Lipid-extracted Microalgae Research Articles

Mesophilic and thermophilic anaerobic digestion of lipid-extracted microalgae N. gaditana for methane production

In the last years, a huge effort has been made to make biodiesel production from microalgae a feasible option. Besides the potential of biodiesel for replacing fossil fuels as a cleaner alternative, some limitations have still to be overcome. Among them, the low energy yields of the process and the high-energy requirements of the harvesting and drying steps lead to a high cost per litre of fuel. In this context, anaerobic digestion of the microalgal biomass after lipid extraction can improve the energy balance of the process, by producing methane and revalorizing a waste generated during biodiesel production. In this study, the production of biogas by anaerobic digestion of the marine microalgae Nannochloropsis gaditana after oil extraction was studied. As the hydrolysis is known to be the rate-limiting step of anaerobic digestion of solid substrates, the influence of the process temperature on this step was assessed. For this purpose, two different anaerobic sludges, i.e. mesophilic (35 °C) and thermophilic (55 °C), were used as inocula for different batch tests and for two continuous anaerobic bioreactors. The influence of the oil extraction process on the structural integrity of the microalgae was also studied. The results obtained from scanning electron microscopy and flow cytometry showed that the lipid extraction did not cause cell lysis, but the structure of their surface was affected by the process. Batch assays showed that thermophilic conditions did not improve the biomethane potentials when compared to mesophilic conditions. Two continuous anaerobic bioreactors operated for 120 days confirmed the batch results. However, measurements of the chemical oxygen demand showed that the soluble fraction in the thermophilic reactor was higher than under mesophilic conditions, indicating an improved hydrolysis step. This was confirmed by the results of scanning electron microscopy and flow cytometry, which suggested a more intense disintegration of microalgae in the thermophilic reactor, indicating a greater degree of hydrolysis. Nevertheless, this advantage of thermophilic temperatures over mesophilic conditions did not improve the methane productivity.

Optimization of operating variables for polyphenol extraction from lipid-extracted microalgae using acid-catalyzed hot-water extraction

Microalgae are a promising source of lipids for biodiesel production, which has the potential to replace fossil fuels without affecting the supply of foods and crop products. From the production of biodiesel, a solid waste known as lipid-extracted microalgae (LEM) is generated as a byproduct, and it is considered a rich source of antioxidant compounds, mainly polyphenols. In this study, optimization of the process variables of acid-catalyzed hot-water extraction was performed at low temperature in order to increase the production of polyphenols from LEM (Tetraselmis KCTC 12236BP). A statistically based method was used to optimize key variables, including extraction temperature, time, and sulfuric acid concentration. The results indicated that all process variables had significant effects on the extraction of polyphenols (p < 0.05). For instance, polyphenol levels increased in accordance with the increase of the three variables. The most economical optimal conditions were 100°C, 46.8 min, and 0.32 N H2SO4, under which polyphenol yield was 8.04 mg gallic acid equivalent (GAE)/g dry matter (DM), 5.2-fold higher than that from hot-water extraction without optimization.

Two-stage co-fermentation of lipid-extracted microalgae waste with food waste leachate: A viable way to reduce the inhibitory effect of leftover organic solvent and recover additional energy

This study aimed to mitigate the inhibitory effect of leftover organic solvent (CH3Cl, chloroform) on anaerobic digestion of lipid-extracted microalgae waste (LEMW). Food waste leachate (FWL) was added as a co-substrate and a two-stage fermentation process (H2 production with acidogenesis + methanogenesis) was adopted. The result of the first batch experiment, conducted in the absence of chloroform, showed that as the FWL addition ratio increased up to 60% on chemical oxygen demand (COD) basis, there was a gradual increase in the amount of H2 produced. At 80% addition, however, there was a huge drop in H2 yield, accompanied by a drop in pH. In the presence of chloroform (100–900 mg CH3Cl/L), the mixture (LEMW:FWL = 40:60) exhibited a much higher tolerance than that of LEMW alone, which could be ascribed to the co-metabolic degradation of chloroform by FWL addition. At 600 and 900 mg CHCl3/L, the degradation efficiency dropped below 40% for LEMW alone, while it was maintained above 90% in the mixture. A H2 yield of 36 mL H2/g COD, equivalent to 2.6% of the energy content in the feedstock, was attained from the mixture at 600 mg CHCl3/L. The H2 fermented effluent was then fed to a continuous methanogenic reactor at HRT 40 d, and 82% of energy content in the feedstock was further gained in the form of CH4. Although the energy gained from H2 production was negligible, most of the chloroform was degraded during acidogenesis, which resulted in a successful CH4 conversion.

탈지미세조류로부터 폴리페놀 생산 증대를 위한 열수추출 조건 최적화

합성 항산화제에 대한 대체제로 천연 항산화제에 대한 연구가 활발히 진행되고 있으며 미세조류는 천연 항산화제의 원료로 많은 관심을 받고 있다. 본 연구에서는 탈지미세조류에서 총 폴리페놀(TPC) 추출증대를 위해 추출용매, 온도, 시간, 고액비율과 에탄올 첨가 농도 최적화를 수행하였다. 열수와 유기용매 추출성능을 비교했을 때, 열수추출이 유기용매 보다 우수한 성능을 보였으며 온도 증가에 따라 추출성능도 비례하여 증가함을 보였다. 열수에 의한 추출이 에탄올 용액 추출(>98%)에 비해 우수한 성능을 보였으며40% 에탄올 용액을 이용한 열수 추출이 가장 우수한 추출 효과를 보였다. 추출조건10 min, <TEX>$100^{\circ}C$</TEX>, 40% 에탄올 열수추출에서 최대 폴리페놀 농도인 3.35 mg GAE (gallic acid equivalent)/g DM을 얻을 수 있었다. 지질 추출을 위한 유기용매 전처리 공정이 선수행 되었음에도 불구하고 탈지미세조류(Tetraselmis KCTC 12236BP)의 폴리페놀 농도가 다른 탈지이전 미세조류와 동등한 수준임을 확인할 수 있어 탈지미세조류가 천연 폴리페놀의 원료로서 적합함을 확인 할 수 있었다. 또한, 고액추출을 모사하기 위해 Peleg 모델을 이용해 예측한 폴리페놀 농도가 실험에 의해 얻어진 값과 높은 일치도를 보임으로 모델을 이용한 모사가 폴리페놀 추출 모사에 유용함을 증명할 수 있었다. The search for natural antioxidants as alternatives to synthetic products is growing. Microalgae have emerged as a source of natural antioxidants with significant and diverse health-promoting properties. In this study, the effects of hot-water extraction conditions on total polyphenol compounds (TPC) production were investigated for lipid extracted microalgae (LEA). In order to enhance the polyphenol productivity, the extraction variables including solvents, temperature, time and ethanol concentration were optimized. The results showed hot-water extraction provided a higher extraction efficiency than the organic solvents and extraction at high temperatures showed a better extraction efficiency. While hot-water extract showed a higher extraction efficiency compared to 98% ethanol extraction, the mixture of water and ethanol (40:60 v/v) showed the highest production of polyphenols. The maximum polyphenols of 3.35 mg GAE (gallic acid equivalent)/g DM were obtained at the optimized extraction time of 10 min, <TEX>$100^{\circ}C$</TEX> and 40% ethanol, respectively. Although Tetraselmis KCTC 12236BP was preprocessed by hexane to remove lipid for bio-diesel production, the results showed LEA contains relatively high level of polyphenols compared to untreated microalgae which can be used in the production of value-added materials. The predictions obtained from the developed Peleg's model were compared with the experimental data under the same operating conditions. The predicted and experimental data were consistent, indicating the reliability of the model.

Evaluation of different strategies to produce biofuels from Nannochloropsis oculata and Chlorella vulgaris

The lipid extraction using hexane and methanol:hexane increased the biodegradability of Nannochloropsis oculata by 36% and 24% respectively. Moreover, hexane increased the methane production from raw microalgae, from 253±11 to 313±9 mLCH4/gVS. Methanol:hexane did not affect the methane production, which yielded 254±10 mLCH4/gVS, mainly due to the significant changes in the biomass composition.On the other hand, the lipid extraction failed to increase the biodegradability of Chlorella vulgaris, which resulted around 44% for raw and lipid-extracted microalgae. The methane productions were 219±6, 202±1 and 200±4 mLCH4/gVS from raw and pre-treated microalgae using hexane and methanol:hexane respectively.Regarding the lipid extraction yields, using methanol:hexane the yields were 4.7 and 3.7 times higher for N. oculata and C. vulgaris than using hexane. The biodiesel yields were also higher using methanol:hexane, 2.4 and 1.9 times than using hexane. However, the biodiesel composition was unaffected by the solvent.The substrate to inoculum ratio influenced raw N. oculata digestion. At 1:1 VSSubstrate:VSInoculum, the methane production throughout the first days decreased but not the ultimate methane production. C. vulgaris digestion was unaffected, probably due to the biomass characteristics.Finally, the co-digestion of microalgae and sewage sludge showed no synergy, nor inhibition.

Retrofitting hetrotrophically cultivated algae biomass as pyrolytic feedstock for biogas, bio-char and bio-oil production encompassing biorefinery

Algal biomass grown hetrotrophically in domestic wastewater was evaluated as pyrolytic feedstock for harnessing biogas, bio-oil and bio-char. Freshly harvested microalgae (MA) and lipid extracted microalgae (LEMA) were pyrolysed in packed bed reactor in the presence and absence of sand as additive. MA (without sand additive) depicted higher biogas (420ml/g; 800°C; 3h) and bio-oil (0.70ml/g; 500°C; 3h). Sand addition enhanced biogas production (210ml/g; 600°C; 2h) in LEMA operation. The composition of bio-gas and bio-oil was found to depend on the nature of feedstock as well as the process conditions viz., pyrolytic-temperature, retention time and presence of additive. Sand additive improved the H2 composition while pyrolytic temperature increment caused a decline in CO2 fraction. Bio-char productivity increased with increasing temperature specifically with LEMA. Integration of thermo-chemical process with microalgae cultivation showed to yield multiple resources and accounts for environmental sustainability in the bio-refinery framework.

Inhibitory effect of chloroform on fermentative hydrogen and methane production from lipid-extracted microalgae

To improve the sustainability of microalgae as a bioenergy feedstock, lipid-extracted microalgae (LEM) are often further treated by anaerobic digestion (AD). However, the residual chloroform used for extracting lipids as a solvent could inhibit this process, an aspect that has not been studied to date. In this study, the inhibitory effect of chloroform on H2 and CH4 production was investigated by performing batch tests. To prepare the feedstock, Chlorella vulgaris was ultrasonicated and the supernatant was discarded after centrifugation. In case of H2 production, it was found that the H2 yield fell to almost half that of the control (15.6 mL H2/g CODadded) at 100 mg CHCl3/L. The reason for the decrease of the H2 yield with the increase of chloroform level was due to the change of metabolites from acetate and butyrate to lactate via a non-hydrogenic reaction. In comparison with H2 production, a much more severe inhibitory effect of chloroform on CH4 production was observed. The inhibitor concentration (IC30, 60, and 90) on H2 production was 138, 319, and 622 mg CHCl3/L, respectively, while concentrations of 15, 37, and 86 mg CHCl3/L were obtained on CH4 production. When the chloroform concentration was ≥25 mg/L on CH4 production, more than 2 g COD/L of organic acids remained, resulting in a decrease of CH4 yield. These findings indicate that the residual chloroform in LEM should be seriously considered to prevent possible microbial inhibition when designing a process for additional energy recovery from microalgae via AD.

Effects of co-products on the life-cycle impacts of microalgal biodiesel

Microalgal biodiesel production has been investigated for decades, yet it is not commercially available. Part of the problem is that the production process is energy and chemical intensive due, in part, to the high portion of microalgal biomass left as residues. This study investigated cradle-to-gate life-cycle environmental impacts from six different scenarios of microalgal biodiesel and its co-products. Ozone depletion, global warming, photochemical smog formation, acidification and eutrophication potentials were assessed using the Tool for the Reduction and Assessment of Chemical and other environmental Impacts (TRACI). Monte Carlo Analysis was conducted to investigate the processes with major contribution in each impact category. The market opportunity for each co-product was examined based on supply, demand and prices of the products that could potentially be substituted by the co-products. The results indicated that the scenario with the least life-cycle environmental impacts in all the five impact categories with the highest net energy ratio was the scenario utilizing a multitude of co-products including bioethanol from lipid-extracted microalgae (LEA), biomethane (to produce electricity and heat) from simultaneous saccharification-fermentation (SSF) residues, land-applied material from SSF residue anaerobic digestion (AD) solid digestate, recycling nutrients from SSF residue AD liquid digestate and CO2 recovered from SSF process contributed. Decreasing the energy consumption of the centrifuge in the land-applied material production process and increasing the lipid content of microalgae can reduce environmental footprints of the co-products. The same scenario also had the highest total income indicating their potential as co-products in the market.

Development of hydrothermal liquefaction and upgrading technologies for lipid-extracted algae conversion to liquid fuels

Bench-scale tests were performed for lipid-extracted microalgae (LEA) conversion to liquid fuels via hydrothermal liquefaction (HTL) and upgrading processes. Process simulation and economic analysis for a large-scale LEA HTL and upgrading system were developed based on the best available experimental results. The system assumed an LEA feed rate of 608dry metric tons/day and that the feedstock was converted to a crude HTL bio-oil and further upgraded via hydrotreating and hydrocracking to produce liquid fuels, mainly alkanes. Performance and cost results demonstrated that HTL and upgrading is effective for converting LEA to liquid fuels. The liquid fuels annual yield was estimated to be 26.9million gallon gasoline-equivalent (GGE) and the overall energy efficiency on a higher heating value (HHV) basis was estimated to be 69.5%. The variation range of the minimum fuel selling price (MFSP) was estimated to be $2.07 to $7.11/GGE by combining the effects of selected process factors. Key factors affecting the production cost were identified to be the LEA feedstock cost, final products yields, and the upgrading equipment cost. The impact of plant scale on MFSP was also investigated.