Process development for carbon dioxide removal from biogas and lipid production by oleaginous microalgae cultivation

Abstract

Oleaginous microalgae has high CO2 removal rate which could be used for biogas purification effectively. They also can accumulate high lipid content >20% dry basis and has potential to be utilized as biodiesel feedstocks. This study aimed to develop the efficient process of microalgae cultivation for biogas purification coupling with lipid production as well as the development of harvesting process, immobilization of microalgae for repeat-use and the ability of microalgae for phytoremediation of industrial wastewater. There are main six parts in this study. Part I is the cultivation of several oleaginous microalgae using biogas and the evaluation of their ability to remove CO2 and improve methane content in biogas. All oleaginous microalgae could effectively remove CO2 in biogas (>90%) and accumulate lipid content in range the range of 24-42%. Among the species tested, Scenedesmus sp. was most effective in CO2 removal. The optimal conditions for both biogas purification and lipid production were: gas flow rate of 0.3 L h-1 per Lmedium, inoculum sized at 107 microalgal cells mL-1, added with KNO3 0.8 g L-1 as nitrogen source and illuminated at 5.5 klux light intensity. Under these conditions, methane content in biogas was increased from 60% up to >90% corresponding to high CO2 removal rate of 5.097 g-CO2 day-1 per 1 L-medium coupled with lipid productivity of 88.57 mg L-1 day-1. In addition, with the strategy of stepwiseincreasing gas flow rate the final biomass and lipid productivity were 1.25 and 1.79 folds increased. The microalgal lipids were composed of fatty acids with fuel properties of high oxidation stability and high ignition quality. Part II is the further improvement of Scenedesmus sp. performance in biogas upgrading and lipid production by the strategies of step-wise increasing of growth factor levels. Three important growth factors for microalgae included light intensity, nitrogen source and CO2 flow rate. The stepwise-increasing of CO2 flow rate was suitable for cell growth and lipid production while the stepwise increasing of light intensity was more suitable for CO2 removal efficiency. Among the strategies attempted, the simultaneous stepwise-increasing of all three growth factors most effectively enhanced the performance of microalgae. Through this strategy, >96% of CO2 was continuously removed from biogas and the CH4 content in the purified biogas was >98%. This process also generated microalgal biomass at 4.40 g L-1 with a lipid content of 34.10%. The CO2 removal rate by this process was as high as 6.50 g- CO2 day-1 per 1 L microalgal culture. The microalgal lipids contained long chain fatty acids (C16-C18) >94% and their prospect fuel properties indicated their suitable use as biodiesel feedstocks. Part III is the optimization of the conditions for simultaneous biogas purification and pretreatment of anaerobic digester effluent from palm oil mill by immobilized oleaginous microalga Scenedesmus sp. in alginate gel beads. The optimal culture conditions for immobilized microalga were: the use of initial cell concentration at 106 cells mL-1 and bead volume to medium volume ratio at 25% v/v. The optimal conditions for simultaneous biogas purification and pretreatment of secondary effluent were: the use of diluted effluent at 4:1 and light intensity at 9.5 klux. Through these conditions, 88.46% of CO2 was removed from biogas and the methane content was increased more than 95%. The CO2 removal rate was 4.63 g- CO2 day-1 per 1 L-medium. After process operation, the immobilized microalgae effectively removed COD >71% and all of nitrogen and phosphorus. The final microalgal biomass obtained was 2.98 g L-1 with high lipid content of 35.92%. The pigments including chlorophylls and carotenoids in biomass were 45.97 and 26.06 mg g-1 biomass, respectively. Fatty acid compositions of microalgal lipids were C16-C18 (>98%). Part IV aimed to increase the lipid content of oleaginous microalgae via nutrient starvations and optimize the cost-effective harvesting process. Two locally isolated oleaginous microalgae from Songkhla Lake in Thailand were identified as Micractinium reisseri SIT04 and Scenedesmus obliquus SIT06. Starvation of either ferrous or phosphorus did not significantly affect cell growth but the starvation of nitrogen did limit cell growth of both strains. However, the nitrogen starvation stimulated lipid content of both strains by 1.5-1.6 folds which were higher than the lipid content increased by ferrous and phosphorus starvation (1.2 folds). S. obliquus SIT06 could grow and accumulated higher lipid content. The lipid accumulated during nitrogen starvation contained higher content of saturated fatty acids. The harvesting process through bioflocculation was optimized by Response Surface Methodology (RSM). The maximum flocculation efficiency greater than 99% was achieved using minimum dosage of chitosan at 64 mg L-1 which are fairly costeffective at estimated chitosan around 0.098 Bath per gram microalgae biomass. Part V aimed to optimize the photoautotrophic cultivation of S. obliquus SIT06 using RSM and to harvest microalgal cells by co-pelletization with filamentous fungi. The optimal conditions for photoautotrophic cultivation of S. obliquus SIT06 were: pH of 8.0, NaNO3 as a nitrogen source at concentration of 1.1 g L-1, and light intensity of 87 μmol proton m-2 s-1. Under these conditions, the highest microalgal biomass obtained was 1.99 g L-1 with a high lipid content of 40.86%. To simplify harvesting process of microalgal cells, pellet-forming filamentous fungi were inoculated into the late log-phase of microalgae culture. Among the fungi tested, Cunninghamella echinulata TPU 4652 most effectively harvested the microalgal cells with the highest flocculation efficiency of 92.7%. Moreover, the biomass and lipids of microalgae-fungi pellets were as high as 4.45 and 1.21 g L-1, respectively. The extracted lipids were mainly composed of C16:0, C18:0 and C18:1, and their estimated fuel properties meet with the international standards indicating their potential use as biodiesel feedstocks. Part VI is the development of the rapid method for harvesting and immobilizing oleaginous microalgae using pellet-forming filamentous fungi. Among the fungi tested, Trichoderma reesei QM 9414 showed superior pellet forming ability under shaking speed at 100 rpm. Its pellets were used to harvest oleaginous microalga Scenedesmus sp. With increasing volume ratio of fungal pellets to microalgae culture up to 1:2, >94% of microalgal cells were rapidly harvested within 10 min. The ratio of fungal pellets could manipulate both harvesting time and initial concentration of microalgal cells in the pellets. The microalgae-fungal pellets were successfully used as immobilized cells for effective phytoremediation of secondary effluent from seafood processing plants under nonsterile condition. The chemical oxygen demand, total nitrogen, and total phosphorus removal were >74%, >44%, and >93%, respectively. The scanning electron microscopy showed that the microalgal cells were not only entrapped in the pellets but also got attached to the fungal hyphae with sticky exopolysaccharides, possibly secreted by the fungi. The extracted lipids from the pellets were mainly composed of C16-C18 (>83%) with their suitability as biodiesel feedstocks. This study has shown that oleaginous microalgae are the promising microorganisms that can be used not only for effective biogas purification but also production of lipids with high potential as biodiesel feedstocks. The effective strategies to increase microalgal growth, lipid content, CO2 removal efficiency as well as the effective harvesting process and innovative immobilization of microalgae have been proposed. The oleaginous microalgae also show high ability for being used in phytoremediation of industrial wastewater. The extracted lipids from both microalgae and microalgae-fungal pellets have similar fatty acid compositions with those of plant oils. This study may contribute greatly to the biogas industry and the industrialized microalgae based biofuel production.