Microalgae Producing Energy Through Emissions | Tomorrow Today

Ways of harvesting microalgae for biodiesel production

Microalgae can be harvested by four different methods these are,

1. Filtering
2. Flocculation
3. Sedimentation
4. Centrifugation

These different ways of harvesting have their own distinct advantage and disadvantage. Let me give you their highlights one by one.

1. 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.

   
   
2. 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.

   
   
3. 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.
4. 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.

Ideal culture conditions to grow Algae

Algae are green plants, primarily they requires sunlight, carbon dioxide and mineral nutrients. On the other hand they also requires optimum environmental conditions such as the right pH and temperature. Algae grow and thrive mostly in water and moist environmental conditions. Depending on the algae species, algae can grow either in fresh water or sea water. The majority of microalgae thrive in fresh water, though many micro algae can grow also in sea waters too. Most of the macroalgae are the dominant types of green plants in the seas and oceans.

Sunlight requirements

Algae which lives in deep waters require very low intensity of sunlight, but algae species which lives on the surface of the water or in a very shallow waters require direct sunlight. In most modern photobioreactors light is provided artificially from fluorescent lamps. This kind of lamps are good option to control the intensity of light based on the particular species of algae.

Carbon dioxide and other gases

Algae consumes very huge amount of carbon dioxide, than terrestrial green plants. For algae cultured under photo bioreactors and open ponds, carbon dioxide is supplied artificially in the form of bubbles. The optimum concentration of carbon dioxide to grow algae for biodiesel purpose is up to 3%.
Oxygen is the byproduct of photosynthesis as a result very high concentration of oxygen in the water can be toxic to the algae for it reduces photosynthesis efficiency. To avoid oxygen toxicity the water should be constantly stirred and recycled to remove the excess oxygen.
Some photobioreactiors get their carbon dioxide from flue gas. This type of gas contain up to 15% of carbon dioxide. In addition to this it is also a mixture of many other gases such as nitrogen dioxide, sulphur dioxide, etc. Most algae can make use of this extra mixture of gases as a nutrient sources.

Mineral nutrients, Vitamins, and hormonal requirements

Because of the diverse nature of algae species in their nutrient requirements, it is very difficult to generalize the amount and rate of each nutrient required by algae. But most scientists agree that all algae needs a set of 14 elements, these are Nitrogen, Phosphorus, Magnesium, Potassium, Sulphur, Calcium, Molibdium, Zinc, Iron, Copper, Manganese, Oxygen, Hydrogen and Carbon. In addition, some elements are required by some or more algae, these are Cobalt, Iodine, Sodium, Boron, Chlorine, Silicon and Bromine. For example diatoms need an additional silicon for optimum growth. All algae uptake dissolved inorganic nutrients such as nitrates, nitrites, ammonium and phosphates.

Vitamins such as vitamin B12 and thiamine and growth regulators could give better yield of algae. The concentration of the vitamins and growth regulators and the method of applications will depend on the particular species of algae.

pH requirements

The ideal pH for most algae species is between 7 to 9 pH levels. The culture medium should be kept alkaline with an average pH of 8. Most algae are very sensitive to acidic culture medium and they perform very badly under acidic conditions.