Growth characteristics of Chlorella sorokiniana in airlift and bubble column photobioreactors

Abstract

The present study investigated the feasibility of bioCO₂ sequestration using Chlorella sorokiniana. It was found that 5% CO₂ (v/v) in air was the most suitable concentration for the growth of this organism. At this concentration, the maximum rate of CO₂ sequestered and the biomass obtained were found to be 1.21 g L⁻¹ d⁻¹ and 4.4 g L⁻¹ respectively. Modeling and simulation of the growth profile was obtained using the logistic equation. Further, at higher CO₂ concentrations, pH drop in the growth media, TAP [-acetate], was prevented by replacing NH₄Cl by NaNO_3. Additionally, the study evaluated the performance of two reactors namely: bubble column and airlift reactor based on their growth profile and transport properties like K_La and mixing time. The growth profile was better in airlift reactor and it provides cyclic axial mixing of media. K_La of downcomer was significantly lower than the riser in airlift reactor.

Introduction

Global warming has reached an alarming level due to increase in CO₂ concentration in the atmosphere. Recent studies conducted at Mauna Loa Observatory (Hawaii, US) in December, 2011 found that the concentration of CO₂ in air was nearly 391 ppmv. Thus, it is imperative to identify and improve the process of CO₂ sequestration. In view of this, microalgae have been identified as a potential candidate for sequestering CO₂. Besides mitigating CO_2, they are known to have several other uses. They can be used for the production of biofuels (e.g. biodiesel, bioethanol, biohydrogen) and other important products like industrial biofilters, food products and secondary metabolites (Loubiere et al., 2009, Kumar et al., 2011).

Microalgae, Chlorella sorokinina, is playing an important role as food and feed because of the multiuse of its biomass previously known to be a rich source of carbohydrate, vitamins, and proteins. The high protein content makes it a suitable raw material for the production of single cell protein (Mahasneh, 1997) while the high vitamin contents makes it a suitable feed for aquaculture systems (Gapasin et al., 1998). Besides, under sulfur deprived condition (Chader et al., 2009) C. sorokiniana is also known to produce clean energy, biohydrogen. Additionally, it has been used for the production of commercially important antioxidants like lutein, α/β carotene, α/β tocopherol, zeaxanthin (Matsukawa et al., 2000).

Several factors are known to affect the CO₂ sequestration process; like choice of photobioreactor, culture/strain, temperature, pH, light intensity, culture density, concentration of CO₂, SO_x and NO_x, CO₂ mass transfer, O₂ accumulation etc. (Kumar et al., 2011). Among these, most notably, a suitable photobioreactor is essential for improved CO₂ sequestration and better utilization of light. Ease of operation, scalability, lower land requirement, higher biomass productivity and cost effectiveness are some of the significant features of an ideal photobioreactor (Kumar et al., 2011). Airlift and bubble column were considered as ideal photobioreactors for the present study because they are known to possess all the above-mentioned properties. In the past, both airlift and bubble column photobioreactors have been studied extensively for the cultivation of shear sensitive microorganisms (Barbosa et al., 2003; Chisti, 1989, Suh and Lee, 2001). Similarly, Ranjbar et al. (2008) and Harker et al. (1996) performed studies on airlift photobioreactor for astaxanthin and carotenoids production respectively. However, till date, these photobioreactors have not been completely exploited for the cultivation of photosynthetic microorganisms. A comparative analysis of both the photobioreactors based on the growth profile of the organism, mixing time and volumetric mass transfer coefficient may provide a thorough and comprehensive knowledge of both of the reactors. However, only few studies are available on the interaction of CO₂ mass transfer, light availability, hydrodynamic stress in the airlift photobioreactor (Chisti and Young, 1993; Contreras et al., 1998, Sánchez Miron et al., 2004, Hulatt and Thomas, 2011).

It is well known that alga grows efficiently only at optimal CO₂ concentrations while, any further increase or decrease of CO₂ concentration limits its growth (Chiu et al., 2008). Similarly, pH is another factor limiting the growth of the microalgae. Mostly the microalgae maintain neutral or slightly alkaline cytosolic pH, as many enzymes are highly pH dependent and become inactive in acidic pH (Gimmler, 2001). Moreover, choice of nitrogen source in the medium is critical as it serves as a nitrogen source as well as, controls the pH of the medium. TAP medium containing NH₄Cl has been extensively used by the researchers for the growth of the biomass that can be used for the hydrogen production (Skjanes et al., 2008). However, use of NaNO₃ as nitrogen source is considered as better than the NH₄Cl especially in presence of high CO₂ concentration in the reactor. Chen et al. (2011) reported that inhibition of cell growth at higher CO₂ concentration in the presence of NH₄Cl may be due inability of cells to regulate passive diffusion of the NH_3, which is in equilibrium with ${\text{NH}}_4^{+}$, across the plasma membrane. Moreover, consumption of NaNO₃ by algae may increase the pH of the medium as also, it may balance the pH drop due to high dissolved CO₂ and carbonic acids (Hulatt and Thomas, 2011).

Thus, the present study aimed to determine the growth characteristics of C. sorokiniana in airlift and bubble column photobioreactors based on different aspects like, mixing time, K_La, and growth profile. Besides, the study investigated the effects of factors like pH and CO₂ stress on algal growth profile. Further, an attempt was made to modify the existing TAP [-acetate] medium by substituting NH₄Cl by NaNO₃ to improvise algal cultivation at higher CO₂ concentrations.