As outlined above, using wastewater nutrients rather than synthetic nutrients improves the sustainability of microalgae biomass production. On the other hand, using microalgae rather than conventional wastewater treatment technology may also result in a more sustainable method for treating wastewaters. In conventional wastewater treatment, N and P are removed from the wastewater without being reused: N is removed primarily by denitrification and is lost to the atmosphere as N2, while P is removed from wastewater by precipitation with metal salts and disposed of in landfills. When wastewater is treated using microalgae, N and P are not only removed from the wastewater, but can also be reused to produce extra biomass. As N and P are extremely valuable resources to our society, initiatives are increasingly being taken to not only remove but also reuse N and P from wastewater (Dawson and Hilton 2011; Cordell et al. 2011; Elser 2012). Combining wastewater treatment with microalgae biomass production can achieve parts of this goal.

Eutrophication of lakes, rivers, wetlands, and coastal waters is a major envi­ronmental issue. To reduce eutrophication, regulations for discharge of effluents from wastewater treatment plants are becoming stricter. In the EU, for instance, discharge limits for wastewater have recently been decreased to 1 mg L-1 for P and to 10 mg L-1 for N (Oliveira and Machado 2013). Conventional technologies have difficulties in removing N and P from wastewater down to these levels. Residual concentrations of N and P in effluent from conventional wastewater treatment plants

are often quite high, high enough to cause eutrophication in receiving natural ecosystems. Microalgae have half-saturation constants for uptake of N and P that are well below the strictest limits. Therefore, the use of microalgae to remove nutrients from wastewater will certainly lead to lower N and P concentrations in the effluent, and less eutrophication of aquatic ecosystems.

Wastewater contains large amounts of organic matter, and it is important that it is oxidized before the effluent is discharged into the environment. In modern wastewater treatment plants, electromechanical air blowers supply oxygen that allows bacterial oxidation. This process consumes a lot of energy, and it is the major contributor to the capital and operational costs of modern wastewater treatment plants. If a proper cultivation design is developed, microalgae can produce sufficient oxygen for bac­terial oxidation of organic matter. Microalgae-based systems are equally effective as electrical air blowers for oxidation of organic matter, but have a much lower cost (Owen 1982; Craggs et al. 2013). The CO2 that is produced during degradation of organic matter can also be used as a carbon source in microalgal photosynthesis. Some microalgae are mixotrophic and can contribute to the degradation of organic matter from wastewater. This mixotrophic growth based on organic matter present in wastewater can even boost microalgal biomass production (Bhatnagar et al. 2011).

It is clear that combining microalgae production with wastewater treatment not only improves the sustainability of microalgae production but also that of waste­water treatment (Sturm and Lamer 2011). Beal et al. (2012) and Menger-Krug et al. (2012) showed that combined wastewater treatment and microalgae production has a much better energy balance than both processes operating separately. Combining microalgae biomass production with wastewater treatment would also make mic­roalgae biofuel production economically more attractive, as additional income can be generated from the treatment of wastewater (Lundquist et al. 2010; Pittman et al.

2011) . Combining microalgae production with wastewater treatment, however, is also a challenge because both processes need to be optimized simultaneously. On the one hand, the productivity and biochemical composition (e. g., lipid content) of the microalgae should be optimal. On the other hand, the quality of the wastewater effluent should comply with national water treatment standards (e. g., biological oxygen demand removal, N and P removal).

5.13 Conclusions

The high demand of microalgae for N and P poses an important environmental burden on microalgal biofuels. This environmental impact can be avoided by replacing synthetic fertilizer with N and P from wastewater. It is feasible to use wastewater as a source of N and P because microalgae have been used for many years in wastewater treatment (in facultative ponds or HRAPs). The resource base provided by wastewater nutrients is theoretically large enough to produce a similar amount of biomass as the global production of rice or wheat, yet it is not large enough to produce enough microalgal biomass to replace fossil fuels. Climatic and geographical factors limit the potential to use wastewater for microalgae produc­tion. Using wastewater as a source of nutrients rather than synthetic fertilizer poses several challenges. The N and P demand of microalgae should be matched with the variable supply of these nutrients by wastewater. Wastewater contains many types of contaminants that can interfere with the production of microalgal biomass and/or with the valorization of certain microalgal biomass fractions. The high pH that is typical of microalgal cultures may result in nutrient losses (precipitation of P and volatilization of N). Further research is needed to overcome some of these chal­lenges. Combining microalgae production with wastewater treatment not only improves the sustainability of microalgal biofuels but also increases efficiency of wastewater treatment because microalgae-based wastewater treatment has a lower energy demand, can result in a better effluent quality, and is a way to recycle valuable nutrients from the wastewater.