Microalgae can be harvested by four different methods these are,
- Filtering
- Flocculation
- Sedimentation
- Centrifugation
These different ways of harvesting have their own distinct advantage and disadvantage. Let me give you their highlights one by one.
- Filtering or Filtration: Species of microalgae such as Spirulina and other microalgaes which have a size of more than 55 micro meters can be easily filtered out and harvested by using filtration techniques. The filters have a maximum pore size of up to 50 micro meters in diameters; the filtration is performed by applying intense pressure or in a vacuum.
- Flocculation: flocculation in short means to create an aggregate. In microalgae harvesting flocculation is used to create a dense mass or aggregate of microalgae easier to remove. In the harvesting of microalgae flocculation can be done in two ways. The first one is called Bioflocculation, it is done by implementing non algal microbial cultures; bioflocculation can also be done by altering certain physical conditions such as temperature and culture media. The second method of flocculation is Chemical flocculation. In this method chemicals like ferric chloride, ferric sulfate, aluminum sulfate, polymeric flocculants and chitosan to cause the formation of aggregates or clumping together of the microalgae. The only disadvantage of chemical flocculation is the cost of the chemicals for large scale harvesting can be very expensive.
- Sedimentation: Sedimentation is the simplest method that requires no additional techniques or costs. In sedimentation the suspended microalgae are deposited by the force of gravity at the bottom which is easier to harvest. But sedimentation could take days even weeks for the microalgae to be drugged down and settled at the bottom of the tank.
- Centrifugation: This is the most common method used to harvest microalgae in large volume at a relatively short period of time. But compared to the above three methods, it costs very much and it has the highest energy consumption.
For better efficiency of the harvest of microalgae, some time flocculation and filtration can be combined for better results. Higher efficiency of filtration will be achieved, if flocculation is performed prior to filtration.
And for even better harvesting efficiency of microalgae, all the four methods can be combined in a step by step procedure as such,
- Step one: flocculation either chemical or bioflocculation
- Step two: sedimentation and harvesting the settled down microalgae after few hours 5 to 6 hrs
- Step three: Filtering the remaining unsettled microalgae and water mixture
- Step four (Optional step): Centrifugation. But this is an optional step.
By following the above four steps the cost of harvesting can be reduced very much. The fourth step is optional, because when step three is completed almost 90% of the microalgae are harvested. And almost 85% of the cost of biodiesel production from microalgae came from costs involved with centrifugation and its huge demand for energy.
