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11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

tion, desalination, fixed biofilms, ultraviolet radiation, and chlorination which are

expensive and complex. The major disadvantages of conventional treatment include

(i) inconsistency depending on the nutrients to be removed, (ii) high operation cost,

(iii) generation of secondary pollution by chemical processes, (iv) loss of valuable

potential nutrients, and (v) incomplete utilization of resources.

Wastewater treatment should be efficient and cost effective. For the past few

years, research on the use of microalgae emerged as a sustainable approach to

eliminate water pollutants [2]. The advantages of biological wastewater treatment

using microalgae include bioremediation in situ, zero production of secondary

waste, and economic viability.

11.1.1

Microalgae

Microalgae are photosynthetic, unicellular, or multicellular organisms representing

diverse species with varied physiology and growth requirements. It is reported that

there are ∼80 000 different microalgal species on earth and 40 000 species have been

studied so far. Microalgae may be prokaryotic or eukaryotic and can be divided into

different phyla.

Chlorophyta: They may be marine, freshwater, or terrestrial, which may be unicel-

lular or and multicellular, autotrophic with photosynthetic pigments (chlorophyll

a, b, c, d, and e; carotenoid a and b; xanthophylls; pheophytin a and b; and bac-

teriochlorophylls) and hence also known as green microalgae. Their growth rate

is ∼100 times higher than terrestrial plants, and their biomass can double in less

than 24 hours which is attributed to their simple structure and high cell surface:

volume ratio resulting in high rate of nutrient absorption, such as in Chlorella

vulgaris, Dunaliella salina, and Haematococcus pluvialis.

Rhodophyta: They may be unicellular and multicellular. Multicellular species are

common and are found in marine environments while only a few are freshwater

unicellular species. The stromal side of thylakoids are lined with phycocyanin and

phycoerythrin as observed in Porphyridium cruentum and Rhodella reticulata.

Haptophyta: They are mainly marine, unicellular, and existed in colonies. They

have one or two chloroplasts containing pyrenoids out of which Isochrysis galbana

and Pavlova salina are the important ones.

Dinophyta: They are unicellular and mainly marine, while some species are found

in freshwater. Half of the total species in this phylum are true autotrophs, and

the other half are heterotrophs with no chloroplast. Example: Crypthecodinium

cohnii.

Ochrophyta: They include diatoms that are the smallest unicellular and most abun-

dant marine phytoplanktons.

Microalgae flourish in varying habitats, which may be aquatic or terrestrial, and

withstand a wide range of environmental variations including nutrient depletion,

salinity, drought, light, osmotic pressure, temperature, and ultraviolet radiation.

They can even optimize lipid production that is the raw material for biodiesel