• Energy Research

Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.

AbstractMicroalgae 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

Lignocellulosic biomass utilization is challenging due to the presence of several carbon sources. Thus, microorganisms with different sugar preferences can be used in co-cultures to overcome this hurdle. This work addressed the simultaneous production of lactic acid (LA) from C5 sugars (i.e., xylose) and bioethanol from C6 (i. e., glucose/fructose) with Bacillus coagulans and Kluyveromyces marxianus, respectively. Sequential inoculation and co-inoculation of microorganisms were also compared. At pH 6, co-inoculation in synthetic media resulted in higher bioethanol (0.51 g/g) and LA (0.98 g/g) yields than sequential inoculation. Furthermore, when using lignocellulosic hydrolysates obtained after enzymatic hydrolysis of 20 % w/w pomegranate peels (PP), 92 % and 98 % of the theoretical maximum bioethanol and LA, respectively, were obtained. This study demonstrated the efficient bioethanol/LA co-generation despite the different optimum fermentation conditions of microorganisms and will pave the way to consider co-cultures for improving process efficiency in lignocellulosic biorefineries.

Abstract Research into the development of renewable and sustainable fuels has been a major concern during last decades. Microalgae, as a potential resource, have gained great attention for energy purposes. In this context, anaerobic digestion seems to be the most direct energy generation process. Nevertheless, the efficiency of this process is hampered due to the hard cell wall of some microalgae. In order to enhance its anaerobic biodegradability, the present research investigated the effect of thermal pretreatment at two temperatures (70 and 90 °C) applied to Scenedesmus biomass. No differences were detected in terms of organic matter or ammonium release upon the two tested temperatures. Nevertheless, a different fact was observed for their anaerobic biodegradability. While raw and pretreated at 70 °C microalgae attained 22–24% anaerobic biodegradability, microalgae pretreated at 90 °C achieved anaerobic biodegradability of 48%. Even though similar profiles were obtained for both temperatures along the pretreatment period, the damage caused in the cell wall at 90 °C seemed to be greater and rendered this substrate readily degradable for anaerobic digestion.

AbstractLignocellulosic ethanol production requires high substrate concentrations for its cost-competitiveness. This implies the presence of high concentrations of insoluble solids (IS) at the initial stages of the process, which may limit the fermentation performance of the corresponding microorganism. The presence of 40–60% IS (w/w) resulted in lower glucose consumption rates and reduced ethanol volumetric productivities of Saccharomyces cerevisiae F12. Yeast cells exposed to IS exhibited a wrinkled cell surface and a reduced mean cell size due to cavity formation. In addition, the intracellular levels of reactive oxygen species (ROS) increased up to 40%. These ROS levels increased up to 70% when both lignocellulose-derived inhibitors and IS were simultaneously present. The general stress response mechanisms (e.g. DDR2, TPS1 or ZWF1 genes, trehalose and glycogen biosynthesis, and DNA repair mechanisms) were found repressed, and ROS formation could not be counteracted by the induction of the genes involved in repairing the oxidative damage such as glutathione, thioredoxin and methionine scavenging systems (e.g. CTA1, GRX4, MXR1, and TSA1; and the repression of cell cycle progression, CLN3). Overall, these results clearly show the role of IS as an important microbial stress factor that affect yeast cells at physical, physiological, and molecular levels.

Ultrasound at 20Hz was applied at different energy levels (Es) to treat Scenedesmus biomass, and organic matter solubilization, particle size distribution, cell disruption and biochemical methane potential were evaluated. An Es of 35.5 and 47.2MJ/kg resulted in floc deagglomeration but no improvement in methane production compared to untreated biomass. At an Es of 128.9, cell wall disruption was observed together with a 3.1-fold organic matter solubilization and an approximately 2-fold methane production in comparison with untreated biomass. Thermal pretreatment at 80°C caused cell wall disruption and improved anaerobic biodegradability 1.6-fold compared to untreated biomass. Since sonication caused a temperature increase in samples to as high as 85°C, it is likely that thermal effects accounted for much of the observed changes in the biomass. Given that ultrasound treatment at the highest Es studied only increased methane production by 1.2-fold over thermal treatment at 80°C, the higher energy requirement of sonication might not justify the use of this approach over thermal treatment.

AbstractAmong biofuel production processes using microalgal biomass, biogas generation seems to be the least complex. This review summarizes information regarding anaerobic digestion of different microalgae species. Various operational parameters and microalgae characteristics (macromolecular distribution and cell wall) are reviewed in the light of their effects on methane production. Additionally, the enhancement of methane production rates achievable by applying biomass pre‐treatments and codigestion of substrates is also reported. The review finally covers the so‐claimed similarities of microalgal biomass and activated sludge as a substrate for anaerobic digestion. © 2011 Society of Chemical Industry and John Wiley & Sons Ltd

Biogas production is one of the means to produce a biofuel from microalgae. Biomass consisting mainly of Scenedesmus sp. was thermally pretreated and optimum pretreatment length (1 h) and temperature (90 °C) was selected. Different chemical composition among batches stored at 4 °C for different lengths of time resulted in organic matter hydrolysis percentages ranging from 3% to 7%. The lower percentages were attributed to cell wall thickening observed during storage for 45 days. The different hydrolysis percentages did not cause differences in anaerobic digestion. Pretreatment of Scenedesmus sp. at 90 °C for 1h increased methane production 2.9 and 3.4-fold at organic loading rates (OLR) of 1 and 2.5 kg COD m(-3) day(-1), respectively. Regardless the OLR, inhibition caused by organic overloading or ammonia toxicity were not detected. Despite enhanced methane production, anaerobic biodegradability of this biomass remained low (32%). Therefore, this microalga is not a suitable feedstock for biogas production unless a more suitable pretreatment can be found.