This is because wastewater contains high concentration of nitrate
and phosphate that promote the growth of microalgae and at the
same time, microalgae can act as a purifier to further remove the
nutrients from wastewater before discharging to water sources.
Nevertheless, the key challenges of using wastewater are susceptible
contamination and inconsistency of nutrients compositions;
these factors will directly retard the growth of microalgae. On
the other hand, using organic fertilizer as a nutrient source offers
an alternative and convenient way to minimize the contamination
level in water. Organic fertilizer can be obtained through a series of
composting and refining processes from waste materials such as
manure, sewage sludge, biomass and food [22]. Although CO2 is released
during composting process, however, the amount is lower
compared to the production of inorganic fertilizer. It was estimated
that 4–82 kg of CO2 and 4–67 kg of CO2 could be saved by composting
one tonne of food waste and garden waste, respectively [23]. In
addition, the emitted CO2 from composting process can be recycled
as an alternative carbon source to cultivate microalgae. Normally,
organic fertilizers available in the market are made in dry pellet
form for easy transportation and storage. Hence, recycle and reuse
the waste materials to cultivate microalgae for biodiesel production
appears as a greener solution to drive the industry towards
sustainable growth.