"Algae For Entreprenuers" Free Report!

Algae harvesting techniques range from low-tech, to extremely high tech. This is one of the biggest challenges in the race to mass commercialization of algae biofuels.

Overview of Algae Harvesting Technologies

In order to be used, algae need to be separated, or harvested, from the pond or growth tank contents. Pond solution or tank contents are pumped in a large centrifuge vessel which moves at high speeds to press algae against the walls. The filter size in the centrifuge is only a few microns therefore water and other solutions are forced out through the spacing leaving algae as the residue. This method of separation is extensively used for micro algae.

Small Scale Methods:

Straining through paper and/or cloth filters: This is a method which will work on a small scale, but becomes problematic on a larger scale. Quite simply the algae culture is strained through a very fine

mesh filter. The filter allows smaller algae to flow through while trapping the larger alga. One advantage to using this system is the strained algae culture is still viable and can be used as a “starter” for your next batch. Disadvantages are the filters get clogged easily and have to be periodically maintained or replaced.

Using a cream separator: In this scenario, similar to the method described above but on a smaller scale. These household cream separators can be bought inexpensively, and used easily. Disadvantages are once again in the scale up; this method is difficult to use on a large scale and professional quality equipment is needed.

Large Scale Methods:

Flocculation and Sedimentation

Chemical flocculation is a method which uses chemicals like lime, alum, or chitosan to separate algae in the pond or growth tank. When these chemicals are added, they cause neutralization of the algae resulting in formation of algae clumps. This method is very costly due to the use of chemicals.

Flocculation and Dissolved Air Flotation

Flocculation and Dissolved Air Flotation (DAF) was established for sewage treatment and later studied in algae harvesting ^. Flocculation is used to increase the size of the algae, and then air is bubbled through the suspension causing the algal clusters to float to the surface. The algae-rich top layer is scraped off to a slurry tank for further processing.

Filtration

Solid/liquid filtration technologies are well studied, and filtration without prior flocculation can be used to harvest and dewater algae . Microalgae and cyanobacteria present unique filtration challenges because most strains considered for energy feed stocks have cell diameters less than 10 μm. Filtration is conceptually simple but potentially very expensive, and can be optimized through further understanding

of several issues related to this method.

Centrifugation

Centrifugation is widely used in industrial suspension separations and has been investigated in algal harvesting .²⁴ the efficiency is dependent on the selected species (as related to size).

• Centrifugation technologies must consider large initial capital equipment investments, operating costs, and high throughput processing of large quantities of water and algae.
• The current level of centrifugation technology makes this approach cost-prohibitive for most of the envisioned large-scale algae bio refineries.
• Significant cost and energy savings must be realized before any widespread implementation of this approach can be carried out.

Other Harvesting Techniques

A number of other techniques at various stages of R&D have been proposed to harvest and dewater microalgae. These include, but are not limited to, the use of organisms growing on immobilized substrates where the amount of initial water is controlled and the growth substrate can be easily removed; acoustic focusing to concentrate algae at nodes; manipulation of electric fields; and bio harvesting, where fuel precursors are harvested from higher organisms (e.g., shrimp and tilapia) grown with algae.