Microalgae harvesting

Introduction

 

Microalgae are a family of microscopic unicellular organisms, which exist individually or in groups. There is a vast array of different species of microalgae, many of which have a great potential to provide protein, lipids, vitamins, and carotenoids. Studies have also showed the digestibility of some microalgae strains by carnivorous fish species, which suggest a potential application of microalgae in producing the fish feed. The most common methods of conducting microalgae cultivation is using open pond or closed bioreactor systems. The important parameters that need to be considered in both systems are efficient use of sunlight, and efficient mass transfer. When the biomass reaches a certain density or at regular intervals, microalgae need to be harvested.

 

 

Microalgae harvesting

 

When algal strains reach cell density between 0.5-7 g/l, they are ready to be harvested. Depending on the type, the strain’s average sizes algae globules ranges between 1 to 100 micron. Separating the algae from the culture fluid remains a major challenge to industrial scale processing, mainly due to the small size of the algal cells and relatively dilutes cultures that require the processing of large fluid volumes. The initial harvesting step is not only costly, but also affects following processes. Conventional techniques that are used today for extracting the algae from the culture fluid during harvesting involve sedimentation, flocculation, membrane filtration, and centrifugation.

 

Sedimentation is the most common technique for harvesting large algae. This method is however  not suitable for microalgae.

 

Flocculation is a common preparatory step prior to other concentration methods. Aggregation of cells by flocculation is an efficient method for possible pre-concentration before centrifugation.

 

Membrane filtering is promising method but has limited commercial experience.

 

The use of centrifuges involves both high investment and operational costs driven by the energy consumption. This method though is preferred among the industrial applications as it is capable of separating different types of microalgae that can vary in size, morphology, density and physiology.

 

In order to reduce dewatering costs alternative methods need to be considered.

 

 

Use of hydrocyclones for microalgae harvesting

 

Few studies investigated use of hydrocyclones for the dewatering process of microalgae biomass. Results show that this type of equipment is capable of separating significant amount of culture fluid from the algae strains. Certain types of microalgae are sensitive to shear forces. Therefore, combination of low shear pumps and hydrocyclones is an attractive alternative, which may reduce overall costs of harvesting process. Certain low shear centrifugal pumps can even promote coalescence of the algae strains, thus replacing the flocculation step during the pretreatment process. Aggregated algae flocs would be removed more efficiently with the use of the hydrocyclones.

 

The evaluation of energy consumption by different separation equipment is shown below.

 

Table 1. – Device energy requirements for processing microalgae culture fluid.

Table 1. – Device energy requirements for processing microalgae culture fluid.

 

Table 1 shows that hydrocyclones are the most energy efficient type of equipment compared to the other listed technologies. Together with mechanical simplicity and no moving parts, they offer low maintenance and high reliability.

 

The important aspect to remember is that hydrocyclones have a lower limit of particle sizes that can be efficiently separated. Therefore, the utilization of low shear pumps for increasing feeding pressure of the culture fluid prior entering the hydrocyclones is essential for the overall effective and cost-efficient harvesting process of microalgae.

 

 

Additional Reading

 

Barros, A.I., Gonçalves, A.L., Simoes, M. et al. Harvesting techniques applied to microalgae: A review. Published by Renewable and Sustainable Energy Reviews, Elsevier, V. 41, January 2015, pp. 1489-1500. http://dx.doi.org/10.1016/j.rser.2014.09.037

 

Brotzman, R., Hydrocyclone Separation of Targeted Algal Intermediates and Products. Report. Argonne National Laboratory, 2015. https://www.energy.gov/sites/prod/files/2015/04/f21/algae_laible_133100.pdf

 

Mohn, F.H., 1980. Experiences and strategies in the recovery of biomass from mass cultures of microalgae.  In: Algae Biomass. Shelef, G. & Soeder, C. J. (Ed.) Elsevier Amsterdam, 547-571.

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