Biomedical Engineering Reference
In-Depth Information
wastewater (Gomez Villa et al . , 2005). The pollutants are recovered from the system
by harvesting biomass (Adey et  al . , 1996). Aside from microalgal biomass build-
up, luxury reserved materials in the form of pigments, protein, antioxidants, amino
acids, and other bioactive compounds make them ideal for stripping nutrients. High-
rate wastewater treatment of hazardous or organic pollutants has been carried out
by microalgae with special attributes. The most widely studied microalgal strains
are Chlorella , Scenedsmus, and Ankistrodesmus species, in which various industry
effluents were used, such as paper industry wastewaters, olive oil production waste-
water, and mill wastewaters (Ghasemi et  al . , 2011; Rawat et  al . , 2011). Microalgal
strain selection plays an important role in HRAP wastewater treatment. Microalgal
collections house only a few thousand different microalgal strains that can efficiently
support wastewater treatment and biomass production for value-added by-products
and meet near-future demands for alternate biofuels. Therefore, we need to concen-
trate on effective microalgal strains in combination with recent advances in genetic
engineering and material science to fix the problem.
12.5 HIGH-RATE ALGAL PONDS (HRAPS)
The three general types of maturation ponds employed in wastewater treatment are
facultative ponds, anaerobic ponds, and the most common, waste stabilization ponds.
Aerobic ponds, also known as high-rate ponds, are shallow and completely oxygen-
ated (Oswald, 1978). High-rate algal ponds (HRAPs) were developed beginning in the
1950s as an alternative to unmixed oxidation ponds for BOD, suspended solids, and
pathogen removal (Rawat et al . , 2011). They constitute a low-cost, low-maintenance
technology for the remediation of various types of effluents (De Godos et al . , 2010).
HRAPs exhibit better performance when compared to anaerobic, aerobic, and facul-
tative ponds using the same influent. The co-habitation of photosynthetic algae and
heterotrophic bacteria is referred to as HRAP symbiosis. HRAPs have been used for
the treatment of a variety of wastewaters, including domestic wastewater, piggery
and animal wastewaters, agricultural runoff, and mine drainage and zinc refinery
wastewater (Rawat et al . , 2011). The utilization of microalgae for the assimilation of
nitrogen and phosphorus at low concentrations presents a sustainable alternative to
the use of existing treatment systems, as the nitrogen and phosphorus can be recov-
ered from the algal biomass for reuse (Boelee et al . , 2011). HRAPs are designed to
promote algal growth, and the technology generally consists of mechanically mixed
shallow raceway ponds (Olguın, 2003; García et al . , 2006). A large paddlewheel vane
pump is used to create a channel velocity sufficient for gentle mixing. The ponds are
generally 2 to 3 m wide, 0.1 to 0.4 m deep, and range from 1,000 to 5,000 m 2 in area,
depending on the scale of application (García et  al . , 2006; De Godos et  al . , 2009;
Rawat et al . , 2011). The hydraulic retention time of such systems is generally in the
range of 4 to 10 days, depending on climatic conditions. Continuous mixing is pro-
vided to keep the cells in suspension and reduce the shading effect, thereby exposing
the algae to light periodically, even in denser cultures. The most common design that
has proven successful on a large scale is the single-loop paddlewheel mixed. Due to
the energy cost dependence on velocity, most ponds have been operated at velocities
from 10 to 30 cm s −1 (Olguın, 2003; Rawat   et al . , 2011). The mode of action of the
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