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 oxygen­ated (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 facul­tative 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 recov­ered 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 (Olguin, 2003; Garcia 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 m2 in area, depending on the scale of application (Garcia 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 pro­vided 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 (Olguin, 2003; Rawat et al., 2011). The mode of action of the

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 (Olguin, 2003). Algal photosynthesis provides oxygen for the decomposi­tion 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 (Garcia 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 tech­nologies, thus making them ideal for use by small rural communities (Garcia et al., 2006). HRAPs have been successfully used in the remediation of pig­gery effluent and also the effluent from aquaculture systems (Olguin, 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).