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11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

Non-stirred Ponds Nonstirred ponds that are 1.5 m deep on an average have the

simplest of media to large scale cultivation facilities. The disadvantages of this

culture system include zooplankton predation, contamination of algae, as well as

pathogen invasion.

11.3.2.2

Closed Systems

The disadvantages of open-pond systems can be minimized or removed in closed

pond systems known as bioreactors. A photobioreactor is a reactor utilizing light for

cultivation of phototrophic microorganisms (algae and cyanobacteria). Optimiza-

tion of photobioreactor system increases biomass, whereas better mixing and air

delivery can be achieved by shifting to cylindrical ceramic diffusers but optimization

of algal density and mixing rate can make this system highly efficient and productive.

Better aeration helps in higher mass transfer thereby removing oxygen that can be

detrimental at high concentrations. Reduction of algal contamination and water loss

due to evaporation are the main advantages of this type of culture system. Although

algal biomass can be significantly improved in the photobioreactor culture system,

the major limitations are high capital, operating, and maintenance costs. The vari-

ous types of photobioreactors designed for algal culture are tubular, flat plate, and

plastic bag photobioreactor.

In addition to the above culture techniques, there are other alternative culture and

treatment systems including hyper concentrated cultures, immobilized system, dial-

ysis cultures, photobioreactor, stabilization ponds and anaerobic ponds, facultative

ponds, and maturation ponds.

11.4

Algae as a Source of Bioenergy

Biofuel conversion technology is hindered in microalgae due to the resistant cell

wall. Pre-treatment is essential to disintegrate the cell wall and enhance energy out-

put. Pretreatment is of three types – physical (mechanical and thermal), chemical,

and biological. Physical pretreatments have been considered as the most effective

in breaking down microalgal cells by disintegrating the crystalline structures in cell

wall using mechanical tension (microwave and sonication) and heat. Thermal pre-

treatment is commonly studied among the various types of physical pretreatments,

but its efficiency varies according to the species of microalgae. Microwave pretreat-

ment and sonication are independent of the microalgae species but require high

energy [35]. Chemical pretreatments use alkali or acid reagents to break down poly-

mers within the cell wall. Chemical pretreatments combined with heat are highly

effective. Biological pretreatments use hydrolytic enzymes to breakdown cell wall

components. It was proposed that the use of an enzymatic mixture (lipase, cellulose,

α-amylase, xylanase, and protease) to pretreat Rhizoclonium, resulted in increased

methane production over physical pretreatment [36]. They can be used to improve

biofuel production from microalgae but, the method depends on the algal species,

their growth, and energy demands.