Process optimization for cultivation and harvesting of microalgae: light and nutrients for growth and chitosan for harvesting

Abstract

Recently, microalgae biomass is being considered as a useful source of biofuel production. Slow growth and low biomass recovery are the major challenges in algae-derived biofuels. In this study, it is focused to increase the growth rate and biomass recovery of microalgae through various approaches. In the first step, the growth rate of a microalga (Chlorella vulgaris) was increased by controlling nutrients. Various nutrients such as nitrate, phosphate, yeast extract, ferric ion, and methanol were used. Among tested nutrients, only nitrogen (25 mg/L) and methanol (30 mg/L) could increase the growth rate. In the second step, the interactive effect of light and nutrients on microalgae growth was studied via two-stage cultivation system. In stage-1, the nutrients and light were supplied at minimal levels (1 dose of BG-11 medium as nutrients and 27 umoles/m2/s of light intensity). In stage-2, higher levels of nutrients (1-2×BG-11 medium) and light (27-40 umoles/m2/s), in different combinations, were employed. The lowest doubling time (14 hrs) or the highest growth rate was achieved at 1×BG-11 as nutrients and 40 umoles/m2/s of light intensity. In the second part of this study, chitosan was used as a biopolymer, for the harvesting of Chlorella vulgaris. Several factors control the microalgal harvesting efficiency such as mixing speed and time, solution pH, type of solvent, settling time, and the concentration of flocculant. The highest harvesting efficiency (99%) was achieved in the following conditions: culture pH 6.0, mixing speed 300 rpm, mixing time 1 min, settling time 10 min, and chitosan dose of 120 mg/L. The harvesting efficiency was dependent on the solubility of chitosan also. Among various chitosan solvents, hydrochloric acid (0.1 M) showed the highest solubility, and thus the maximum harvesting efficiency. To save the cost of flocculant, harvesting efficiency of Chlorella vulgaris was evaluated without any flocculant aid, called self-flocculation. Chlor...