Microalgae are regarded as a potential biomass source for biofuel purposes. With regard to bioethanol production, microalgae seem to overcome traditional substrate drawbacks. Enzymatic activities are ...responsible for carbon allocation and hence for carbohydrate profiles. Enzyme activities may be manipulated by metabolic engineering; however, this goal may also be achieved by controlling environmental conditions of the culture system. We outline the key-enzymes as well as the main operational conditions applied to microalgae growth (inorganic nutrient supplementation, irradiance and temperature) that affect carbohydrate synthesis on microalgae and cyanobacteria. Normally, harsh conditions are needed for such a goal and thus, arrested microalgae growth may occur. Potential strategies to avoid arrested growth, while enhancing carbohydrate accumulation, were also pointed out in this review.
With increasing concerns regarding energy and environment, algae biofuel is generating considerable interest around the world. Nevertheless, the harvesting step required before downstream biomass ...processing is a major bottleneck. Commonly employed methods include addition of chemicals or use of mechanical equipment that increase dramatically the biofuel production cost. This review deals with naturally occurring processes that can help offset those costs by causing microalgae flocculation. Interaction theories are briefly reviewed. In addition, operational parameters such as pH, irradiance, nutrients, dissolved oxygen, and temperature effect on microalgae flocculation are evaluated. Finally, microalgae flocculation is also considered from an ecological point of view by taking advantage of their interaction with other microorganisms.
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•Successful acclimation of methanogenic culture to extreme ammonia levels in CSTR.•Efficient utilization of a 3rd generation biomass as biomethanation substrate.•The most abundant ...bacterium (Shinella spp.) was not affected by the ammonia levels.•C. ultunense increased significantly its abundance during the acclimation process.•Methanosarcina spp. was the most abundant methanogen at the highest ammonia levels.
Acclimatized anaerobic communities to high ammonia levels can offer a solution to the ammonia toxicity problem in biogas reactors. In the current study, a stepwise acclimation strategy up to 10g NH4+-N L−1, was performed in mesophilic (37±1°C) continuously stirred tank reactors. The reactors were co-digesting (20/80 based on volatile solid) cattle slurry and microalgae, a protein-rich, 3rd generation biomass. Throughout the acclimation period, methane production was stable with more than 95% of the uninhibited yield. Next generation 16S rRNA gene sequencing revealed a dramatic microbiome change throughout the ammonia acclimation process. Clostridium ultunense, a syntrophic acetate oxidizing bacteria, increased significantly alongside with hydrogenotrophic methanogen Methanoculleus spp., indicating strong hydrogenotrophic methanogenic activity at extreme ammonia levels (>7g NH4+-N L−1). Overall, this study demonstrated for the first time that acclimation of methanogenic communities to extreme ammonia levels in continuous AD process is possible, by developing a specialised acclimation AD microbiome.
•Eucalyptus globulus was successfully fractionated by uncatalyzed steam explosion.•Soluble hemicelluloses derived compounds were obtained at good yield.•Solid phases from pretreatment showed high ...enzymatic susceptibility (>99%).•SSF of solid phase led to competitive results (51g ethanol/L, ethanol yield 91%).
Second generation ethanol was obtained from steam exploded Eucalyptus globulus. The wood was subjected to steam explosion without external catalyst addition, in order to alter the feedstock structure and to solubilise the hemicellulosic fraction. The experimental work was carried out at severities (So) in the range 3.52–4.83, and the treated solids were subjected to enzymatic hydrolysis. The effects of temperature (°C) and processing time (min) on the sugar yield (glucose and xylose) obtained in enzymatic hydrolysis and on the kinetic parameters describing the saccharification were measured. In most of experiments, cellulose was almost quantitatively retained in solid phase. Autohydrolysis experiments performed under selected conditions (195°C and 34min) enabled the recovery of 18.1g sugars/100g oven-dry raw material in liquor, and led to spent solids highly susceptible to enzymatic hydrolysis (for which cellulose conversions up to 99.5% were reached). Simultaneous Saccharification and Fermentation of solids from selected autohydrolysis experiments led to media containing 51g ethanol/L (corresponding to 91% cellulose conversion into ethanol).
•Efficiency of anaerobic digestion is hampered due to microalgae cell wall.•Organic matter solubilisation and improved biodegradability was achieved by all thermal pretreatments.•Thermal ...pretreatments enhanced hydrolysis rate constant and thus faster biogas production.•Carbohydrates solubilised provided a fairly close estimation for methane production.•Best case scenario was achieved by subjecting biomass to 160°C which resulted in 64% methane yield enhancement.
This study investigates the effect of high pressure thermal hydrolysis on organic matter solubilisation and biogas production from Chlorella vulgaris biomass. Microalgae biomass was subjected to three temperatures, namely 140, 160, and 180°C and two heating times (10 and 20min). Results showed that carbohydrates release prevailed over proteins. Carbohydrates were solubilised concomitantly with increasing temperatures. According to the infrared spectra and monomeric sugars determined in the pretreated medium, temperatures applied clearly affected the solubilisation of structural carbohydrates of the microalgae cell wall. Likewise, thermal pretreatment provided enhanced methane production with regard to the raw algal biomass. Enhanced hydrolysis rate constant supported faster biogas production. Regardless the heating time employed, increasing temperatures depicted increasing methane production. Even thought, organic matter solubilisation was greater at 180°C, the anaerobic biodegradability did not show the same trend. This fact was ascribed to the formation of reaction products that hampered methane production. Best case scenario was achieved by subjecting biomass to 160°C which resulted in 64% methane yield enhancement.
Biogas generation is the least complex technology to transform microalgae biomass into bioenergy. Since hydrolysis has been pointed out as the rate limiting stage of anaerobic digestion, the main ...challenge for an efficient biogas production is the optimization of cell wall disruption/hydrolysis. Among all tested pretreatments, enzymatic treatments were demonstrated not only very effective in disruption/hydrolysis but they also revealed the impact of microalgae macromolecular composition in the anaerobic process. Although carbohydrates have been traditionally recognized as the polymers responsible for the low microalgae digestibility, protease addition resulted in the highest organic matter solubilization and the highest methane production. However, protein solubilization during the pretreatment can result in anaerobic digestion inhibition due to the release of large amounts of ammonium nitrogen. The possible solutions to overcome these negative effects include the reduction of protein biomass levels by culturing the microalgae in low nitrogen media and the use of ammonia tolerant anaerobic inocula. Overall, this review is intended to evidence the relevance of microalgae proteins in different stages of anaerobic digestion, namely hydrolysis and methanogenesis.
•Efficiency of anaerobic digestion is hampered due to microalgae cell wall.•Improved biodegradability was achieved by all the evaluated pretreatments.•Thermal treatment for 40min and 120°C supported ...the highest methane production.•Higher biomass concentration is required to obtain a positive energy balance.
To enhance the anaerobic digestion of Chlorella vulgaris, thermochemical pretreatments were conducted. All pretreatments markedly improved solubilisation of carbohydrates. Thermal treatments and thermal treatments combined with alkali resulted in 5-fold increase of soluble carbohydrates while thermal treatment with acid addition enhanced by 7-fold. On the other hand, proteins were only solubilized with thermo-alkaline conditions applied. Likewise, all the pretreatments tested improved methane production. Highest anaerobic digestion was accomplished by thermal treatment at 120°C for 40min without any chemical addition. As a matter of fact, hydrolysis constant rate was doubled under this condition. According to the energetic analysis, energy input was higher than the extra energy gain at the solid concentration employed. Nevertheless, higher biomass organic load pretreatment may be an option to achieve positive energetic balances.
Olive crop and olive oil industry generates several residues, i.e., olive tree pruning biomass (OTPB), extracted olive pomace (EOP) and olive leaves (OL) that could be used to produce high-added ...value products in an integrated biorefinery. OTPB is generated in the field as a result of pruning operation to remove old branches; EOP is the main residue of the pomace olive oil extracting industry after extraction with hexane of residual oil contained in olive pomace; and OL comes from the olive cleaning process carried out at olive mills, where small branches and leaves are separated by density. In this work, an analysis of the potential of OTPB, EOP and OL residues was addressed by estimating the production volumes at national level and the spatial distribution of these residues using geographic information system software. Information provided by public institutions and personal surveys to the industries was evaluated. Moreover, chemical analysis of the residues was undertaken and the results used to make a first assessment of valorization into biofuels such as bioethanol and bio based chemicals. Results show that close to 4.2 million tons/year of EOP, OL and OTPB derived from olive oil industry and olive tree cultivation in Spain could be available as a raw material for biorefineries in Spain. The analysis of the chemical characteristics indicates the relevant potential of these feedstocks for the production of bioethanol and other compounds such as phenols based on suitable processing and conversion routes, although techno-economic evaluations must be tackled to refine this approach.
Rapeseed straw constitutes an agricultural residue with great potential as feedstock for ethanol production. In this work, uncatalyzed steam explosion was carried out as a pretreatment to increase ...the enzymatic digestibility of rapeseed straw. Experimental statistical design and response surface methodology were used to evaluate the influence of the temperature (185-215°C) and the process time (2.5-7.5min). According to the rotatable central composite design applied, 215°C and 7.5min were confirmed to be the optimal conditions, considering the maximization of enzymatic hydrolysis yield as optimization criterion. These conditions led to a maximum yield of 72.3%, equivalent to 81% of potential glucose in pretreated solid. Different configurations for bioethanol production from steam exploded rapeseed straw were investigated using the pretreated solid obtained under optimal conditions as a substrate. As a relevant result, concentrations of ethanol as high as 43.6g/L (5.5% by volume) were obtained as a consequence of using 20% (w/v) solid loading, equivalent to 12.4g ethanol/100g biomass.
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