Phycoremediation by High-Rate Algal Ponds (HRAPs) - Biotechnological Applications of Microalgae - page 188

HRAP occurs directly via growth of algae and harvesting of biomass and indirectly
by ammonia-nitrate volatilization and orthophosphate precipitation via a change
in pH. Algal photosynthesis thus controls the efficiency of nitrate and phosphate
removal (Olguın, 2003). Algal photosynthesis provides oxygen for the decomposi-
tion of organic matter by aerobic heterotrophic bacteria, allowing for a reduction in
organic matter coupled with the removal of nitrogen and phosphorus due to uptake
by the algae (García et al . , 2006). The biomass produced as a result can be harvested
and used for the production of biofuels via various pathways (Park et al . , 2011b).
These systems are simple to operate when compared to conventional tech-
nologies, thus making them ideal for use by small rural communities (García
et al . , 2006). HRAPs have been successfully used in the remediation of pig-
gery effluent and also the effluent from aquaculture systems (Olguın, 2003). The
combination of wastewater treatment and biofuel production is receiving much
more interest than previously, owing to the advantageous implications of such a
combination. However, fundamental large-scale research must be undertaken in
order to optimize algal production and maintain high-quality effluent standards
(Park et al . , 2011a).
12.5.1 n utrient r eMoval
The removal of nitrogen and phosphorus from wastewater is essential in prevent-
ing ecological damage to receiving water bodies. Phosphorus is particularly dif-
ficult to remove (Pittman et al . , 2011). Chemical precipitation is currently the main
commercial process for removing phosphorus from wastewater. Biological removal
efficiencies vary from 20% to 30% for most organisms (de-Bashan et al . , 2004).
The phosphorus is then converted into activated sludge that cannot be fully recycled
and is buried in landfills or treated to render sludge fertilizer. Microalgae are effec-
tive in removing nitrogen, phosphorus, and toxic metals from wastewater, thus mak-
ing them ideal candidates for nutrient removal and recovery (Pittman et al . , 2011).
Microalgal uptake of phosphorus has been shown to be as efficient as chemical treat-
ment (Pittman et al . , 2011).
Carbon, nitrogen, phosphorus, and sulfur are essential growth requirements for
most microalgae (Chisti, 2007; Tsai et al . , 2011; Zeng et al . , 2011). These elements
are commonly found in domestic wastewater in concentrations that support microal-
gal cultivation. Minimal nutritional requirements can be estimated using the approx-
imate molecular formula of the microalgal biomass, that is, CO 0.48 H 1.83 N 0.11 P 0.01
(Chisti, 2007; Putt et al . , 2011). Nitrogen is the critical factor for the growth and lipid
content regulation of microalgae. Phosphorus, although required in smaller amounts,
must be supplied in excess as it complexes with metal ions and is thus not fully bio-
available for cell uptake (Chisti, 2007). Microalgae naturally utilize suitable nutri-
ents and energy sources from their environment, thereby optimizing the efficiency
of utilization for growth and survival. They are resilient organisms, in that a single
species may be able to undergo various types of metabolism, depending on the avail-
able nutrients for growth as well as other environmental factors (Amaro et al., 2011).
Nitrogen is utilized in the form of nitrate and ammonia, with ammonia being used
preferentially in the presence of both chemical species (Feng et al . , 2011).