The removal of nitrogen and phosphorus from wastewater is essential in prevent­ing ecological damage to receiving water bodies. Phosphorus is particularly dif­ficult to remove (Pittman et al., 2011). Chemical precipitation is currently the main commercial process for removing phosphorus from wastewater. Biological removal efficiencies vary from 20% to 30% for most organisms (de-Bashan et al., 2004). The phosphorus is then converted into activated sludge that cannot be fully recycled and is buried in landfills or treated to render sludge fertilizer. Microalgae are effec­tive in removing nitrogen, phosphorus, and toxic metals from wastewater, thus mak­ing them ideal candidates for nutrient removal and recovery (Pittman et al., 2011). Microalgal uptake of phosphorus has been shown to be as efficient as chemical treat­ment (Pittman et al., 2011).

Carbon, nitrogen, phosphorus, and sulfur are essential growth requirements for most microalgae (Chisti, 2007; Tsai et al., 2011; Zeng et al., 2011). These elements are commonly found in domestic wastewater in concentrations that support microal­gal cultivation. Minimal nutritional requirements can be estimated using the approx­imate molecular formula of the microalgal biomass, that is, CO048H183N0.nP0.01 (Chisti, 2007; Putt et al., 2011). Nitrogen is the critical factor for the growth and lipid content regulation of microalgae. Phosphorus, although required in smaller amounts, must be supplied in excess as it complexes with metal ions and is thus not fully bio­available for cell uptake (Chisti, 2007). Microalgae naturally utilize suitable nutri­ents and energy sources from their environment, thereby optimizing the efficiency of utilization for growth and survival. They are resilient organisms, in that a single species may be able to undergo various types of metabolism, depending on the avail­able nutrients for growth as well as other environmental factors (Amaro et al., 2011). Nitrogen is utilized in the form of nitrate and ammonia, with ammonia being used preferentially in the presence of both chemical species (Feng et al., 2011).

Phototrophic cultivation uses sunlight and CO2 as an inorganic carbon source for energy production and growth (Mata et al., 2010). Phototrophic cultivation is less prone to contamination than other types of cultivation. Heterotrophic growth occurs in the absence of light using organic carbon sources such as glucose, acetate, glycerol, fructose, sucrose, lactose, galactose, and mannose (Amaro et al., 2011). Organisms that are able to undergo mixotrophic growth have the ability to photosyn — thesize or use organic substrates as a carbon source. Mixotrophic production reduces photo-inhibition and decreases the loss of biomass due to dark-phase respiration (Brennan and Owende, 2010; Pittman et al., 2011). Organic carbon sources in waste­water allow microalgae to undergo mixotrophic growth followed by phototrophic growth. This effectively removes nutrients while improving biomass and potential lipid productivity (Feng et al., 2011).

The efficiency of nutrient removal depends on the species of algae cultivated and has been shown to be influenced positively by the cultivation of algal strains that are tolerant to certain extremes, such as extreme temperatures, quick sedimentation, or the ability to grow mixotrophically (Olguin, 2003). Choosing a strain for cultivation in HRAPs should preferentially (1) have a high growth rate, (2) have a high protein concentration when grown under nutrient-limited conditions, (3) be used for animal/ fish feed, (4) have the ability to tolerate high nutrient levels, (5) produce a value-added product, (6) be able to grow mixotrophically, and (7) be easily harvested (Sheehan et al., 1998; Olguin, 2003; Rawat et al., 2011). Chlorella vulgaris, Haematococcus pluvialis, and Arthrospira (Spirulina) platensis, among others, are examples of spe­cies that can grow under photo-autotrophic, heterotrophic, and mixotrophic condi­tions (Amaro et al., 2011).

Microalgal wastewater treatment has the potential to significantly reduce the costs of treatment when compared to conventional chemical methods; this is par­tially achieved by negation of the requirement for mechanical aeration as microalgae produce oxygen via the process of photosynthesis (Pittman et al., 2011). The simul­taneous treatment of wastewater and production of biomass reduces the cost of both processes (Brennan and Owende, 2010; Christenson and Sims, 2011). Furthermore, the production of biofuel in conjunction with wastewater treatment has been put for­ward as the most viable method for biofuel production from microalgae in the near future (Brennan and Owende, 2010).

Several studies have proven the potential for nutrient removal from synthetic waste­water by microalgal biomass production. Phosphorus removal of 98% and total ammo­nia removal has been achieved by (Martinez et al., 2000) using Scenedesmus obliquus. Boelee et al. (2011) demonstrated simultaneous removal of nitrate and phosphate to

2.2 mg L-1 and 0.15 mg L-1, respectively, using microalgal biofilms. Su et al. (2012) reported phosphorus removal efficiency of algae to be 89%. Certain photosynthetic bacteria and green microalgae such as Rhodobacter sphaeroides and Chlorella soro — kiniana can, under heterotrophic conditions, remove high concentrations of organic acids (>1,000 mg L-1) and ammonia (400 mg L-1) (Olguin, 2003). The bacterial removal of substances such as polycyclic aromatic hydrocarbons, organic solvents, and phenolic compounds may be assisted by the use of microalgae that produce the oxygen required for bacterial action. Heavy-metal biosorption may be achieved by microalgae grown under phototrophic conditions (Brennan and Owende, 2010).