Phycoremediation by High-Rate Algal Ponds (HRAPs) - Biotechnological Applications of Microalgae - page 190

12.5.2 F aCtors a FFeCtinG h iGh -r ate a lGae p onds
The efficiency of HRAPs depends on a variety of factors. Microalgal growth in
HRAPs is similar to the production of biomass on artificial media. CO 2 , mixing,
good light availability and penetration, and essential nutrient content, pH, and tem-
perature are among the most important factors in achieving high biomass production
and effective nutrient removal (García et al . , 2006; Pittman et al . , 2011). Biotic factors
such as synergistic bacteria, predatory zooplankton, and pathogenic bacteria may also
affect the growth of microalgae. The variables will differ, depending on the type of
wastewater and from one wastewater treatment site to another (Pittman et al., 2011).
Nutrient content (nitrogen and phosphorus) in wastewater can be significantly higher
than in conventional media. Nitrogen in wastewater is generally in the form of ammo-
nia, which can inhibit algal growth at high concentrations (Pittman et al., 2011).
Carbon is assimilated from the atmosphere and CO 2 produced by the oxidation of
organic matter. The photosynthetic growth of algae utilizes CO 2 as a carbon source
for growth while producing oxygen as a by-product, which is utilized by bacteria to
mineralize organic matter and produce CO 2 , which is consumed by algal photosyn-
thesis. This aids in the reduction of greenhouse gas emissions (Munoz and Guieysse,
2006; Ansa et al., 2011; Park et al . , 2011a). HRAPs are generally carbon limited and
must be supplemented, potentially by utilization of flue gas for the improvement of
nutrient removal efficiency (De Godos et al., 2010). The diurnal cycle affects pho-
tosynthetic activity and thereby pH and nutrient removal efficiency (García et al . ,
2006). The dissolved CO 2 concentration has a direct effect on the pH of the sys-
tem, as it is acidic in nature when dissolved in water. The cultivation pH directly
affects the bioavailability of nutrients such as ammonia and phosphate. It may also
aid in the proliferation of nitrifying bacteria (Craggs, 2005; De Godos et al . , 2010).
Both the pH and dissolved oxygen (DO) peak at midday due to the maximization of
photosynthetic efficiency and thereby the removal of CO 2 and an increase in DO of
>200% saturation (García et al . , 2006; Park et al . , 2011a). The consumption of CO 2
and carbonic acid by photosynthesis increases the pH to basic levels (>11), thereby
enhancing nutrient removal via the volatilization of ammonia and phosphorus pre-
cipitation (Craggs, 2005; Su et al . , 2012). At night, the removal efficiency decreases
and may cease due to inadequate oxygen for aerobic respiration. Furthermore, the
lower pH at night decreases nitrogen and phosphorus removal due to pH-dependant
processes (Garcia et al . , 2006; De Godos et al . , 2010). High pH may also reduce
nutrient utilization via significant inhibition of algal growth due to ammonia toxicity.
Furthermore, a pH above 8.3 increasingly inhibits the bacterial activity and thereby
the oxidation of organic matter by heterotrophic bacteria (Craggs, 2005; Ansa et al . ,
2011). The optimal pH for many freshwater algal species is 8, above or below which
productivity decreases (Kong et al., 2010). Some algae are, however, capable of
growth at pH > 10, such as Amphora sp. and Ankistrodesmus sp. (Park et al . , 2011a).
The pH stability in HRAPs is brought about by the balance of CO 2 capture from the
air, bacterial respiration, and algal CO 2 uptake (Su et al . , 2012).
The productivity of algal cultures—and thus nutrient removal—is light and tem-
perature dependent. Photosynthesis increases with an increase in light intensity until
the maximum rate is achieved at light saturation in the absence of nutrient limitation