wastewater are currently being developed for new systems design as well as to improve the understanding of the involved biokinetic processes. However, mathematical models able to describe the complexity of microalgal cultures are still not a common practice. The aim of the present study is to present and calibrate a new mechanistic model built in COMSOL Multiphysics™ platform for the description of microalgae growth. Carbon-limited algal growth, transfer of gases to the atmosphere; and photorespiration, photosynthesis kinetics and photoinhibition are included. The model considers the growth of microalgae as a function of light intensity and temperature, as well as availability of nitrogen and other nutrients. The model was calibrated using experimental data from a case study based on the cultivation of microalgae species in synthetic culture medium. The model was able to reproduce experimental data. Simulation results show the potential of the model to predict microalgae growth and production, nutrient uptake, and the influence of temperature, light intensity and pH on biokinetic processes of microalgae.Peer ReviewedPostprint (author's final draft

Coupling microalgae culture and anaerobic digestion : Treatment and valorisation within environmental biorefinery

L’utilisation des microalgues dans les filières bioénergies est une thématique qui connait un développement remarquable ces dernières années. Si dans une perspective d’exploitation de masse, elles permettent de répondre plus favorablement aux contraintes qui pèsent sur l’exploitation des biocarburants de première et de deuxième génération, elles se heurtent également à la question de la demande en éléments nutritifs mais aussi à un bilan énergétique défavorable. En conséquence, il apparait difficile de répondre à une exigence de durabilité attendue pour ce nouveau gisement. Ce travail de thèse s’est intéressé à une solution permettant à la fois de recycler les éléments nutritifs présents dans la biomasse et de fournir de l’énergie au système de production voire de transformation : la digestion anaérobie. Les travaux se sont particulièrement focalisés sur l’intégration de la production de microalgues et de la méthanisation au travers de la conversion énergétique de cette biomasse et de la mobilisation des éléments nutritifs vers la culture à l’échelle du laboratoire et à l’échelle pilote. Après avoir identifié les contraintes associées à la biodégradabilité anaérobie des microalgues et les stratégies d’optimisation, nous avons mis en évidence que le potentiel énergétique est contraint par la qualité propre des cellules et une capacité de résistance à la dégradation biologique. L’application de stratégies d’optimisation de cette étape de conversion via l’utilisation de prétraitement thermique a montré qu’il est possible d’augmenter les rendements de production d’énergie et d’éléments minéraux mobilisables vers la culture. L’utilisation d’un écosystème naturel microalgue-bactérie destiné à la production en milieu ouvert et qui utilise un digestat synthétique comme milieu de culture a révélé le rôle déterminant de la flore bactérienne associée en interaction avec les microalgues. Ces résultats ont été évalués dans un système de production à l’échelle pilote préindustrielle en conditions extérieures, conçu et opéré spécifiquement pour répondre à cette problématique. Les caractéristiques propres du bassin de culture déterminent le comportement hydrodynamique du milieu et le comportement physique et écologique de la population phytoplanctonique mobilisée. L’étude de la dynamique des communautés microbiennes, eucaryotes et procaryotes, confirme le potentiel de résilience et de production d’un écosystème complexe soumis aux contraintes de son environnement. Les résultats de ces travaux ouvrent des perspectives de gestion et d’optimisation des procédés intégrant l’algoculture et la méthanisation qui peuvent répondre plus largement à des problématiques environnementales et de production de molécules d’intérêt au-delà des filières énergétiques.In recent years, there has been an explosion of interest in the use of microalgae as a source of bioenergy. Mass cultivation of microalgae for bioenergy production promises several advantages compared to first and second generation biofuels. However, similar difficulties in terms of nutrient requirements and an unfavourable energy balance are faced. As a consequence, achieving the sustainable levels of microalgal culture required to implement this strategy in the longer term appears problematic. The work presented in this thesis focuses on anaerobic digestion; a solution which allows both recycling of nutrients and supply of energy to the production and downstream processes. In particular, the studies presented here have been directed towards the integration of microalgal culture and methanisation, at both the laboratory and pilot scale. The guiding principle used is conversion of biomass and provision of nutrients to the culture. We first identified the constraints and potential strategies associated with the aerobic biodegradability of microalgae. Next, we demonstrated that the energetic potential of cells is limited by their quality as well as their level of resistance to biological degradation. We have shown that it is possible to optimise the conversion step, increasing energy yields and nutrient mineralisation via a strategy of thermal pretreatment The use of a natural microalgae-bacteria ecosystem which uses a synthetic digestate as culture media, revealed a key role for bacterial flora interacting with microalgae. These results were further tested in a pilot-level production system specifically designed to address these questions. The evidence suggests that the characteristics of the culture pond determine both the hydrodynamic behaviour of the culture and the physical and ecological behaviour of the phytoplanktonic population. A study of the dynamics of the microbial, eukaryotic and prokaryotic communities suggests the presence of a resilient and complex ecosystem, which is influenced by variations in its environment. The results of this work provide opportunities for management and optimisation of processes integrating microalgae cultivation and methanisation beyond bioenergy production, for example liquid wastes treatment and production of high-value byproducts

Couplage des cultures de microalgues avec la méthanisation : traitement et valorisation de la matière et de l’énergie dans le cadre de la bioraffinerie environnementale

In recent years, there has been an explosion of interest in the use of microalgae as a source of bioenergy. Mass cultivation of microalgae for bioenergy production promises several advantages compared to first and second generation biofuels. However, similar difficulties in terms of nutrient requirements and an unfavourable energy balance are faced. As a consequence, achieving the sustainable levels of microalgal culture required to implement this strategy in the longer term appears problematic. The work presented in this thesis focuses on anaerobic digestion; a solution which allows both recycling of nutrients and supply of energy to the production and downstream processes. In particular, the studies presented here have been directed towards the integration of microalgal culture and methanisation, at both the laboratory and pilot scale. The guiding principle used is conversion of biomass and provision of nutrients to the culture. We first identified the constraints and potential strategies associated with the aerobic biodegradability of microalgae. Next, we demonstrated that the energetic potential of cells is limited by their quality as well as their level of resistance to biological degradation. We have shown that it is possible to optimise the conversion step, increasing energy yields and nutrient mineralisation via a strategy of thermal pretreatment The use of a natural microalgae-bacteria ecosystem which uses a synthetic digestate as culture media, revealed a key role for bacterial flora interacting with microalgae. These results were further tested in a pilot-level production system specifically designed to address these questions. The evidence suggests that the characteristics of the culture pond determine both the hydrodynamic behaviour of the culture and the physical and ecological behaviour of the phytoplanktonic population. A study of the dynamics of the microbial, eukaryotic and prokaryotic communities suggests the presence of a resilient and complex ecosystem, which is influenced by variations in its environment. The results of this work provide opportunities for management and optimisation of processes integrating microalgae cultivation and methanisation beyond bioenergy production, for example liquid wastes treatment and production of high-value byproducts.L’utilisation des microalgues dans les filières bioénergies est une thématique qui connait un développement remarquable ces dernières années. Si dans une perspective d’exploitation de masse, elles permettent de répondre plus favorablement aux contraintes qui pèsent sur l’exploitation des biocarburants de première et de deuxième génération, elles se heurtent également à la question de la demande en éléments nutritifs mais aussi à un bilan énergétique défavorable. En conséquence, il apparait difficile de répondre à une exigence de durabilité attendue pour ce nouveau gisement. Ce travail de thèse s’est intéressé à une solution permettant à la fois de recycler les éléments nutritifs présents dans la biomasse et de fournir de l’énergie au système de production voire de transformation : la digestion anaérobie. Les travaux se sont particulièrement focalisés sur l’intégration de la production de microalgues et de la méthanisation au travers de la conversion énergétique de cette biomasse et de la mobilisation des éléments nutritifs vers la culture à l’échelle du laboratoire et à l’échelle pilote. Après avoir identifié les contraintes associées à la biodégradabilité anaérobie des microalgues et les stratégies d’optimisation, nous avons mis en évidence que le potentiel énergétique est contraint par la qualité propre des cellules et une capacité de résistance à la dégradation biologique. L’application de stratégies d’optimisation de cette étape de conversion via l’utilisation de prétraitement thermique a montré qu’il est possible d’augmenter les rendements de production d’énergie et d’éléments minéraux mobilisables vers la culture. L’utilisation d’un écosystème naturel microalgue-bactérie destiné à la production en milieu ouvert et qui utilise un digestat synthétique comme milieu de culture a révélé le rôle déterminant de la flore bactérienne associée en interaction avec les microalgues. Ces résultats ont été évalués dans un système de production à l’échelle pilote préindustrielle en conditions extérieures, conçu et opéré spécifiquement pour répondre à cette problématique. Les caractéristiques propres du bassin de culture déterminent le comportement hydrodynamique du milieu et le comportement physique et écologique de la population phytoplanctonique mobilisée. L’étude de la dynamique des communautés microbiennes, eucaryotes et procaryotes, confirme le potentiel de résilience et de production d’un écosystème complexe soumis aux contraintes de son environnement. Les résultats de ces travaux ouvrent des perspectives de gestion et d’optimisation des procédés intégrant l’algoculture et la méthanisation qui peuvent répondre plus largement à des problématiques environnementales et de production de molécules d’intérêt au-delà des filières énergétiques

Les microalgues, promesses et défis

Colloque « Les biotechnologies pour relever le défi du carbone renouvelable » Toulouse 18/04/2013Microalgae are currently the topic of a growing interest due to their high and numerous potentialities. Internationally, scientists and industries are deeply involved looking at fields of application such as energy production, human and animal nutrition, treatment of anthropogenic waste. There are still a lot of scientific and technological bottlenecks and challenges that need to be solved and a high potential for innovations between agro-ecological and techno-economical appproaches to succeed in creating an industrial sector. This article aims to provide an overview of the spectrum of potentialies and present some INRA work on the subject.Les microalgues suscitent aujourd’hui un intérêt grandissant tant les applications qui convergent autour de ces cellules sont nombreuses. De la production d’énergie à la nutrition humaine et animale, en passant par le traitement des rejets anthropiques, chercheurs et industriels s’investissent dans ces vastes domaines d’application. Il y a aujourd’hui une multitude de verrous scientifiques et technologiques qui méritent d’être levés, carrefours d’innovations entre approches agro-écologiques et visions techno-économiques, pour parvenir à créer une filière industrielle sur le sujet. Cet article se propose de dresser un panorama du spectre des possibles et de présenter certains travaux de l’INRA sur le sujet

CO2 addition to increase biomass production and control microalgae species in high rate algal ponds treating wastewater

Challenges regarding microalgal cultivation need to be solved in order to enhance microalgae potential as a feedstock for biofuel, bioenergy, and bioproducts. The optimization of the operating strategy in high rate algal ponds treating wastewater still requires research on microalgal ecosystem response to variations in nutrients availability. For this reason, the aim of this study was to determine the effect of CO2 addition on microalgal population diversity and wastewater treatment performance. To this end, batch and continuous experiments were carried out in an experimental plant constituted by four high rate algal ponds (500 L each) treating urban wastewater with and without pH regulation. As expected, CO2 addition induced a significant increase in biomass concentration (between 66 and 100%). Moreover, a positive effect on microalgal biomass concentration was observed, reducing the effect of the variation in influent wastewater characteristics. Concerning the microalgal populations, the variation of inorganic carbon availability induced a shift in the dominant microalgae species. In spite of this, no variations were observed in terms of wastewater treatment efficiency. Taking together, this study highlighted the positive effect of CO2 addition to increase biomass production and control microalgae species in high rate algal ponds treating wastewater.Peer Reviewe

Un zeste de mathématiques pour les biocarburants de demain

National audienceDes modèles mathématiques ont été développés pour représenter, sous forme d'équations aux dérivées partielles, la dynamique de l'écoulement dans le bassin ainsi que l'évolution des concentrations des espèces biologiques. Une fois résolus informatiquement, ces modèles permettent d'obtenir des simulations numériques et donnent accès à des quantités physiques utiles. On peut ainsi améliorer l'agitation générée par la roue à aubes ou encore optimiser les espèces de micro-algues cultivées dans le bassin, tout en réduisant la dépense énergétique associée à la production. Même si les résultats des calculs doivent être utilisés en complément de résultats expérimentaux, ils sont plus rapides et moins coûteux à obtenir, et permettent d'appréhender les contraintes et les impacts de la production de masse à grande échelle. Les simulateurs ainsi développés seront particulièrement utiles pour développer et rationaliser la filière industrielle, comprenant un ensemble d'acteurs économiques qui mèneront ces petites algues du fond de leur bassin au carburateur de nos véhicules du futur