Microalgae play an important role in many wastewater treatment facilities around the world. In developing economies in tropical and subtropical countries, waste­water is often treated using facultative ponds or oxidation ponds (Duncan 2004; Rahman et al. 2012). These consist of relatively deep and non-mixed ponds that are spontaneously colonized by microalgae. In these ponds, microalgae serve mainly to supply oxygen for the aerobic oxidation of organic matter present in the wastewater. Because these ponds are relatively deep and are poorly mixed, microalgal pro­ductivity is relatively low; only about 10 ton of dry biomass ha-1 year-1. The microalgal biomass is not harvested at the end of the wastewater treatment process, and either settles to the bottom of the pond or is washed out of the ponds. Because the microalgal biomass is not harvested, removal of nutrients from the wastewater by the microalgae is inefficient.

At the end of the 1950s, high-rate algal ponds (HRAPs) were proposed as an alternative to facultative ponds (Oswald and Golueke 1960). HRAPs are raceway — type ponds in which the water is mixed by a paddle wheel. Compared to facultative ponds, HRAPs are much shallower and better mixed and, as a result, have higher microalgal productivity, about 30 ton dry biomass ha-1 year-1. The productivity of HRAP’s can be further increased with CO2 addition (Craggs et al. 2013). Because microalgal productivity is higher, a larger volume of wastewater can be treated on the same land area when compared to facultative ponds. Akin to facultative ponds, microalgae in HRAPs supply oxygen for the aerobic oxidation of organic matter, and if the microalgal biomass is harvested at the end of the treatment process, the microalgae remove nutrients from the wastewater. Harvesting of the microalgal biomass, however, is costly and many HRAPs today do not harvest the biomass. Although HRAPs are used in wastewater treatment plants around the world, the technology is much less widespread than oxidation ponds or conventional elec­tromechanical wastewater treatment systems (Craggs et al. 2013).