Cultivation and Harvesting of Freshwater Microalgal Biomass in Different Wastewaters for Biofuel Production

Abstract

Microalgae as a source of lipids, carbohydrates and proteins can be used for biodiesel, bioethanol and biobutanol production, respectively. Microalgal biomass have received a substantial attention in research focused on chemical and bioenergy production. The most significant barriers for economical biofuel production using microalgae are the technological and engineering aspects, such as isolation, selection of potent microalgae species, and high cost of microalgae cultivation and harvesting steps. Selection of microalgae strains and optimization of the culture condition are key growth parameters to maximize biomass production and nutrient uptake from wastewater. In this research work, different freshwater microalgae species were isolated and selected to be cultivated in wastewater under different abiotic factors including salt stress and phytohormones. Maximum dry cell weight of microalgal biomass and lipid productivity were obtained at 100 mmol/L NaCl. Nitrogen was completely removed within 10 days as a result of algal growth promoted by the addition of 200-400 mmol/L NaCl. Phosphorus removal increased from 77 to 84% as the concentration of NaCl increased from 100 to 400 mmol/L. The highest removal (66%) of total inorganic carbon was obtained with the addition of 200 mmol/L NaCl. Additional NaCl in the wastewater resulted in an increased glycerol yield by microalgae (4-fold) which is an osmoregulant. Phytohormones, Indole-3-acetic acid (IAA) and Diethyl aminoethyl hexanoate (DAH) significantly influenced the growth and biochemical properties of microalgae. IAA and DAH at 10-5 M enhanced the microalgal growth by 1.9-and 2.5-fold, respectively. IAA controls important physiological processes in plants including cell enlargement, tissue differentiation and responses to light and gravity, while DAH promotes cell division. The primary role of DAH in cell division might be responsible for the higher microalgal growth under the influence of DAH compared to IAA in this study. The highest carbohydrate content (33 and 34%) achieved at 10-8 M and 10-5 M of IAA and DAH, respectively. While, the highest protein content (34 and 35%) obtained at 10-8 M of IAA and DAH, respectively. Total FAME content of microalgae increased upto 100 mg/g-DCW under the influence of both phytohormones. A novel application of acid mine drainge (AMD), an iron and aluminum metal ions rich natural source for biomass harvesting of microalgae species was investigated. Maximum flocculation of microalgae occurred with 10% dosage of AMD. Zeta potential (ZP) was increased from -10.66 to 1.77 and -13.19 to 1.33 for S. obliquus and C. vulgaris, respectively. Scanning electron microscope with energy-dispersive X-ray of the microalgae floc confirmed the sweeping floc formation mechanism upon the addition of AMD. Comparing with other harvesting methods, AMD could be an effective option for economic harvesting of microalgal biomass. This research work demonstrated that the application of salt stress, wastewater and phytohormones for enhancing the microalgal growth along with the biochemical content, and harvesting the generated biomass using AMD could be an appropriate option for the economic cultivation and harvesting strategy for the biofuel production.