Elsevier

Bioresource Technology

Volume 110, April 2012, Pages 496-502
Bioresource Technology

Evaluation of flocculation induced by pH increase for harvesting microalgae and reuse of flocculated medium

https://doi.org/10.1016/j.biortech.2012.01.101Get rights and content

Abstract

Flocculation induced by pH increase for harvesting microalgae and reuse of flocculated medium were evaluated. Increasing the medium pH value induced the highest flocculation efficiency of up to 90% for freshwater microalgae (Chlorella vulgaris, Scenedesmus sp., Chlorococcum sp.) with low/medium biomass concentrations and marine microalgae (Nannochloropsis oculata, Phaeodactylum tricornutum). The mechanism may be explained that Mg2+ in the growth medium hydrolyzed to form magnesium hydroxide precipitate, which coagulated microalgal cells by sweeping flocculation and charge neutralization. Additionally, this study revealed that the microalgal biomass concentrations and released polysaccharide (RPS) from microalgae could influence the flocculation efficiencies. Furthermore, neutralizing pH and then supplementing nutrients allowed the flocculated medium to maintain an approximate growth yield to that of the fresh medium in algal cultivation. These results suggest that the method presented here is effective, and allows the reuse of the flocculated medium, thereby contributing to the economic production from algae to biodiesel.

Highlights

► A flocculation method by pH increase was found effective for harvesting microalgae. ► Biomass concentration and RPS concentration influenced the flocculation efficiencies. ► Flocculated medium could be reused by neutralizing pH and supplementing nutrients. ► Reused medium maintained an approximate growth yield to that of the fresh medium.

Introduction

Today about 80% of global energy demand is produced from fossil fuels. However, with the depletion of fossil fuels, governments and research institutions are making a great effort to develop new fuels (Brennan and Owende, 2010). In this regard, biofuels are rapidly being developed. Unfortunately, biodiesel from sources such as plantation oil crops, waste vegetable oil and animal fat cannot realistically satisfy the existing demand for fuels (Clarens et al., 2010). Some species of microalgae, with high growth rate and high lipid content, appear to be attractive alternatives as a feedstock for biodiesel production (Chisti, 2007, Hu et al., 2008, Halim et al., 2011).

Microalgal cells, with a size range between 5 and 50 μm, always form stable suspensions in growth medium due to their negative surface charge. The separation and recovery of microalgal biomass from growth medium is a critical step in the microalgal biomass production process, which accounts for about 20–30% of the total production cost (Gudin and Therpenier, 1986). So it is necessary to develop cost-effective technologies that would permit efficient harvesting. There are several methods that have been developed for harvesting microalgae (Chen et al., 2011): centrifugation (Knuckey et al., 2006), foam fractionation (Gsordas and Wang, 2004), filtration (Zhang et al., 2010a, Zhang et al., 2010b), and flocculation (Rossingol et al., 1999).

Among the above methods, flocculation is considered to be an effective and convenient process, which allows rapid treatment of large quantities of microalgae cultures (Oh et al., 2001). Flocculation is the coalescence of separate suspended microalgal cells into larger loosely attached conglomerates. Firstly, the suspended cells aggregate into larger particles via the interaction of the flocculant with the surface charge of the cells. Then, the aggregates coalesce into large flocs that settle out of suspension (Knuckey et al., 2006). A large number of chemical products have been tested as flocculants including various inorganic multivalent metal salts (Duan and Gregory, 2003) and organic polymer/polyelectrolyte (Vandamme et al., 2010). In addition, some microbes have been applied to flocculating certain microalgae recently (Lee et al., 2009, Salim et al., 2011, Kim et al., 2011). Furthermore, electrolytic flocculation, another flocculation technique in which the electrolysis of the microalgal suspension leads to the flocculation of microalgae, has also been developed (Gao et al., 2010, Ilhan et al., 2008).

It is well known that flocculation of microalgal biomass is particularly sensitive to pH of culture suspension. pH increase enhances the flocculation efficiency substantially by promoting the precipitation of added flocculants (McCausland et al., 1999). In fact, pH increase may also influence the charge of microalgal cells (Danquah et al., 2009) and change the existing forms of metal cations in culture suspension due to their hydrolysis (Gregory and Duan, 2001). In this respect, flocculation simply by pH increase could be an attractive alternative because it is low-cost, low energy consumption, non-toxic to microalgal cells and it also eliminates the use of flocculants. Another outstanding advantage of this method is that the growth medium can be recycled after the microalgal flocculation since no flocculants are used and the medium is not contaminated. However, this method was only tested to a very small number of microalgal strains and has rarely been reported to date (Harith et al., 2009, Lee et al., 1998). Especially, few researches investigated the effects of the key factors, such as pH value, microalgal biomass concentration and the concentration of RPS on the flocculation by pH increase; and the detailed flocculation mechanism was also not fully understood yet.

In the present study, the effectiveness of flocculation by pH increase for three freshwater microalgae, namely Chlorella vulgaris, Scenedesmus sp. and Chlorococcum sp. and two marine microalgae, namely Phaeodactylum tricornutum and Nannochloropsis oculata were evaluated. The detailed flocculation mechanism of this method was studied and the influences of the key parameters were systematically examined. Furthermore, the reuse of the flocculated growth medium for cultivation was also investigated.

Section snippets

Microalgal strains and culture conditions

All microalgal strains were obtained from the Laboratory of Microalgal Bioenergy & Biotechnology, Research Center of Hydrobiology at Jinan University. C. vulgaris, Scenedesmus sp. and Chlorococcum sp. were grown in a BG-11 medium containing the following components: NaNO3 (1.5 g L−1); K2HPO4·3H2O (40 mg L−1); MgSO4·7H2O (75 mg L−1); CaCl2·2H2O (36 mg L−1); NaCO3 (20 mg L−1) FeCl3·6H2O (3.15 mg L−1); citric acid (6 mg L−1) and 1 mL of microelements composed of H3BO3 (2.86 mg L−1), MnCl2·4H2O (1.81 mg L−1), ZnSO4·7H

Flocculation of microalgal cells by pH increase

The flocculation efficiency was investigated as a function of pH variations in Fig. 1a and b. An alkaline flocculation zone was observed for each microalgal species. The freshwater microalgal cells began to agglomerate when the pH was adjusted up to about 8.6. As pH increased to 10.6, the efficiencies were greatly raised to more than 90% and reached plateaus. The flocculation for the two species of marine microalgae was initiated at pH 8.0 and 8.2 respectively. And the highest efficiencies

Conclusions

A flocculation method induced by pH increase was found effective for harvesting microalgae. The mechanism involved the formation of magnesium hydroxide precipitate, sweeping flocculation and charge neutralization. The flocculation efficiencies decreased considerably with the increase of biomass concentration and RPS concentration. The flocculated medium could be reused, thereby minimizing the demand for water and reducing the cost of biodiesel production from algae. Finally, the method was only

Acknowledgements

This study was sponsored by Natural Science Foundation of Guangdong (No. S2011040001667), the Fundamental Research Funds for the Central Universities (No. 21611310), and the National High Technology Research and Development Program of China (863 Program) (No. 2009AA064401).

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These authors contributed equally to this paper.

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