Some researchers have suggested that the production of algal biodiesel would produce greenhouse emissions, increase the water footprint, and require more energy than the production of biofuels from corn and canola feedstocks (Park et al., 2011a). These could, however, offset the use of wastewater as a nutrient source. Other researchers concluded that algal biodiesel production would be energetically viable but feedstock inputs would account for almost half of the energy produced, thus making the process not economically viable. The use of wastewater will offset this, giving a net increase in the net energy ratio (NER) (Sturm and Lamer, 2011). Conversion of algal biomass to energy via a multistage biorefinery process, including lipid extraction for biodiesel, utilization of residual biomass for combustion, and anaerobic digestion of biosolids, has the poten­tial to provide a significant amount of energy in the region of 4,610 kW-h d-1 to 48,000 kW-h d-1 (Sheehan et al., 1998; Sturm and Lamer, 2011). The energy requirement of conventional wastewater treatment is significantly higher than that of high-rate algal ponds. The Advanced Integrated Wastewater Ponds System (AIWPS), designed by Oswald and Green, LLC, requires up to 91% less energy (kW-h kg-1 BOD removed) than conventional systems (Olguin, 2003; Rawat et al., 2011).

Microalgal oxygen release provides the oxygen required for the proliferation of heterotrophic bacteria, thus negating the requirement for mechanical aera­tion as in conventional wastewater treatment. Conventional wastewater treatment costs approximately four times more than the use of HRAPs (Rawat et al., 2011). The AIWPS consists of advanced facultative ponds with anaerobic digestion pits, HRAPs, algal settling ponds, and maturation ponds in series (Craggs, 2005). This system requires 50 times more land area than conventional wastewater treatment viz. activated sludge, not taking into account the land area required for waste activated sludge disposal. Capital costs and operational costs of the AIWPS are half and less than one-fifth that of activated sludge, respectively (Park et al., 2011a). The supply of nutrients, water, and CO2 contributes from 10% to 30% of the total cost of commercial algal production (Benemann, 2008). Much of the cost of wastewater HRAPs is covered by the cost of wastewater treatment (Table 12.2). The costs of algal production and harvesting using wastewater treatment HRAPs have less environmental impact in terms of water footprint, energy, and tertilizer use. Recycling of growth media is used as a method of minimizing costs. Recycling can, however, cause a reduction in algal productiv­ity due to the increase in contamination and/or the accumulation of inhibitory metabolites (Park et al., 2011a).

12.2 CONCLUSION

Researchers are in general agreement that the use of wastewater treatment HRAPs is the only economical method currently available for algal production of biofuels. There are significant benefits to the use of wastewater HRAPs for the effective, low — cost treatment of wastewaters and algal biomass production for biofuels generation. There is, however, still a great need to optimize conditions for algal growth and nutrient removal under prevailing climatic conditions. Large-scale lipid optimiza­tion and harvesting of algal biomass still remains a challenge, and improvements in this area will subsequently decrease the overall cost of algae production and reme­diation of wastewater.

ACKNOWLEDGMENTS

The authors hereby acknowledge the National Research Foundation (South Africa) for financial contribution.

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