Extensive requirement of fresh water is the basic challenge to produce algal bio­mass. For instance, approximately 1.5 L of water per kilogram of biofuel produced. In CPBs, water use may be much larger due to losses either in the form of evapora­tion in open cultivation systems or for water usage for cooling (Wijffels and Barbosa 2010). In open systems, the annual water consumption in RPs for microalgae bio­mass production is in the range of 11-13 million L ha-1 (Chinnasamy et al. 2010). This highpoints the significance of reusing waste water (sourced from industrial, municipal effluents) that enables nutrient recycling, subsequently lowering the cost of production (Santiago et al. 2013).

18.3.1 Microalgae Based Bioremediation

Microalgae based bioremediation have been widely studied to remove pollutants (bio-extraction) from water in the last four decades (Ryther et al. 1972; Kuyucak and Volesky 1988; Romero-Gonzalez et al. 2001; Fu and Wang 2011). Several studies have been conducted to present this proof-of-concept in this scenario (Table 18.3). Three microalgal species were cultivated in heavy metal contami­nated water, either nutrient addition or without nutrients. All species accumulated heavy metals concentration as high as up to 8 % of their dry mass. Interestingly, growth rates were also increased by two folds, when cultured in contaminated water as compared to fresh water (Saunders et al. 2012). In another study, two algal species, including Scenedesmus sp. and Chlorella sp. were cultivated favor­ably on anaerobic sludge centrate representing the higher nutrient uptake (phos­phorous and ammonia) and average growth rate obtained was 3.3 ± 1.5 g dry biomass m-2 day-1 in this case (Dalrymple et al. 2013). Algae grown on anaerobic sludge centrate showed growth productivity rate of 12.8 g dry biomass m-2 day-1 (Zhou et al. 2012b) . Similarly, growth rate of 3 g dry biomass m-2 day-1 for Chlorella sp. grown on wastewater was reported by Woertz et al. (2009). In addi­tion, an unusually higher growth rate (13 g of dry biomass m-2 day-1) was reported in a batch culture study. It was shown that at the end of 14-day batch culture 94 % ammonia, 89 % total nitrogen, and 81 % total phosphorous were removed by algal species from municipal waste water (Li et al. 2011a).

Although Chlorella sp. can produce lipid contents up to 30 % yet lipids contents decrease by two folds when grown in high strength nitrogen media (usually waste water contains high nitrogen). It is believed that lipid production is considerably reduced under such situations because of low C:N ratio. In gen­eral, high lipid contents are achieved when the microalgae are “starved” of nitrogen (Illman et al. 2000; Chisti 2007). So, nitrogen is the key nutrient to be considered while we are interested to enhance the lipid productivity of our test microalgae.