ABSTRACT
The type of harvesting employed depends heavily on the specific strain of organisms used in mass culture. Filtration, which operates under pressure or vacuum, is a preferred method for harvesting relatively large filamentous microalgae such as Spirulina platensis. For small cells of microalgae such as Chlorella and Dunaliella, conventional filtration is not practical, while membrane microfiltration and ultrafiltration are the possible alternatives. In case of gravity filtration, problem of clogging can be resolved by using vibrating screens which also increases the filtration rate. But cell rupturing might occur due to the cells rubbing against each other and filter. Therefore, with an increase in organic load in pond culture, possibility of bacterial contamination also increases. However, despite its various problems, filtering is currently the best method for optimal algal harvesting and concentration. The last step in the production of the algal biomass is drying of the dewatered slurry to a moisture content of 12-15 per cent. Various methods of
drying such as spray-drying, sun-drying, freeze-drying, fluidised bed drying, refractance window dehydration and vacuum-drying exist today. Dehydrating algae mass with a thin layer drum dryer yield is an excellent product from Scenedesmus. Drying algae on drum dryer has dual advantage of sterilising samples and breaking cell wall. An inexpensive and simple solar device can also be used for drying microalgal biomass with 90 per cent moisture content. Commercially, spray-drying is the most utilised drying method to produce pharmaceutical and food grade biomass. Despite the disadvantages of high capital and operational costs, the process is useful for obtaining a very pure and safe product. Algae oils are extracted in a number of ways that varies from simple to complex. Many times depending on process of extraction, algae need not be dried before oil extraction. While the simplest of all is mechanical crushing, the choice of method depends on several factors such as the volume of oil to be extracted, requirement and investment limits. Many commercial manufacturers of vegetable oil (olive or corn oil) use a combination of mechanical pressing and chemical solvents in extracting oil.
