Biomedical Engineering Reference
In-Depth Information
(Park et al . , 2011a). Damage to the light receptors (photo-inhibition) occurs beyond
the point of light saturation, thereby reducing productivity (Richmond, 2004). The
potential for photo-inhibition to occur is more prevalent during the summer months,
resulting in photosynthesis ceasing at midday (Olguin, 2003). With an increase in
culture density, there is an increase in the shading effect. An algal concentration of
300 g TSS m −3 will absorb all the available light in the top 15 cm of the pond. Mixing
is thus essential in reducing this effect (Ansa et al . , 2011; Park et al . , 2011a). Algal
productivity increases with an increase in temperature. For most species of microal-
gae under optimal culture conditions, optimal temperatures vary between 28°C and
35°C. The optimal temperature varies with nutrient and light limitation. An increase
in temperature above the optimal level results in photorespiration, which reduces the
overall productivity (Sheehan et al., 1998). Sudden changes in temperature can result
in a substantial decline in algal growth. Temperature also affects the pH, oxygen,
and CO 2 solubility, as well as the ionic equilibrium (Park et al . , 2011a).
HRAPs are susceptible to contamination by native algae and grazing by zooplank-
ton and other algal pathogens. Attempts to grow algae as monocultures in HRAPs have
failed due to said contamination (Sheehan et al . , 1998; Park et al . , 2011a). Protozoa and
rotifers have the ability to reduce algal concentrations to very low levels in a period
of just a few days (Benemann, 2008). Daphnia has the ability to reduce chlorophyll
by 99% within a few days. Fungal parasites and viral infections have the ability to
induce algal cell structure changes, and changes in diversity and succession, thereby
reducing algal populations significantly (Park et al . , 2011a; Rawat et al . , 2011). Control
of grazers and parasites may be achieved by physical methods such as filtration, low
DO concentration and high organic loading rates, and chemical treatments such as
the application of chemicals that mimic invertebrate hormones, increase the pH, and
increase the free ammonia concentration. The most practical method of zooplankton
control is the adjustment of the pH to 11, as many zooplanktons have the ability to
tolerate low DO levels for extended periods of time. The toxic effects of high pH are
augmented by the increase in free ammonia brought about by volatilization of ammo-
nia at high pH. The effects of inhibitory substances on parasitic fungi require elucida-
tion, and no general treatments for fungal control currently exist (Park et al . , 2011a).
12.5.3 e FFiCienCy oF w astewater t reatMent and a lGal G rowth
The assimilation of nitrogen and phosphorus into algal and bacterial biomass is
seen as advantageous due to the recycling potential of the nutrients via biomass
treatment. Unicellular microalgae are found to be the most efficient and most pre-
dominant in wastewater treatment ponds (Pittman et al . , 2011). The use of combined
algae-bacteria cultures increases the nitrogen accumulation efficiency; for exam-
ple, in the treatment of acetonitrile, 53% ammonia was assimilated into biomass
as compared to only 26% in a bacterial system under the same conditions. Under
optimal conditions, 100% removal can be achieved (Su et al . , 2011). The increased
removal efficiency of nutrients may be attributed to the algal requirement of high
amounts of nitrogen and phosphorus for the production of proteins, nucleic acids,
and phospholipids, which account for 45% to 60% of the algal dry weight (Munoz
and Guieysse, 2006). Su et al . (2011) demonstrated COD, ammonia, and phosphate
Search WWH ::




Custom Search