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11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

Most of the industrial wastewaters contain high concentrations of heavy metal

pollutants and organic chemical pollutants like hydrocarbons, biocides, and

surfactants rather than nitrogen and phosphorus. Textile industries generate a

massive volume of waste when discharged into water bodies posing risk to the

aquatic ecosystem. Heavy metals like chromium (Cr), arsenic (As), copper (Cu), and

zinc (Zn) are the common constituents of textile wastewater. The concentrations of

nitrogen and phosphorus, however, vary in textile wastewaters depending on the

source [7]. In textile wastewater, the amount of TN varies between 21 and 57 mg/l

and TP varies from 1.0 to 9.7 mg/l [8]. Besides, chemical oxygen demand (COD) and

biological oxygen demand (BOD) also vary due to the structural variations in the

dyes and their metabolites used. Water bodies are also highly polluted by leachates

from landfills and dump yards, containing high levels of hazardous micropollutants

along with nitrogen and phosphorus (∼100 mg/l) in their inorganic forms [9].

11.2

Microalgae for Wastewater Treatment

Wastewaters from industries and polluted rivers have an elevated amount of nutri-

ents such as carbon, nitrogen, phosphorous, and other minerals. Some of the major

elements required for the propagation of microalgae include nitrogen, phosphorus,

and carbon. Studies have been carried out on the treatment of industrial, domestic,

agricultural wastewaters, and eutrophicated lakes using microalgae in addition

to wastewaters from aquacultures, fish farms, wineries, domestic discharges, and

industries [10]. Microalgae can absorb different types of pollutants (biosorption)

due to a series of independent metabolic processes, electrostatic interaction, ion

exchange, complexation, chelation, and micro-precipitation, and occurs essentially

within the dead or inactive cell walls. The microalgal cell composition is fibrous and

amorphous complex with different types of polysaccharides and functional groups

having the ability to capture heavy metals. Microalgae-based wastewater treatment

is driven by unlimited solar energy, CO2, and nutrients from wastewater itself.

Besides, microalgae produce extracellular biosurfactants as a result of metabolic

degradation which are employed in tertiary treatment due to its efficiency in

sequestering nutrients and heavy metals and production of secondary metabolites

preventing pathogen growth.

Phosphorus, a major nutrient leading to eutrophication is removed by precip-

itation of the effluent to form an insoluble solid fraction or transformed into an

activated sludge which is not recyclable. Microalgae are highly efficient in removing

nitrogen, phosphorus, and toxic heavy metals and therefore can be employed during

the tertiary treatment of wastewater. It is a greener and sustainable alternative to

the current energy-intensive and expensive technologies as they produce oxygen

in situ needed for mineralization of water pollutants. Microalgae-based wastewater

treatment is highly recommended for developing countries as the oxygen generated

from photosynthetic microalgae reduces the cost of mechanical aeration in the pond

treatment. Wastewater treatment using microalgae provides an opportunity for

efficient recycling of nutrients. For example, recovered algal biomass enriched with