Centrifugation is a widely employed technique for harvesting and dewatering microalgae biomass. Microalgae are considered a promising source of sustainable biomass due to their ability to grow rapidly, utilize CO2, and produce valuable bioproducts, such as biofuels, feed, food, and high-value chemicals. However, efficient harvesting and dewatering of microalgae remain significant challenges in large-scale operations. Algae cells are small and have low settling velocities, making traditional solid-liquid separation techniques less effective.
Centrifugation uses the principle of sedimentation under the influence of centrifugal force. The process accelerates the settling velocity of particles, enabling rapid separation of biomass from the culture medium. The efficiency of centrifugation depends on factors such as centrifugal force, particle size and density, medium viscosity, and temperature.
Several types of centrifuges are used for microalgae harvesting and dewatering, including disc stack centrifuges, decanter centrifuges, and tubular bowl centrifuges. Each type has its advantages and limitations in terms of separation efficiency, throughput capacity, energy consumption, capital cost, and operational complexity.
Disc Stack Centrifuges
Disc stack centrifuges are high-speed separators that use a series of conical discs stacked vertically inside a rotating bowl. The culture medium is fed into the center of the bowl, where it spreads out over the disc stack. As the liquid moves through the disc stack, algae cells are subjected to high centrifugal forces (up to 20,000 g) and rapidly settle on the discs.
The concentrated biomass forms a sludge layer on the disc stack surfaces and is periodically ejected from the centrifuge by an automatic self-cleaning mechanism. The clarified medium is discharged continuously from the top of the bowl. Disc stack centrifuges offer high separation efficiency and short residence times but require relatively high energy input and capital investment.
Decanter Centrifuges
Decanter centrifuges are continuous-feed separators that use a horizontal, cylindrical bowl with a conical end and a rotating screw conveyor inside. The culture medium is fed into the bowl, and the algae cells are subjected to centrifugal forces (up to 10,000 g) as the bowl rotates at high speed. The biomass settles on the bowl wall, forming a cake layer.
The screw conveyor continuously scrapes the biomass cake towards the conical end of the bowl, where it is discharged as a concentrated sludge. The clarified medium is discharged through adjustable weirs at the opposite end of the bowl. Decanter centrifuges have lower separation efficiency than disc stack centrifuges but offer higher throughput capacity, lower energy consumption, and simpler operation.
Tubular Bowl Centrifuges
Tubular bowl centrifuges use a vertical, cylindrical bowl with a stack of conical inserts or a helical coil inside. The culture medium is fed into the bottom of the bowl, and algae cells are subjected to centrifugal forces (up to 15,000 g) as the bowl rotates at high speed. The biomass settles on the bowl wall and forms a cake layer, which is periodically removed manually or by an automatic discharge mechanism.
The clarified medium is discharged continuously from the top of the bowl through an overflow weir or a paring disc. Tubular bowl centrifuges offer intermediate separation efficiency and throughput capacity compared to disc stack and decanter centrifuges but have lower capital cost and operational complexity.
Centrifugation is an effective method for microalgae harvesting and dewatering, but its feasibility in large-scale operations depends on factors such as energy consumption, capital investment, and operational complexity. Researchers are exploring alternative methods like flocculation, flotation, and membrane filtration to improve the overall efficiency and sustainability of microalgae biomass production.
