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11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

of C. vulgaris was 79.6% and 71.7% in sewage and slaughterhouse wastewater,

respectively, whereas Spirulina sp. reduced COD by 71.7% in dairy processing

wastewater [16]. The COD removal efficiency of species of Neochloris, Chlorella,

and Chlorococcum in a river polluted with pharmaceutical wastes was found to be

90%, 84.5%, and 88.4%, respectively [4].

11.2.3

Nutrients (Nitrogen and Phosphorus)

Nitrogen and phosphorus are the two major nutrients in wastewaters, wherein

total nitrogen can be removed by nitrification and denitrification using microal-

gae. Different species of microalgae assimilate different organic and inorganic

compounds containing nitrogen in autotrophic and heterotrophic conditions.

Inorganic nitrogen species include nitrite (NO²⁻), nitrate (NO³⁻), and ammonia

(NH⁴⁺). Microalgae usually take up nitrogen in the form of ammonia via glutamine

synthetase and glutamine-2-oxoglutarate amido transferase pathway (GS/GOGAT

pathway). Ammonia mostly in the form of glutamine move across the cell mem-

brane and finally assimilate into amino acids to synthesize proteins, which requires

less energy to reduce and assimilate as compared to NO²⁻and NO³⁻[8]. Phosphorus

is usually present as orthophosphate (PO4

3−) in wastewater that are vital for cell

membrane, DNA, RNA, and ATP. Microalgae can also absorb extra phosphorus

under in high phosphate condition and stored as polyphosphate which is used as

an internal buffer under conditions of low phosphorus.

Several studies have been conducted to investigate the efficiency of microalgae in

removing nitrogen and phosphorus from wastewater. C. pyrenoidosa removed 62%

of TN and 87% of TP in textile wastewater [17], C. vulgaris removed 30–95% of TN

and 20–55% of TP in agro-industrial wastewater [18], Spirogyra sp. removes 95% of

TN and 90% of TP in textile wastewater and Cladophora sp. removed 93% of TN and

88% of TP in textile wastewater [19]. C. vulgaris was used to remove over 90% of TN

and 80% of TP from the primary treated sewage [20], whereas an earlier reported the

elimination of TN (50.2%) and TP (85.7%) in industrial wastewater using microalgae

[21]. Chromochloris zofingiensis removed 68–81% of TN and 90–100% of TP in piggery

wastewater [22].

11.2.4

Heavy Metals

Microalgae absorb heavy metals (metal biosorption) in two phases. The first phase

is called passive biosorption where the cell surface of the microalgae interacts with

heavy metals. The functional groups of cell surface have metal-binding groups

like amines, hydroxyls, carboxylates, phosphates, and sulfates providing multiple

active sites for metal ions. This phase takes place rapidly by ion exchange, physical

adsorption, complexation, or inorganic micro-precipitation mechanisms. The

second phase, called active biosorption, is where metal ions enter the microalgal

cell through the cell membrane. This stage is dependent on the metabolism of

the organism and slower compared to the first phase. A study on the cellular

distribution of microalgae revealed that large amounts of metal ions attached to