2. What are microalgae? Microalgae are prokaryotic or eukaryotic
photosynthetic microorganisms that can grow rapidly and live in
harsh conditions due to their unicellular or simple multicellular
structure. Examples: Prokaryotic microorganisms: Cyanobacteria
(Cyanophyceae) Eukaryotic microalgae: Green algae (chlorophyta) and
diatoms (Bacillariophyta) 3. Biology of microalgae Algae are
recognised as one of the oldest life-forms. They are primitive
plants (thallophytes), i.e. lacking roots, stems and leaves, have
no sterile covering of cells around the reproductive cells and have
chlorophyll a as their primary photosynthetic pigment. Algae
structures are primarily for energy conversion without any
development beyond cells, and their simple development allows them
to adapt to prevailing environmental conditions and prosper in the
long term. 4. Biofuels cosmetics Pharmaceuticals Nutrition
Aquaculture Food additives Pollution prevention Biotechnology areas
of microalgae 5. Biodiesel from microalgae why? Energy is of vital
importance to society and human. Biomass energy, as a green and
renewable resource, has been considered to be one of the best ways
to solve the global energy crisis. Microalgae is an economical and
potential raw material of biomass energy, because it does not
require a large area of land for cultivation, exhibits short growth
period, possesses a high growth rate and contains more high-lipid
materials than food crops. 6. Biodiesel from microalgae why? Recent
studies have demonstrated that microalgae has been widely regarded
as one of the most promising raw materials of biofuels. However,
lack of an economical, efficient and convenient method to harvest
microalgae is a bottleneck to boost their full- scale application.
7. Technologies for the production of microalgal biomass production
Under natural growth conditions phototrophic algae absorb sunlight,
and assimilate carbon dioxide from the air and nutrients from the
aquatic habitats. Therefore, as far as possible, artificial
production should attempt to replicate and enhance the optimum
natural growth conditions. The use of natural conditions for
commercial algae production has the advantage of using sunlight as
a free natural resource . However, this may be limited by available
sunlight due to diurnal cycles and the seasonal variations; thereby
limiting the viability of commercial production to areas with high
solar radiation. For outdoor algae production systems, light is
generally the limiting factor . 8. Open ponds Open ponds are the
most widely used system for large- scale outdoor microalgae
cultivation. Commercially economical. Easy to build and operate.
Depending on their size, shape, type of agitation and inclination,
the open pond systems can be classified into (a) raceway pond, (b)
circular pond, and (c) sloped pond 9. Figure 1 Three different
designs of open pond systems 10. Enclosed PBR Two major types of
enclosed PBR are tubular and plate types. Due to enclosed structure
and relative controllable environment, enclosed PBR can reach high
cell density and easy to maintain monoculture. 11. Hybrid systems
Other types of systems are an internally illuminated photo
bioreactor (Helix PBR) developed by Origin oil company. The light
array rotates vertically that allows algae growth in deep media and
provides agitation. The light array consists of blue, red and white
lights, which are the wavelengths the algae prefer. 12. Harvesting
and drying of algal biomass There are currently several harvesting
methods, including mechanical, electrical, biological and chemical
based. In mechanical based methods, microalgal cells are harvested
by mechanical external forces, such as centrifugation, filtration,
sedimentation, dissolved air flotation and usage of attached algae
biofilms and ultrafiltration membranes. 13. Harvesting and drying
of algal biomass (1) Bulk harvestingaimed at separation of biomass
from the bulk suspension. The concentration factors for this
operation are generally 100800 times to reach 27% total solid
matter. This will depend on the initial biomass concentration and
technologies employed, including flocculation, flotation or gravity
sedimentation. (2) Thickeningthe aim is to concentrate the slurry
through techniques such as centrifugation, filtration and
ultrasonic aggregation, hence, are generally a more energy
intensive step than bulk harvesting. 14. Flocculation and
ultrasonic aggregation Microalgae cells carry a negative charge
that prevents natural aggregation of cells in suspension, addition
of flocculants such as multivalent cations and cationic polymers
neutralises or reduces the negative charge. It may also physically
link one or more particles through a process called bridging, to
facilitate the aggregation. Multivalent metal salts like ferric
chloride (FeCl3), aluminium sulphate (Al2 (SO4)3) and ferric
sulphate (Fe2 (SO4)3) are suitable flocculants. 15. Harvesting by
flotation Flotation methods are based on the trapping of algae
cells using dispersed micro-air bubbles and therefore, unlike
flocculation, do not require any addition of chemicals. Some
strains naturally float at the surface of the water as the
microalgal lipid content increase. 16. Gravity and centrifugal
sedimentation Gravity and centrifugation sedimentation methods are
based on Stokes Law, i.e. settling characteristics of suspended
solids is determined by density and radius of algae cells (Stokes
radius) and sedimentation velocity. Gravity sedimentation is the
most common harvesting technique for algae biomass in wastewater
treatment because of the large volumes treated and the low value of
the biomass generated. 17. Biomass filtration A conventional
filtration process is most appropriate for harvesting of relatively
large (>70 mm) microalgae such as Coelastrum and Spirulina. It
cannot be used to harvest algae species approaching bacterial
dimensions (
