Effect of food wastewater on biomass production by a green microalga Scenedesmus obliquus for bioenergy generation

Effect of food wastewater (FW) on the biomass, lipid and carbohydrate production by a green microalga Scenedesmus obliquus cultivated in Bold's Basal Medium (BBM) was investigated. Different dilution ratios (0.5-10%) of BBM either with FW or salt solution (NaCl) or sea water (SW) were evaluated. S. obliquus showed the highest growth (0.41 g L(-1)), lipid productivity (13.3 mg L(-1) day L(-1)), carbohydrate productivity (14.7 mg L(-1) day L(-1)) and nutrient removal (38.9 mg TN L(-1) and 12.1 mg TP L(-1)) with 1% FW after 6 days of cultivation. The FW promoted algal autoflocculation due to formation of inorganic precipitates at an alkali pH. Fatty acid methyl ester analysis revealed that the palmitic and oleic acid contents were increased up to 8% with FW. Application of FW improved the growth, lipid/carbohydrate productivity and biomass recovery efficiency of S. obliquus, which can be exploited for cost effective production of microalgae biomass.

Variations of organic matters and microbial community in thermophilic anaerobic digestion of waste activated sludge with the addition of ferric salts

Ferric salts will influence the thermophilic anaerobic digestion of waste activated sludge (WAS). FeCl3 was found to contribute to the anaerobic digestion process with a cumulative biogas production of 357 mL/gVS, 79.6% higher than that in the control group, and Fe2(SO4)3 had no distinct impact, while Fe(NO3)3 inhibited the methanogenesis process. A favorable balance between the release of organic matters from WAS and consumption rate was established after dosing FeCl3 from the perspective of variations of soluble COD, volatile fatty acids (VFAs) and the dissolved organic matters (DOM) assessed by EEM fluorescence spectroscopy and fluorescence regional integration (FRI) technique. Conversely, the system with Fe(NO3)3 achieved an unsuitable substrates environment. Pyrosequencing revealed that the anaerobic digestion system with FeCl3 enriched Coprothermobacter for proteins fermentation and Methanosarcina for methanogenesis with the values of 18.7% and 63.2%, respectively, while that with the supplementation of Fe(NO3)3 obtained the lowest relative abundance.

Development and validation of a screening procedure of microalgae for biodiesel production: application to the genus of marine microalgae Nannochloropsis

Nannochloropsis has emerged as a promising alga for biodiesel production. However, the genus consists of 6 species and hundreds of strains making strain selection a challenge. Furthermore, oil productivity is instrumental to economic viability of any algal strain for industrial production, which is dependent on growth rate and oil content. In most cases, these two parameters have been studied independently. Thus, the goal of this study is to provide a combined method for evaluating strain performance in specially designed photobioreactors together with an in-depth lipidomic analyses. The nine strains of Nannochloropsis tested showed considerable variations in productivity and lipidomics highlighting the importance of strain selection. Finally, Nannochloropsis gaditana CCMP527 and Nannochloropsis salina CCMP537 emerged as the two most promising strains, with an oil content of 37 and 27 dry wt% after 11-day nitrogen starvation, respectively, resulting in TAG productivity of 13×10(-3) and 18×10(-3) kg m(-3) d(-1), respectively.

Temperature-dependent transformation of biogas-producing microbial communities points to the increased importance of hydrogenotrophic methanogenesis under thermophilic operation

Stability of biogas production is highly dependent on the microbial community composition of the bioreactors. This composition is basically determined by the nature of biomass substrate and the physical-chemical parameters of the anaerobic digestion. Operational temperature is a major factor in the determination of the anaerobic degradation process. Next-generation sequencing (NGS)-based metagenomic approach was used to monitor the organization and operation of the microbial community throughout an experiment where mesophilic reactors (37°C) were gradually switched to thermophilic (55°C) operation. Temperature adaptation resulted in a clearly thermophilic community having a generally decreased complexity compared to the mesophilic system. A temporary destabilization of the system was observed, indicating a lag phase in the community development in response to temperature stress. Increased role of hydrogenotrophic methanogens under thermophilic conditions was shown, as well as considerably elevated levels of Fe-hydrogenases and hydrogen producer bacteria were observed in the thermophilic system.

Rapid bio-methanation of syngas in a reverse membrane bioreactor: membrane encased microorganisms

The performance of a novel reverse membrane bioreactor (RMBR) with encased microorganisms for syngas bio-methanation as well as a co-digestion process of syngas and organic substances was examined. The sachets were placed in the reactors and examined in repeated batch mode. Different temperatures and short retention time were studied. The digesting sludge encased in the PVDF membranes was able to convert syngas into methane at a retention time of 1 day and displayed a similar performance as the free cells in batch fermentation. The co-digestion of syngas and organic substances by the RMBR (the encased cells) showed a good performance without any observed negative effects. At thermophilic conditions, there was a higher conversion of pure syngas and co-digestion using the encased cells compared to at mesophilic conditions.

Ultraviolet and 5'fluorodeoxyuridine induced random mutagenesis in Chlorella vulgaris and its impact on fatty acid profile: a new insight on lipid-metabolizing genes and structural characterization of related proteins

The present study was aimed at randomly mutating the microalga, Chlorella vulgaris, in order to alter its cellular behaviour towards increased lipid production for efficient biodiesel production from algal biomass. Individual mutants from ultraviolet light (UV-1 (30 s exposure), UV-2 (60 s exposure) and UV-3 (90 s exposure)) and 5'fluorodeoxyuridine (5'FDU-1 (0.25 mM) and 5'FDU-2 (0.50 mM)) exposed cells were identified to explore an alternative method for lipid enhancement. A marginally significant decrease in biomass in the UV mutants; marked increase in the lipid content in UV-2 and 5'FDU-1 mutants; significant increase in saturated fatty acids level, especially in UV-2 mutant; insignificant increase in lipid production when these mutants were subjected to an additional stress of nitrogen starvation and predominantly enhanced level of unsaturated fatty acids in all the strains except UV-2 were noted. Chloroplast ultrastructural alterations and defective biosynthesis of chloroplast specific lipid constituents were observed in the mutants. Modelling of three-dimensional structures of acetyl coA carboxylase (ACCase), omega-6, plastid delta-12 and microsomal delta-12 fatty acid desaturases for the first time and ligand-interaction studies greatly substantiated our findings. A replacement of leucine by a serine residue in the acetyl coA carboxylase gene of UV-2 mutant suggests the reason behind lipid enhancement in UV-2 mutant. Higher activity of ACCase in UV-2 and 5'FDU-1 strongly proves the functional consequences of gene mutation to lipid production. In conclusion, algal mutants exhibited significant impact on biodiesel production through structural alterations in the lipid-metabolizing genes, thereby enhancing lipid production and saturated fatty acid levels.

Development and physiological characterization of cellobiose-consuming Yarrowia lipolytica

Yarrowia lipolytica is a promising production host for a wide range of molecules, but limited sugar consumption abilities prevent utilization of an abundant source of renewable feedstocks. In this study we created a Y. lipolytica strain capable of utilizing cellobiose as a sole carbon source by using endogenous promoters to express the cellodextrin transporter cdt-1 and intracellular β-glucosidase gh1-1 from Neurospora crassa. The engineered strain was also capable of simultaneous co-consumption of glucose and cellobiose. Although cellobiose was consumed slower than glucose when engineered strains were cultured with excess nitrogen, culturing with limited nitrogen led to cellobiose consumption rates comparable to those of glucose. Under limited nitrogen conditions, the engineered strain produced citric acid as a major product and we observed greater citric acid yields from cellobiose (0.37 g/g) than glucose (0.28 g/g). Culturing with a sole carbon source of either glucose or cellobiose induced additional differences on cell physiology and metabolism and a link is suggested to evasion of glucose-sensing mechanisms through intracellular creation and consumption of glucose. We ultimately applied this cellobiose-utilization system to produce citric acid from bioconversion of crystalline cellulose through simultaneous saccharification and fermentation (SSF).

Ultrasound pretreatment for enhanced biogas production from olive mill wastewater

This study investigates applicability of low frequency ultrasound technology to olive mill wastewaters (OMWs) as a pretreatment step prior to anaerobic batch reactors to improve biogas production and methane yield. OMWs originating from three phase processes are characterized with high organic content and complex nature. The treatment of the wastewater is problematic and alternative treatment options should be investigated. In the first part of the study, OMW samples were subjected to ultrasound at a frequency of 20kHz with applied powers varying between 50 and 100W under temperature controlled conditions for different time periods in order to determine the most effective sonication conditions. The level of organic matter solubilization at ultrasound experiments was assessed by calculating the ratio of soluble chemical oxygen demand/total chemical oxygen demand (SCOD/TCOD). The results revealed that the optimum ultrasonic condition for diluted OMW is 20kHz, 0.4W/mL for 10min. The application of ultrasound to OMW increased SCOD/TCOD ratio from 0.59 to 0.79. Statistical analysis (Friedman's tests) show that ultrasound was significantly effective on diluted OMW (p<0.05) in terms of SCOD parameter, but not for raw OMW (p>0.05). For raw OMW, this increase has been found to be limited due to high concentration of suspended solids (SS). In the second part of the study, biogas and methane production rates of anaerobic batch reactor fed with the ultrasound pretreated OMW samples were compared with the results of control reactor fed with untreated OMW in order to determine the effect of sonication. A nonparametric statistical procedure, Mann-Whitney U test, was used to compare biogas and methane production from anaerobic batch reactors for control and ultrasound pretreated samples. Results showed that application of low frequency ultrasound to OMW significantly improved both biogas and methane production in anaerobic batch reactor fed with the wastewater (p<0.05). Anaerobic batch reactor fed with ultrasound pretreated diluted OMW produced approximately 20% more biogas and methane compared with the untreated one (control reactor). The overall results indicated that low frequency ultrasound pretreatment increased soluble COD in OMW and subsequently biogas production.

A biorefining process: Sequential, combinational lignocellulose pretreatment procedure for improving biobutanol production from sugarcane bagasse

Here, for the first time, we designed a sequential, combinatorial lignocellulose pretreatment procedure (SCLPP) for microbial biofuel fermentation to reduce generation of microbial growth inhibitors and furthermore increase sugar yields. We tested this pretreatment process using sugarcane bagasse as substrate and assessed the effectiveness by analysis of biobutanol production through microbial clostridium beijerinckii NCIMB 8052 conversion. Our results showed that there were no inhibitory effects when using the hydrolysates as fermentation substrate. Under the SSF scheme, we observed the highest concentrations of butanol (6.4g/L) and total ABE (11.9g/L), resulting in a higher ABE productivity, compared with the SHF method. These findings suggest that the SCLPP is a feasible method for improving ABE production, lowering microbial inhibitor generation, and ensuring success in the subsequent fermentation process. Therefore, our work demonstrated developing a tractable integrated process that facilitates to increase biofuel production from agricultural residues rich in lignocellulose is feasible.

High-rate hydrogenotrophic methanogenesis for biogas upgrading: the role of anaerobic granules

Hydrogenotrophic methanogenesis has been proved to be a feasible biological method for biogas upgrading. To improve its performance, the feasibility of typical anaerobic granules as the inoculum was investigated in both batch and continuous experiments. The results from batch experiments showed that glucose-acclimated granules seemed to perform better than granules acclimated to acidified products (AP, i.e. acetate, propionate and ethanol) in in situ biogas upgrading systems and a slightly higher H2 consumption rate (1.5 mmol H2 g VSS(-1) h(-1)) was obtained for glucose-acclimated granules. For AP-acclimated granules, the inhibition on anaerobic digestion and pH increase (up to 9.55±0.16) took place, and the upgrading performance was adversely affected. In contrast, better performance for AP-acclimated granules was observed in ex situ systems, possibly due to their higher hydrogenotrophic methanogenic activities (HMA). Moreover, when gas-liquid mass transfer limitations were alleviated, the upgrading performance was significantly improved (three-fold) for both glucose-acclimated and AP-acclimated granules. The HMA of anaerobic granules could be further enhanced to improve biogas upgrading performance via continuous cultivation with H2/CO2 as the sole substrate. During the three months' cultivation, secondary granulation and microbial population shift were observed, but anaerobic granules still remained intact and their HMA increased from 0.2 to 0.6 g COD g VSS(-1) d(-1). It indicated that the formation of hydrogenotrophic methanogenic granules, a new type of anaerobic granules specialized for high-rate hydrogenotrophic methanogenesis and biogas upgrading, might be possible. Conclusively, anaerobic granules showed great potential for biogas upgrading.

The influence of light intensity and photoperiod on duckweed biomass and starch accumulation for bioethanol production

Duckweed has been considered as a valuable feedstock for bioethanol production due to its high biomass and starch production. To investigate the effects of light conditions on duckweed biomass and starch production, Lemna aequinoctialis 6000 was cultivated at different photoperiods (12:12, 16:8 and 24:0h) and light intensities (20, 50, 80, 110, 200 and 400μmolm(-2)s(-1)). The results showed that the duckweed biomass and starch production was increased with increasing light intensity and photoperiod except at 200 and 400μmolm(-2)s(-1). Considering the light cost, 110μmolm(-2)s(-1) was optimum light condition for starch accumulation with the highest maximum growth rate, biomass and starch production of 8.90gm(-2)day(-1), 233.25gm(-2) and 98.70gm(-2), respectively. Moreover, the results suggested that high light induction was a promising method for duckweed starch accumulation. This study provides optimized light conditions for future industrial large-scale duckweed cultivation.

Response surface optimization of the thermal acid pretreatment of sugar beet pulp for bioethanol production using Trichoderma viride and Saccharomyces cerevisiae

BACKGROUND

Worldwide nowadays, relying on the second generation bioethanol from the lignocellulosic feedstock is a mandatory aim. However, one of the major drawbacks for high ethanol yield is the physical and chemical pretreatment of this kind of feedstock. As the pretreatment is a crucial process operation that modifies the lignocellulosic structure and enhances its accessibility for the high cost hydrolytic enzymes in an attempt to maximize the yield of the fermentable sugars. The objective of this work was to optimize and integrate a physicochemical pretreatment of one of the major agricultural wastes in Egypt; the sugar beet pulp (SBP) and the enzymatic saccharification of the pretreated SBP using a whole fungal cells with a separate bioethanol fermentation batch processes to maximize the bioethanol yield.

METHODS AND RESULTS

The response surface methodology was employed in this study to statistically evaluate and optimize the conditions for a thermal acid pretreatment of SBP. The significance and the interaction effects of the concentrations of HCl and SBP and the reaction temperature and time were studied using a three-level central composite design of experiments. A quadratic model equation was obtained to maximize the production of the total reducing sugars. The validity of the predicted model was confirmed. The thermally acid pretreated SBP was further subjected to a solid state fermentation batch process using Trichoderma viride F94. The thermal acid pretreatment and fungal hydrolyzes were integrated with two parallel batch fermentation processes of the produced hydrolyzates using Saccharomyces cerevisiae Y39, that yielded a total of ≈ 48 g/L bioethanol, at a conversion rate of ≈ 0.32 g bioethanol/ g SBP.

CONCLUSION

Applying the proposed integrated process, approximately 97.5 gallon of ethanol would be produced from a ton (dry weight) of SBP.

Salinity induced oxidative stress enhanced biofuel production potential of microalgae Scenedesmus sp. CCNM 1077

Microalgal biomass is considered as potential feedstock for biofuel production. Enhancement of biomass, lipid and carbohydrate contents in microalgae is important for the commercialization of microalgal biofuels. In the present study, salinity stress induced physiological and biochemical changes in microalgae Scenedesmus sp. CCNM 1077 were studied. During single stage cultivation, 33.13% lipid and 35.91% carbohydrate content was found in 400 mM NaCl grown culture. During two stage cultivation, salinity stress of 400 mM for 3 days resulted in 24.77% lipid (containing 74.87% neutral lipid) along with higher biomass compared to single stage, making it an efficient strategy to enhance biofuel production potential of Scenedesmus sp. CCNM 1077. Apart from biochemical content, stress biomarkers like hydrogen peroxide, lipid peroxidation, ascorbate peroxidase, proline and mineral contents were also studied to understand the role of reactive oxygen species (ROS) mediated lipid accumulation in microalgae Scenedesmus sp. CCNM 1077.

Cyanobacteria and microalgae: a renewable source of bioactive compounds and other chemicals

Microalgae and cyanobacteria are rich sources of many valuable compounds, including important bioactive and biotechnologically relevant chemicals. Their enormous biodiversity, and the consequent variability in the respective biochemical composition, make microalgae cultivations a promising resource for many novel chemically and biologically active molecules and compounds of high commercial value such as lipids and dyes. The nature of the chemicals produced can be manipulated by changing the cultivation media and conditions. Algae are extremely versatile because they can be adapted to a variety of cell culture conditions. They do not require arable land, can be cultivated on saline water and wastewaters, and require much less water than plants. They possess an extremely high growth rate making these microorganisms very attractive for use in biofuel production--some species of algae can achieve around 100 times more oil than oil seeds. In addition, microalgae and cyanobacteria can accumulate various biotoxins and can contribute to mitigate greenhouse gases since they produce biomass through carbon dioxide fixation. In this review, we provide an overview of the application of microalgae in the production of bioactive and other chemicals.

Large-scale biodiesel production using flue gas from coal-fired power plants with Nannochloropsis microalgal biomass in open raceway ponds

The potential use of microalgal biomass as a biofuel source has raised broad interest. Highly effective and economically feasible biomass generating techniques are essential to realize such potential. Flue gas from coal-fired power plants may serve as an inexpensive carbon source for microalgal culture, and it may also facilitate improvement of the environment once the gas is fixed in biomass. In this study, three strains of the genus Nannochloropsis (4-38, KA2 and 75B1) survived this type of culture and bloomed using flue gas from coal-fired power plants in 8000-L open raceway ponds. Lower temperatures and solar irradiation reduced the biomass yield and lipid productivities of these strains. Strain 4-38 performed better than the other two as it contained higher amounts of triacylglycerols and fatty acids, which are used for biodiesel production. Further optimization of the application of flue gas to microalgal culture should be undertaken.

Rapid alkali catalyzed transesterification of microalgae lipids to biodiesel using simultaneous cooling and microwave heating and its optimization

Biodiesel with improved yield was produced from microalgae biomass under simultaneous cooling and microwave heating (SCMH). Nannochloropsis sp. and Tetraselmis sp. which were known to contain higher lipid species were used. The yield obtained using this novel technique was compared with the conventional heating (CH) and microwave heating (MWH) as the control method. The results revealed that the yields obtained using the novel SCMH were higher; Nannochloropsis sp. (83.33%) and Tetraselmis sp. (77.14%) than the control methods. Maximum yields were obtained using SCMH when the microwave was set at 50°C, 800W, 16h of reaction with simultaneous cooling at 15°C; and water content and lipid to methanol ratio in reaction mixture was kept to 0 and 1:12 respectively. GC analysis depicted that the biodiesel produced from this technique has lower carbon components (<19 C) and has both reasonable CN and IV reflecting good ignition and lubricating properties.

Selection of microalgae for biodiesel production in a scalable outdoor photobioreactor in north China

The aim of this study was to identify the most promising species as biodiesel feedstock for large-scale cultivation in north China. Eight species of microalgae, selected on the basis of indoor screening, were tested for lipid productivity and the suitability of their fatty acid profiles for biodiesel production under outdoor conditions. Among them, three species Desmodesmus sp. NMX451, Desmodesmus sp. T28-1 and Scenedesmusobtusus XJ-15 were selected for further characterization due to their possessing higher lipid productivities and favorable biodiesel properties. The best strain was S. obtusus XJ-15, with highest biomass productivity of 20.2gm(-2)d(-1) and highest lipid content of 31.7% in a culture of 140L. S. obtusus XJ-15 was further identified as the best candidate for liquid biofuel production, characterized by average areal growth rate of 23.8gm(-2)d(-1) and stable lipid content of above 31.0% under a scale of 1400L over a season.

Assessing microalgae biorefinery routes for the production of biofuels via hydrothermal liquefaction

The interest in third generation biofuels from microalgae has been rising during the past years. Meanwhile, it seems not economically feasible to grow algae just for biofuels. Co-products with a higher value should be produced by extracting a particular algae fraction to improve the economics of an algae biorefinery. The present study aims at analyzing the influence of two main microalgae components (lipids and proteins) on the composition and quantity of biocrude oil obtained via hydrothermal liquefaction of two strains (Nannochloropsis gaditana and Scenedesmus almeriensis). The algae were liquefied as raw biomass, after extracting lipids and after extracting proteins in microautoclave experiments at different temperatures (300-375°C) for 5 and 15min. The results indicate that extracting the proteins from the microalgae prior to HTL may be interesting to improve the economics of the process while at the same time reducing the nitrogen content of the biocrude oil.

Changes of the microbial population structure in an overloaded fed-batch biogas reactor digesting maize silage

Two parallel, stable operating biogas reactors were fed with increasing amounts of maize silage to monitor microbial community changes caused by overloading. Changes of microorganisms diversity revealed by SSCP (single strand conformation polymorphism) indicating an acidification before and during the pH-value decrease. The earliest indicator was the appearance of a Methanosarcina thermophila-related species. Diversity of dominant fermenting bacteria within Bacteroidetes, Firmicutes and other Bacteria decreased upon overloading. Some species became dominant directly before and during acidification and thus could be suitable as possible indicator organisms for detection of futurity acidification. Those bacteria were related to Prolixibacter bellariivorans and Streptococcus infantarius subsp. infantarius. An early detection of community shifts will allow better feeding management for optimal biogas production.

Co-cultivation of fungal and microalgal cells as an efficient system for harvesting microalgal cells, lipid production and wastewater treatment

The challenges which the large scale microalgal industry is facing are associated with the high cost of key operations such as harvesting, nutrient supply and oil extraction. The high-energy input for harvesting makes current commercial microalgal biodiesel production economically unfeasible and can account for up to 50% of the total cost of biofuel production. Co-cultivation of fungal and microalgal cells is getting increasing attention because of high efficiency of bio-flocculation of microalgal cells with no requirement for added chemicals and low energy inputs. Moreover, some fungal and microalgal strains are well known for their exceptional ability to purify wastewater, generating biomass that represents a renewable and sustainable feedstock for biofuel production. We have screened the flocculation efficiency of the filamentous fungus A. fumigatus against 11 microalgae representing freshwater, marine, small (5 µm), large (over 300 µm), heterotrophic, photoautotrophic, motile and non-motile strains. Some of the strains are commercially used for biofuel production. Lipid production and composition were analysed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources contained in wheat straw and swine wastewater, respectively. Co-cultivation of algae and A. fumigatus cells showed additive and synergistic effects on biomass production, lipid yield and wastewater bioremediation efficiency. Analysis of fungal-algal pellet's fatty acids composition suggested that it can be tailored and optimised through co-cultivating different algae and fungi without the need for genetic modification.

Strategic enhancement of algal biomass, nutrient uptake and lipid through statistical optimization of nutrient supplementation in coupling Scenedesmus obliquus-like microalgae cultivation and municipal wastewater treatment

Supplementing proper nutrients could be a strategy for enhancing algal biomass, nutrients uptake and lipid accumulation in the coupling system of biodiesel production and municipal wastewater treatment. However, there is scant information reporting systematic studies on screening and optimization of key supplemented components in the coupling system. The main factors were scientifically screened and optimized using statistical methods. Plackett-Burman design (PBD) was used to explore the roles of added nutrient factors, whereas response surface methodology (RSM) was employed for optimization. Based on the statistic analysis, the optimum added TP and FeCl3·6H2O concentrations for Scenedesmus obliquus-like microalgae growth, nutrients uptake and lipid accumulation were 4.41 mg L(-1) and 6.48 mg L(-1), respectively. The corresponding biomass, lipid content and TN/TP removal efficiency were 1.46 g L(-1), 36.26% and >99%. The predicted value agreed well with the experimental value, as determined by validation experiments, which confirmed the availability and accuracy of the model.

Effect of substrate pretreatment on particle size distribution in a full-scale research biogas plant

The objective of this study was to investigate the pretreatment effects of high-fibre substrate on particle size distribution in a full-scale agricultural biogas plant (BGP). Two digesters, one fed with pretreated material and one with untreated material, were investigated for a period of 90days. Samples from different positions and heights were taken with a special probe sampling system and put through a wet sieve. The results show that on average 58.0±8.6% of the particles in both digesters are fine fraction (<0.063mm). A higher amount of particles (13.1%) with a length >4mm was measured in the untreated digester. However, the volume distribution over all positions and heights did not show a clear and uniform distribution of particles. These results reveal that substrate pretreatment has an effect on particle size in the fermenting substrate, but due to the uneven distribution mixing, is not homogeneous within the digester.

Pretreatment of microalgae to improve biogas production: a review

Microalgae have been intensively studied as a source of biomass for replacing conventional fossil fuels in the last decade. The optimization of biomass production, harvesting and downstream processing is necessary for enabling its full-scale application. Regarding biofuels, biogas production is limited by the characteristics of microalgae, in particular the complex cell wall structure of most algae species. Therefore, pretreatment methods have been investigated for microalgae cell wall disruption and biomass solubilization before undergoing anaerobic digestion. This paper summarises the state of the art of different pretreatment techniques used for improving microalgae anaerobic biodegradability. Pretreatments were divided into 4 categories: (i) thermal; (ii) mechanical; (iii) chemical and (iv) biological methods. According to experimental results, all of them are effective at increasing biomass solubilization and methane yield, pretreatment effect being species dependent. Pilot-scale research is still missing and would help evaluating the feasibility of full-scale implementation.

Choosing co-substrates to supplement biogas production from animal slurry--a life cycle assessment of the environmental consequences

Biogas production from animal slurry can provide substantial contributions to reach renewable energy targets, yet due to the low methane potential of slurry, biogas plants depend on the addition of co-substrates to make operations profitable. The environmental performance of three underexploited co-substrates, straw, organic household waste and the solid fraction of separated slurry, were assessed against slurry management without biogas production, using LCA methodology. The analysis showed straw, which would have been left on arable fields, to be an environmentally superior co-substrate. Due to its low nutrient content and high methane potential, straw yields the lowest impacts for eutrophication and the highest climate change and fossil depletion savings. Co-substrates diverted from incineration to biogas production had fewer environmental benefits, due to the loss of energy production, which is then produced from conventional fossil fuels. The scenarios can often provide benefits for one impact category while causing impacts in another.

Potential of Monoraphidium minutum for carbon sequestration and lipid production in response to varying growth mode

Mixotrophic growth at flask level and, autotrophic-mixotrophic and autotrophic growth in photobioreactor by utilizing CO2/air/flue gas were checked for the isolated strain of Monoraphidium minutum from polluted habitat. Our study confirmed that it is a saturated fatty acid rich (30.92-68.94%) microalga with lower degree of unsaturation oil quality (42.06-103.99) making it potential biodiesel producing candidate. It showed encouraging biomass productivity (80.3-303.8mgl(-1)day(-1)) with higher total lipid (22.80-46.54%) under optimum glucose, fructose, microalgal biodiesel waste residue and sodium acetate fed mixotrophic conditions. The pH control by intermittent CO2, continuous illumination with 30% flue gas, and utilization of biodiesel glycerin were effective schemes to ameliorate either biomass productivity or % lipids or both of these parameters at photobioreactor scale (7.5L working volume). The modulation of environmental variables (pH control, CO2 and organic substrates concentration) could augment % saturated fatty acids, such as C16:0.