[Retracted] Microalgae as a Renewable Source of Energy: A Niche Opportunity

Table 2

Comparison of commercial and wastewater treatment HRAP.

Factors	Commercial production HRAP	Wastewater treatment HRAP	Reference
Capital, operation, and maintenance cost	Requires heavy investment	Covered by wastewater treatment	[39, 40]
Land use	High	Covered by wastewater treatment	[40, 41]
Commercial availability	Already established for high bioactive compounds	Already established at a small scale for wastewater treatment.	[40, 42]
Most costly parameters	Water, fertilizer, harvesting, and mixing	Covered by wastewater treatment	[41, 43]
Limiting factors for algal growth	Light, temperature, nutrients, CO2 (externally provided)	Light, temperature, nutrients (internally provided by wastewater treatment), CO2 (partially provided by bacteria by the oxidation of organic compounds and by the exhaust gas available in wastewater treatment facilities).	[40, 41, 43]
Algal productivity	High productivity, as high as 30 gm/m^2/d	High productivity is not the main driver, the emphasis is more on waste treatment and the algal biomass is the byproduct	[43–45]
Harvesting	Due to small sized cells <20 µm	Bioflocculation of algal cells achieved by aggregation of algal cells with wastewater bacteria.	[43, 45]
Water footprint	Requires significant amount of water and net water loss via evaporation	Not applicable	[44]
Risk of contamination	Comparatively lower	High	[46]
Algal species	Possible to cultivate single species microalgal cells. However, so far only extremophiles which can survive under extreme conditions are used for raceway algal cultivation	Maybe possible by selective biomass recirculation	[42, 46]