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11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

11.3

Cultivation of Microalgae in Wastewater

11.3.1

Factors Affecting the Growth of Microalgae

The efficiency of wastewater treatment using microalgae greatly depends on their

growth in wastewater and the extent of nutrient uptake depends on pH, TN:TP ratio,

pH, and light.

11.3.1.1

TN:TP Ratio

The TN:TP ratio of wastewater is an important factor for the successful cultivation

of microalgae as it determines the extent of nutrient assimilation, biomass pro-

ductivity, and dominance of microalgal species. Depending on the source, ranging

from 4 : 1 to as high as 40 : 1. TN:TP ratio of 16 : 1, also known as Redfield ratio was

initially considered as a constant for the growth of marine microalgae but 22 : 1 is

now regarded as an optimal ratio after a comprehensive study of oceanic organic

particulates. For freshwater microalgae TN:TP ratio of 6.8 : 1 to 10 : 1 is considered as

an optimal ratio. A nutrient ratio higher than optimal ratio can lead to phosphorus

limitation and lower ratio might result in nitrogen limitation as the structure and

function of the microalgae are greatly affected by the nutrient concentration. TN:TP

ratio, however, varies during microalgal growth.

11.3.1.2

pH

The different cellular processes in microalgae like energy production, composition

and function of cell organelles, enzymes, and proteins depend on the pH of wastew-

ater. The optimal pH for microalgae ranges from pH 7 to 9. However, microalgae

exhibit a high tolerance to pH variation. Chlorella ellipsoidea can thrive in both acidic

and basic pH (4–10). A study showed that C. vulgaris showed best growth rate at

pH 10 [29]. Nutrient uptake is also highly influenced by pH. C. vulgaris showed the

best nitrogen removal efficiency at pH between 7 and 8. Wastewater treated with

microalgae has higher pH due to its high photosynthetic rate, drawing more dis-

solved CO2 leading to high concentrations of carbonates and bicarbonates [10].

11.3.1.3

Light

Light is an important component for photosynthetic microalgae. Like green plants,

they capture light using chlorophyll and convert it into chemical energy in the

form of ATP. Oxygen and other reducing agents convert CO2 to organic molecules.

Different species of microalgae possess different pigments such as chlorophylls,

carotenoids, and phycobilins. Green microalgae (Chlorophyceae) have chlorophylls

a and b while majority of cyanobacteria (blue-green algae) have chlorophyll a,

phycocyanin, and phycoerythrin to help them in absorbing light with varying

wavelength and intensity. A 50% increase in biomass production and better nutrient

removal was reported in Scenedesmus sp. using red and blue lights compared to

white light [30]. Another study showed that exposure to blue light resulted in higher

growth rate in C. vulgaris as compared to the wavelengths of other light (white,

red, and green). A report showed that the intensity of light affected cell growth,