Optimization for Cultivation of Microalgae OUHK

Microalgae can fix CO2 with 10 to 50 times greater efficiency than terrestrial plants. Algae have a much higher growth yield (10 to 100 times higher) compared with other biofuel sources such as corn.

Description

Optimization for cultivation of microalgae Chlorella vulgaris and
lipid production in photobioreactor

Wong Yee-keung, Ho Kin-chung

School of Science and Technology, The Open University of Hong Kong
Department of Biology, Hong Kong Baptist University

Abstract

Microalgae have been used as energy resources in recent decades to mitigate the global energy crisis. As the demand for pure microalgae strains for commercial use increases, designing an effective photobioreactor (PBR) for mass cultivation is important.

Chlorella vulgaris, a local freshwater microalga, was used to study the algal biomass cultivation and lipid production using various PBR configurations (bubbling, air-lift, porous air-lift). The results show that a bubbling column design is a better choice for the cultivation of Chlorella vulgaris than an air-lift one. The highest biomass concentration in the bubbling PBR was 0.78 g/L while the air-lift PBR had a value of 0.09 g/L.

Key operating parameters, including inner-tube length and bubbling flowrate, were then optimized based on biomass production and lipid yield. The highest lipid content was in the porous air-lift PBR and the air-lift PBR with shorter draft tube (35 cm) was also better than a longer one (50 cm) for algal cultivation, but the microalgae attached on the inner tube of PBR always occurred.

The highest biomass concentration could be produced under the highest gas flowrate of 2.7 L/min whereas the lowest dry cell mass was under the lowest gas flowrate of 0.2 L/min. Besides, the biomass production in Day 10 in white LED was the highest (1.25 g/L) while blue LED, red LED and open pond were 0.80 g/L, 0.35 g/L and 0.58 g/L respectively.

Keywords: photobioreactor, Chlorella vulgaris, algal cultivation, lipid production, LED light

Extracts:

  • Microalgae can fix CO2 with 10 to 50 times greater efficiency than terrestrial plants (Kumar et al., 2010; Wang et al., 2012).
  • Algae have a much higher growth yield (10 to 100 times higher) compared with other biofuel sources such as corn (Greenwell et al., 2010). 
  • Posten (2009) claimed that the PBR design is basically derived from the typical surface-to-volume ratio (SVR) of 80 m2/m3 to 100 m2/m3, and the larger the SVR, the higher the distribution of light to the PBR.
  • Microalgae grow effectively in PBRs under optimal biotic and abiotic conditions (i.e. pH, temperature, CO2 exposure, lighting, and nutrients availability) (Acién Fernández et al., 2001; Barbosa et al., 2003; Sánchez Mirón et al., 2000; Yoo et al., 2012).
  • Light was provided by cool-white fluorescent lamps at 9000 lux with a dark/light cycle of 16:8 h for 14 days.
  • A column PBR with a capacity of 16 L was fabricated using transparent acrylic materials. The dimensions of the column PBR were 60.0 cm (height) × 20.0 cm (diameter), with openings at the top and bottom sides. The thickness of the column wall was 0.3 cm.
  • The perforated pipe sparger was located 3 cm from the bottom of the reactor with the supply of ambient air at the flowrates from 1 L/min to 6 L/min.
  • A synthetic culture medium was used, and air flowrate was 1 L/min. The initial cell density was 2.1 × 10e6 cells/mL.
  • Specific growth rate μ (per day) was calculated by using the equation from Wong et al., 2015b, using final and initial biomass concentrations (g/L) on days t1 and t0, respectively.
  • Cell concentration was determined using a 10x lens with the following equation: Cell Concentration = Total number of cells x (20/1.2) x 100 (Wong et al., 2015b).
  • Gas hold-up (ε) was calculated using the equation from Aljabbar, 2010; Molina et al., 1999), using hD as the gas-liquid dispersion height (cm) and hL is the height of gas free liquid (cm).
  • Blue light illumination LED increased cell size and cells grown, and red light was active divisions in small-sized cell, cell concentration and cell mass concentration (Chen et al., 2011; Kim et al., 2014; Koc, 2013; Shu, 2011; Ugwu, 2008; Vunjak-Novakovic, 2005).
  • The bubbling PBR was the better choice for the cultivation of Chlorella vulgaris.
  • The higher flowrate (2.7 L/min) of air supply in bubbling PBRs can cultivate higher biomass production but produce lower lipid content due to the shorter mixing time.
  • The photobioreactor under white LED light condition was the best for biomass and lipid production.