Microalgae, due to their small cell size and rapid growth, have a significant potential as a renewable source of biofuels and biochemicals. They can be cultivated in a variety of environments and do not require arable land or fresh water, making them a sustainable option for biomass production. However, one of the major challenges in microalgae processing is their harvesting and subsequent dewatering. The small size of microalgal cells (typically ranging from 2-20 µm) makes conventional harvesting methods such as centrifugation, filtration, and flotation inefficient and costly.
Centrifugation, which relies on the difference in density between the algal cells and the culture medium, often requires high energy inputs and is not suitable for large-scale operations. Filtration can lead to clogging of the filter media due to the small size and deformability of microalgal cells. Flotation, on the other hand, is affected by the presence of extracellular polymeric substances produced by microalgae which may inhibit bubble formation and attachment to algal cells.
Moreover, once harvested, the high water content of microalgal biomass (about 90-95%) poses further challenges in dewatering. Conventional dewatering methods such as pressure filtration, vacuum filtration, and centrifugal dewatering are energy-intensive and not economically viable at an industrial scale.
Recent research has focused on developing innovative methods for efficient harvesting and dewatering of microalgae. These include bioflocculation, where certain species of bacteria or algae are used to form aggregates with microalgal cells; ultrasonic treatment, which can disrupt cell walls and facilitate separation; and the use of hydrogels or super absorbent polymers that can absorb water from algal suspensions.
Bioflocculation has shown promise as a low-cost and eco-friendly method for harvesting microalgae. Certain bacteria or algae can produce extracellular polysaccharides that bind with microalgal cells, forming larger aggregates that can be easily separated from the culture medium. However, this method requires careful selection of flocculating agents to avoid contamination of the biomass.
Ultrasonic treatment uses sound waves to create pressure changes in the liquid medium, leading to the formation and collapse of microscopic bubbles. This can disrupt the cell walls of microalgae, facilitating their separation from the culture medium. While this method can improve harvesting efficiency, it may also lead to loss of valuable intracellular products.
The use of hydrogels or super absorbent polymers for dewatering has been explored due to their high water absorption capacity. However, these materials need to be removed from the biomass before further processing, adding additional steps and costs.
In conclusion, while small cell size poses significant challenges in microalgae harvesting and dewatering, innovative solutions are being developed to overcome these hurdles. Further research is needed to improve the efficiency and reduce the cost of these methods for large-scale industrial applications.
