How to grow Chlorella?
The cultivation of Chlorella has the longest tradition in Asiatic
countries, especially Japan, where it has been used for human nutrition, pharmaceutics
and cosmetics in an annual amount of several thousands of tonnes. The use has
lately involved the so-called aqua-cultures - in which the alga is a valuable
component of the food chain in directed intensive-breeding colonies for delicate
fish and shrimp species.
Chlorella is usually grown in concrete circular ponds up to 45 m in diameter, with agitation by a rotating arm (resembling a clock dial with the second hand running around), or, in horizontal oblong basins (known as "raceway ponds") in which a 20 - 30 cm thick layer of inorganic nutrient solution with algae, exposed to sunlight and bubbled by carbon dioxide, is continuously moved by means of paddlewheels.
The Třeboň culture technology, developed and optimised since 1960, is very different. It is based on a system of inclined areas on which the algal suspension continuously flows during the day in a very thin layer at an optimum velocity, being thus intensively mixed. The turbulence is determined by the flow velocity and the roughness of the cultivation surface. Intensive turbulence optimizes the changes of light and dark periods of individual algal cells, resulting in a high efficiency of light utilization. During the night, the algal suspension is stored in aerated collecting tanks. The one-sided inclined area of the first bioreactors were covered with closely-spaced transverse baffles, which slowed down the suspension flow velocity and maintained the desired 5-cm layer of algae on the surface . The currently-used bioreactors, on the other hand, are composed of two baffle-free flow areas inclined in opposite directions and arranged in a meandering way . Because the flowing algal layer is only 6 mm thick, the suspension volume per unit area is 50 times smaller and the algal concentration 50 times higher than in the basins . This configuration considerably reduces the energy demands for the circulation of the suspension during its cultivation and for the separation of the algae from the nutrient solution during its harvest.
Optimization of the cultivation technology and the use of high-performance algal strains make it possible to achieve high yields of the Chlorella biomass under suitable climatic conditions. While in Třeboň (located at 50o N latitude), the growing season lasts 150 days and the yield is 16 - 20 tonnes algal dry weight per 1 hectare unit area, in southern Europe or North Africa, where the annual growing season lasts up to 300 days, the yields are up to 3 times higher.
How to process Chlorella?
After the algae have grown sufficiently, to attain a density
of approximately 30 g dry weight per 1 l nutrient medium, the algae are harvested.
The first step is the concentration of the suspension on disc centrifuges. The
concentrated algae are repeatedly washed with water in order to remove remnants
of the nutrient solution and other impurities. To achieve the maximum benefit
of the valuable cell contents that are enclosed in a tough cellulose 'coat'
(a fitting analogy might be the leather 'coat' of a football), we developed
a procedure such that the algae pass through a device in which they are mechanically
disrupted. Although the concentrated suspension contains up to ten billion cells
per millilitre, the proportion of disrupted cells after two to three-minute
disintegration reaches more than 90 %. The grinding of cells increases two to
three-fold their digestibility - achieving values comparable with the digestibility
of pure proteins.
The last step in the processing is the drying. This takes place in spray driers - in which the algal biomass is distributed into micro-droplets whose surface temperature does not surpass 60 oC during the entire 50 seconds for which the drying period lasts. The whole processing procedure is a very gentle one and the dried algal biomass, with the consistency of dried milk powder, thus retains all the valuable components in their original state.