11.3 Cultivation of Microalgae in Wastewater

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carbohydrate, and lipid production, along with increased efficiency of CO2 fixation.

A study was conducted where biomass production and CO2 mitigation efficiency

increases with increase in light intensity from 140 to 540 μmol/m² s and the peak is

obtained at 420 μmol/m² s after which a decrease is observed [31]. It was observed

that microalgae under 24 hours continuous artificial illumination showed higher TP

removal than under a 12 hours light-12 hours dark regime by solar radiation [32].

11.3.2

Algal Culture Systems

The type of culture system depends on the reason for cultivation (biofuels, CO2

mitigation etc.), nutrient source, final products desired, operation, maintenance,

and capital costs. The cultivation system is classified into “open” and “closed”

systems. “Open” systems (ponds, lagoons, and deep channels) are constructed

outdoors, while “closed” system may be established outdoors under sunlight or

indoor under artificial light.

11.3.2.1

Open Systems

The most commonly employed algal cultivation system for commercial scale is the

open systems because of simple operation technique and low cost of construction

[8]. These systems may be natural like ponds, lagoons, and lakes or artificial water

systems like the comprise tanks and man-made ponds. Open systems can be cate-

gorized into stirred and non-stirred ponds depending on the mode of aeration and

nutrient distribution in the medium. Stirred ponds are suitable for abundant light,

good aeration, and nutrients influencing microalgal growth, whereas the non-stirred

ponds are easier to manage and are economical.

Stirred Ponds

High rate algal ponds (HRAPs), also known as raceway ponds, are cir-

cular, largest, and most used forms of stirred ponds, which were devised to combine

biofuel production and wastewater treatment on a large scale almost 50 years ago

[33]. Oswald and his colleagues developed HRAP technology for wastewater treat-

ment demonstrating efficient removal of organic compounds, and nutrients (P, N)

with a reduction in pathogens. HRAPs are shallow (15–25 cm deep) open systems in

closed loop with individual or multiple channels with paddlewheel for water circu-

lation [34]. They run on hydraulic retention under varying time intervals according

to seasons, organic loading rates of 100–150 BOD/ha day and varying depths from

0.25 to 0.6 m [33]. Raceways are economical in comparison to other forms of closed

systems but are low in productivity due to poor mixing efficiency, contamination,

shading effect, and inefficient CO2.

Algae grown in HRAPs assimilate nutrients from wastewater thus helping in

recovery of nutrients. HRAP is a type of advanced pond system which includes

section like anaerobic digestion pits, algal settling, and maturation ponds in succes-

sion. Advanced pond system requires larger land area (∼50 times) than activated

sludge system, one of the most common waste processing technologies, while the

cost of construction of this system is about one-fifth of activated sludge system [33].