Net energy analysis of the production of biodiesel and biogas from the microalgae: Haematococcus pluvialis and Nannochloropsis

Microalgae have been proposed as possible alternative feedstocks for the production of biodiesel because of their high photosynthetic efficiency. The high energy input required for microalgal culture and oil extraction may negate this advantage, however. There is a need to determine whether microalgal biodiesel can deliver more energy than is required to produce it. In this work, net energy analysis was done on systems to produce biodiesel and biogas from two microalgae: Haematococcus pluvialis and Nannochloropsis. Even with very optimistic assumptions regarding the performance of processing units, the results show a large energy deficit for both systems, due mainly to the energy required to culture and dry the microalgae or to disrupt the cell. Some energy savings may be realized from eliminating the fertilizer by the use of wastewater or, in the case of H. pluvialis, recycling some of the algal biomass to eliminate the need for a photobioreactor, but these are insufficient to completely eliminate the deficit. Recommendations are made to develop wet extraction and transesterification technology to make microalgal biodiesel systems viable from an energy standpoint.     

Neochloris oleoabundans grown on anaerobically digested dairy manure for concomitant nutrient removal and biodiesel feedstock production

Microalgae have been investigated as a promising biodiesel feedstock; however, large-scale production is not currently cost-competitive with petroleum diesel, and its environmental impacts have received little attention. Using wastewater to supply nutrients for algal growth obviates synthetic fertilizer use, provides on-site nutrient removal, and reduces greenhouse gas emissions. In this work, anaerobically digested dairy manure was used to grow the oleaginous green alga Neochloris oleoabundans. In batch culture experiments with both synthetic media and anaerobic digester effluent, N. oleoabundans assimilated 90-95% of the initial nitrate and ammonium after 6 d and yielded 10-30% fatty acid methyl esters on a dry weight basis. Cellular lipid content and the N concentration in the growth media were inversely correlated. In addition, the proportion of polyunsaturated fatty acids (i.e. C16:3, C18:2, and C18:3) decreased with N concentration over time while the proportion of C18:1 fatty acid increased. Although N deficiency is likely the primary driver behind lipid accumulation, the influence of culture pH confounded results and requires further study. Other living microorganisms in the digester effluent were not observed to affect algal growth and lipid productivity, though the breakdown of organic nitrogen may have hindered lipid accumulation traditionally achieved through the manipulation of synthetic media. This work highlights the potential for waste-grown mono-algal cultures to produce high quality biodiesel while accomplishing simultaneous wastewater treatment.     

Anaerobic digestion of microalgae residues resulting from the biodiesel production process

The recovery of methane from post transesterified microalgae residues has the potential to improve the renewability of the ‘microalgae biomass to biodiesel’ conversion process as well as reduce its cost and environmental impact. This paper deals with the anaerobic digestion of microalgae biomass residues (post transesterification) using semi-continuously fed reactors. The influence of substrate loading concentrations and hydraulic retention times on the specific methane yield of the anaerobically digested microalgae residues was investigated. The co-digestion of the microalgae residues with glycerol as well as the influence of temperature was also examined. It was found that the hydraulic retention period was the most significant variable affecting methane production from the residues, with periods (>5 days) corresponding to higher energy recovery. The methane yield was also improved by a reduction in the substrate loading rates, with an optimum substrate carbon to nitrogen ratio of 12.44 seen to be required for the digestion process.     

Prospects of biodiesel production from microalgae in India

Energy is essential and vital for development, and the global economy literally runs on energy. The use of fossil fuels as energy is now widely accepted as unsustainable due to depleting resources and also due to the accumulation of greenhouse gases in the environment. Renewable and carbon neutral biodiesel are necessary for environmental and economic sustainability. Biodiesel demand is constantly increasing as the reservoir of fossil fuel are depleting. Unfortunately biodiesel produced from oil crop, waste cooking oil and animal fats are not able to replace fossil fuel. The viability of the first generation biofuels production is however questionable because of the conflict with food supply. Production of biodiesel using microalgae biomass appears to be a viable alternative. The oil productivity of many microalgae exceeds the best producing oil crops. Microalgae are photosynthetic microorganisms which convert sunlight, water and CO₂ to sugars, from which macromolecules, such as lipids and triacylglycerols (TAGs) can be obtained. These TAGs are the promising and sustainable feedstock for biodiesel production. Microalgal biorefinery approach can be used to reduce the cost of making microalgal biodiesel. Microalgal-based carbon sequestration technologies cover the cost of carbon capture and sequestration. The present paper is an attempt to review the potential of microalgal biodiesel in comparison to the agricultural crops and its prospects in India.     

The kinetics of Scenedesmus obliquus microalgae growth utilizing carbon dioxide gas from biogas

Microalgae Scenedesmus obliquus was cultured in a laboratory photobioreactor to determine the efficacy of using biogas as a carbon source for the microalgae's growth. The biogas contained ∼60% CH₄ and ∼40% CO₂, and was derived from an anaerobic digester operating from animal wastes, and an anaerobic reactor utilizing high strength wastewater. The results showed that biogas is a viable carbon source for microalgae growth and that significant portions of the biogas' CO₂ can be utilized for algae growth, resulting in a biogas having a high concentration of methane. This paper develops the kinetic expressions for the algae's growth by assuming an autocatalytic reaction between carbon substrate and microalgae. The maximum specific growth rate and biomass productivity of S. obliquus were 0.56 d⁻¹ and 0.145 g L⁻¹d⁻¹ respectively. The biomass contained 51.8% carbon and higher heating value (HHV) was 22.9 MJ kg⁻¹.     

Application of sweet sorghum for biodiesel production by heterotrophic microalga Chlorella protothecoides

Microalga Chlorella protothecoides can grow heterotrophically with glucose as the carbon source and accumulate high proportion of lipids. The microalgal lipids are suitable for biodiesel production. To further increase lipid yield and reduce biodiesel cost, sweet sorghum juice was investigated as an alternative carbon source to glucose in the present study. When the initial reducing sugar concentration was 10 g L⁻¹ in the culture medium, the dry cell yield and lipid content were 5.1 g L⁻¹ and 52.5% using enzymatic hydrolyzates of sweet sorghum juice as the carbon source after 120 h-culture in flasks. The lipid yield was 35.7% higher than that using glucose. When 3.0 g L⁻¹ yeast extract was added to the medium, the dry cell yield and lipid productivity was increased to 1.2 g L⁻¹ day⁻¹ and 586.8 mg L⁻¹ day⁻¹. Biodiesel produced from the lipid of C. protothecoides through acid catalyzed transesterification was analyzed by GC–MS, and the three most abundant components were oleic acid methyl ester, cetane acid methyl ester and linoleic acid methyl ester. The results indicate that sweet sorghum juice could effectively enhance algal lipid production, and its application may reduce the cost of algae-based biodiesel.     

Validation of a simple gravimetric method for measuring biogas production in laboratory experiments

This work presents a gravimetric method for measuring biogas or methane production from anaerobic reactors, based on measurement of reactor mass loss. Results are most sensitive to error in biogas methane content, and less so to temperature and pressure. To evaluate the method, we applied it and volumetric methods to 133 laboratory-scale batch and semi-continuous reactors, ranging in size from 37 g to 8.0 kg of reacting mass. For most observations, the relative difference between the two methods was <10% when the “true” biogas composition was used in calculations. Small systematic differences observed in some cases were probably due to error in estimates of biogas pressure, temperature, and composition, as well as biogas leakage. Based on theory and observation, it is reasonable to expect relative accuracy better than 15% of the true value.     

Potential of microalgae oil from Dunaliella tertiolecta as a feedstock for biodiesel

Alternative, non-food based biomass fuel feedstock development is vital for our national security, economy and the environment. Microalgae are among the most promising of these alternatives. Microalgal cell growth rates and metabolic products are affected by a combination of environmental parameters. In this work, the influences of light source, light intensity, CO₂ concentration, and photoperiod on the growth of Dunaliella tertiolecta (D. tertiolecta) were studied. The effects of these environmental parameters on the lipid content and fatty acid composition of D. tertiolecta were also investigated. Red light-emitting diodes (LEDs), white LEDs, and fluorescent lights were all found to be effective for algal growth. Increasing light intensity resulted in significantly more rapid algal growth, and increasing the period of light also significantly increased biomass productivity. Similar growth rates were observed for 2%, 4%, and 6% CO₂-concentrations. The different light sources and intensities were found to have no significant effect on FAME composition of D. tertiolecta. Methyl linolenate and methyl palmitate were found to be the major components of FAME produced from D. tertiolecta oil. D. tertiolecta and its derived oils should be a suitable feedstock for biofuel production.     

Pretreatment of lignocellulosic biomass for enhanced biogas production

Lignocellulosic biomass is an abundant organic material that can be used for sustainable production of bioenergy and biofuels such as biogas (about 50–75% CH₄ and 25–50% CO₂). Out of all bioconversion technologies for biofuel and bioenergy production, anaerobic digestion (AD) is a most cost-effective bioconversion technology that has been implemented worldwide for commercial production of electricity, heat, and compressed natural gas (CNG) from organic materials. However, the utilization of lignocellulosic biomass for biogas production via anaerobic digestion has not been widely adopted because the complicated structure of the plant cell wall makes it resistant to microbial attack. Pretreatment of recalcitrant lignocellulosic biomass is essential to achieve high biogas yield in the AD process. A number of different pretreatment techniques involving physical, chemical, and biological approaches have been investigated over the past few decades, but there is no report that systematically compares the performance of these pretreatment methods for application on lignocellulosic biomass for biogas production. This paper reviews the methods that have been studied for pretreatment of lignocellulosic biomass for conversion to biogas. It describes the AD process, structural and compositional properties of lignocellulosic biomass, and various pretreatment techniques, including the pretreatment process, parameters, performance, and advantages vs. drawbacks. This paper concludes with the current status and future research perspectives of pretreatment.     

Investigation of the combined effects of acetate and photobioreactor illuminated fraction in the induction of anoxia for hydrogen production by Chlamydomonas reinhardtii

In the context of hydrogen production by microalgae, the growth of Chlamydomonas reinhardtii was characterized under autotrophic and mixotrophic conditions in a fully controlled photobioreactor (PBR). The combined effect of light transfer conditions, as represented by the illuminated fraction γ, with acetate consumption was observed upon establishment of anoxia. Anoxia was reached in batch cultures when γ was close to 1 (almost fully illuminated culture) in mixotrophic conditions while a value of γ ≈ 0.46 in autotrophic conditions was not sufficient. Based on these results, continuous hydrogen production was established in a cylindrical PBR operated in luminostat with constant illumination and in mixotrophic conditions. Maximum hydrogen gas production was equal to 1.4 ± 0.1 ml_H2 l⁻¹ h⁻¹ for photon flux density of 110 μmol m⁻² s⁻¹ and reactor illuminated fraction of γ = 0.5. Carbon mass balance was realized, emphasizing the necessity to work in strictly autotrophic conditions for hydrogen production with no concomitant CO₂ release.     

Hydrogen gas production by combined systems of Rhodobacter sphaeroides O.U.001 and Halobacterium salinarum in a photobioreactor

Rhodobacter sphaeroides O.U.001 is a photosynthetic non-sulfur bacterium which produces hydrogen from organic compounds under anaerobic conditions. Halobacterium salinarum is an archaeon and lives under extremely halophilic conditions (4 M NaCl). H. salinarum contains a retinal protein bacteriorhodopsin in its purple membrane which acts as a light-driven proton pump. In this study the Rhodobacter sphaeroides O.U.001 culture was combined with different amounts of packed cells of H. salinarum S9 or isolated purple membrane fragments in order to increase the photofermentative hydrogen gas production. The packed cells of H. salinarum have the ability to pump protons upon illumination due to the presence of bacteriorhodopsin. The proton gradient produced may be used for the formation of ATP or protons may be used for H₂ production by R. sphaeroides. Similar to intact cells purple membrane fragments may also form vesicles around certain ions and may act like closed systems.     

Enhancement of biogas production from sunnhemp using alkaline pretreatment

This study investigates enhancing the biogas production of sunnhemp by pretreatment, before the anaerobic digestion and co-digestion processes, to address the complex and recalcitrant structure of the plant. Fresh sunnhemp harvested at a cutting interval of 50 days is used in the study. Five systems (each with a 5 litre useable volume) are operated semi-continuously with five different ratios of the feedstock by feeding separate feedstocks every five days with a hydraulic retention time (HRT) of 40 days. The system operates at room temperature (30 °C). The study uses sunnhemp as 20% of the feedstock and also considers sunnhemp mixed with cow manure at different ratios, with the weighed sunnhemp being pretreated with dilute sodium hydroxide. Pretreatment of sunnhemp before digestion produces a methane (CH₄) yield 89% greater than that of the untreated sunnhemp. It requires 3.597 kg of dry sunnhemp to produce 1 m³ of CH₄ and the annual CH₄ yield per hectare is 19,015 m³. In the pretreatment of sunnhemp before co-digestion, the increased CH₄ yield depends on the amount of pretreated sunnhemp in the feedstocks. However, the %CH₄, the CH₄ production level and the system stability depend on the optimal ratio of the sunnhemp to cow manure. The initially prepared sunnhemp to cow manure ratio is recommended at 10 g:10 g in 80 mL of water. At this ratio, the %CH₄ and the CH₄ yield are 53.84% and 313 kg chemical oxygen demand (COD) removed, respectively, and the COD removal efficiency is 56.4%. Sunnhemp has high potential and it is worth pretreating before producing biogas. Using sunnhemp to produce biogas is recommended to decrease greenhouse gas emissions and mitigate global warming.     

Enhanced biogas production from anaerobic co-digestion of municipal wastewater treatment sludge and fat,oil and grease (FOG) by a modified two-stage thermophilic digester system with selected thermo-chemical pre-treatment

Anaerobic co-digestions with fat, oil and grease (FOG) were investigated in two-stage thermophilic (55 °C) semi-continuous flow co-digestion systems. One two-stage co-digestion system (System I) was modified to incorporate a thermo-chemical pre-treatment of pH = 10 at 55 °C, which was the best pre-treatment condition for FOG co-digestion identified during laboratory-scale biochemical methane potential (BMP) testing. The other two-stage co-digestion system (System II) was operated without a pre-treatment process. The anaerobic digester of each digestion system had a hydraulic retention time (HRT) of 24 days. An organic loading rate (OLR) of 1.83 ± 0.09 g TVS/L·d was applied to each digestion system. It was found that System I effectively enhanced biogas production as the thermo-chemical pre-treatment improved the substrate hydrolysis including increased COD solubilization and VFA concentrations. Overall, the modified System I yielded a 25.14 ± 2.14 L/d biogas production rate, which was substantially higher than the 18.73 ± 1.11 L/d obtained in the System II.     

Biohydrogen production by a novel integration of dark fermentation and mixotrophic microalgae cultivation

Biohydrogen is usually produced via dark fermentation, which generates CO₂ emissions and produces soluble metabolites (e.g., volatile fatty acids) with high chemical oxygen demand (COD) as the by-products, which require further treatments. In this study, mixotrophic culture of an isolated microalga (Chlorella vulgaris ESP6) was utilized to simultaneously consume CO₂ and COD by-products from dark fermentation, converting them to valuable microalgae biomass. Light intensity and food to microorganism (F/M) ratio were adjusted to 150 μmol m⁻² s⁻¹ and F/M ratio, 4.5, respectively, to improve the efficiency of assimilating the soluble metabolites. The mixotrophic microalgae culture could reduce the CO₂ content of dark fermentation effluent from 34% to 5% with nearly 100% consumption of soluble metabolites (mainly butyrate and acetate) in 9 days. The obtained microalgal biomass was hydrolyzed with 1.5% HCl and subsequently used as the substrate for bioH₂ production with Clostridium butyricum CGS5, giving a cumulative H₂ production of 1276 ml/L, a H₂ production rate of 240 ml/L/h, and a H₂ yield of 0.94 mol/mol sugar.     

Land use and second-generation biofuel feedstocks: The unconsidered impacts of Jatropha biodiesel in Rajasthan,India

Governments around the world see biofuels as a common solution to the multiple policy challenges posed by energy insecurity, climate change and falling farmer incomes. The Indian government has enthusiastically adopted a second-generation feedstock – the oilseed-bearing shrub, Jatropha curcas – for an ambitious national biodiesel program. Studies estimating the production capacity and potential land use implications of this program have typically assumed that the ‘waste land’ slated for Jatropha production has no economic value and that no activities of note will be displaced by plantation development. Here we examine the specific local impacts of rapid Jatropha plantation development on rural livelihoods and land use in Rajasthan, India. We find that in Jhadol Tehsil, Jatropha is planted on both government and private land, and has typically displaced grazing and forage collection. For those at the socioeconomic margins, these unconsidered impacts counteract the very benefits that the biofuel programs aim to create. The Rajasthan case demonstrates that local land-use impacts need to be integrated into decision-making for national targets and global biofuel promotion efforts.     

Dark fermentative hydrogen production with crude glycerol from biodiesel industry using indigenous hydrogen-producing bacteria

Glycerol is an inevitable by-product from biodiesel synthesis process and could be a promising feedstock for fermentative hydrogen production. In this study, the feasibility of using crude glycerol from biodiesel industry for biohydrogen production was evaluated using seven isolated hydrogen-producing bacterial strains (Clostridium butyricum, Clostridium pasteurianum, and Klebsiella sp.). Among the strains examined, C. pasteurianum CH4 exhibited the best biohydrogen-producing performance under the optimal conditions of: temperature, 35 °C; initial pH, 7.0; agitation rate, 200 rpm; glycerol concentration, 10 g/l. When using pure glycerol as carbon source for continuous hydrogen fermentation, the average H₂ production rate and H₂ yield were 103.1 ± 8.1 ml/h/l and 0.50 ± 0.02 mol H₂/mol glycerol, respectively. In contrast, when using crude glycerol as the carbon source, the H₂ production rate and H₂ yield was improved to 166.0 ± 8.7 ml/h/l and 0.77 ± 0.05 mol H₂/mol glycerol, respectively. This work demonstrated the high potential of using biodiesel by-product, glycerol, for cost-effective biohydrogen production.     

Chlorella sp.: A new strain with highly saturated fatty acids for biodiesel production in bubble-column photobioreactor

The biodiesel production from a naturally isolated strain of Chlorella in 2 L bubble-column photobioreactor was studied. The microalgal strain was isolated from the rice paddy-field soil samples during a screening program. After 17 days, at the end of exponential phase of growth, the total content of the lipids was extracted. The extracted fatty acids were first esterified and then identified using GC/MS analysis. Several types of fatty acid methyl esters (FAMEs) were identified in the isolated microalga and the presence of saturated fatty acids in Chlorella sp. MCCS 040 was approved. The composition of fatty acids in the studied species of microalga was mainly palmitic acid methyl ester, myristic acid methyl ester, stearic acid methyl ester and undecanoic acid methyl ester. This strain because of its highly saturated fatty acids content can be an ideal candidate for biodiesel production.     

Pretreatment of energy crops with sodium hydroxide and cellulolytic enzymes to increase biogas production

This work presents the influence of alkali pretreatment on the enzymatic hydrolysis and efficiency of anaerobic digestion of lignocellulosic biomass pretreated both in a one- (chemical or enzymatic) and two-step (chemical and enzymatic) process. In this study two species of energy crops were used Miscanthus giganteus and Sida hermaphrodita. The aim of this work was to compare biogas production and methane yield during fermentation of pretreated and untreated energy crops. The results show that alkali pretreatment is necessary for the effective biogas generation from plant material due to high delignification level and significant hemicellulose degradation. The two-step hydrolysis process consisting on the alkali and enzymatic step leads to the release of high concentrations of glucose (about 20 g L⁻¹). The best results were achieved for M. giganteus with biogas production yield of 421.5 Ndm³ kg TS⁻¹ and with methane production yield of 257 Ndm³ kg TS⁻¹.     

A potent lipid producing isolate of Epicoccum purpurascens AUMC5615 and its promising use for biodiesel production

Oils of oleaginous microorganisms are a powerful alternative to vegetable oils for biodiesel production. In this study, the fungus Epicoccum purpurascens AUMC5615 isolated from Egypt showed a potent high lipid content (80% lipid) when grown on 4% sucrose in submerged culture under continuous illumination. Under dark submerged conditions the lipid content has drastically decreased to 12%. In light static conditions, the lipid content was 70%; however, the net lipid yield was significantly lower than that of light submerged cultures because of the decrease in growth under light static conditions in comparison to light submerged cultures. Under dark static conditions the lipid content of the fungus has declined to 30%. These results indicate that light plays a crucial role in the lipid accumulation whereas submersion enhances the growth of the fungus. Concomitantly, the highest yield of carotenoids was obtained under light submerged conditions followed respectively by light static, dark submerged and dark static. This synchronized increase in carotenoids content might be implicated in protecting the high lipid pools in the fungus from peroxidation. Growing the fungus on 4% of crude molasses resulted in a net lipid production of 26.8 g per liter under light submerged conditions. The determination of fatty acids by GC/MS revealed that the major constituents are four saturated fatty acids, hexadecanoic, n-decanoic, dodecanoic and octadecanoic acids. These saturated fatty acids would give valuable stability properties of such fungal biodiesel. The current investigation opens the scope for the possible use of this promising fungal isolate in biodiesel production.     

Bio-methanization of energy crops through mono-digestion for continuous production of renewable biogas

The aim of this laboratory-scale study was to investigate the long-term anaerobic fermentation of an extremely sour substrate, an energy crop, for continuous production of methane (CH₄) as a source of renewable energy. The sugar beet silage was used as the mono-substrate, which had a low pH of around 3.3–3.4, without the addition of manure. The mesophilic biogas digester was operated in a hydraulic retention time (HRT) range between 15 and 9.5 days, and an organic loading rate (OLR) range of between 6.33 and 10 g VS l⁻¹ d⁻¹. The highest specific gas production rate (spec. GPR) and CH₄ content were 0.67 l g VS⁻¹ d⁻¹ and 74%, respectively, obtained at an HRT of 9.5 days and OLR of 6.35 g VS l⁻¹ d⁻¹. The digester worked within the neutral pH range as well. Since this substrate lacked the availability of macro and micro nutrients, and the buffering capacity as well, external supplementation was definitely required to provide a stable and efficient operation, as provided using NH₄Cl and KHCO₃ in this case. The findings of this ongoing long-term fermentation of an extremely acidic biomass substrate without manure addition have reflected crucial information about how to appropriately maintain the operational and particularly the environmental parameters in an agricultural biogas plant.