Aside from mineral elements and nitrogen sources, some wastewaters contain high concentrations of organic carbons for mixotrophic/heterotrophic cultivation of microalgae. Thus, some wastewaters can be used as both carbon and nitrogen sources for cultivation of microalgae. Cultivation of microalgae in wastewater for biodiesel production is highly desirable since it leads to a significant reduction in the production costs and reduction in the demand for freshwater with the con­comitant removal of various contaminants, such as phosphorus, nitrogen, heavy metals, and pathogens from the wastewater.

In addition to carbon, nitrogen, and phosphorus, microalgae also require micronutrients for growth and oil production. Micronutrients required in trace amounts include silica, calcium, magnesium, potassium, iron, manganese, sulfur, zinc, copper, zinc, nickel, lead, chromium, and cobalt (Bao et al. 2008; Ortiz Escobar and Hue 2008; Faridullah et al. 2009; Vu et al. 2009). These nutrients are usually added through the addition of commercial fertilizers, which substantially increase production costs. The concentrations of these essential micronutrients rarely limit algal growth when wastewater is used (Knud-Hansen et al. 1998). Furthermore, many wastewaters such as poultry litter, slaughter house wastes, dairy effluents, swine wastes, municipal wastewater, and effluents from anaerobic digesters are rich in organic nutrients. In addition to supplying these nutrients, the cultivation of microalgae in wastewaters is an efficient method of wastewater treatment (Ogbonna et al. 2000). Hodaifa et al. (2008) recorded 67.4 % reduction in BOD with S. obliquus cultured in diluted (25 %) industrial wastewater from olive oil extraction. Wang et al. (2009) also reported that wastewaters from different stages of treatment are good for cultivation of Chlorella sp. with efficient removal of N, P, and COD. A consortium of 15 native algal isolates removed more than 96 % nutrients from wastewater (Chinnasamy et al. 2010). However, there are variations in the composition of wastewaters and each may be suitable for culti­vation of only a few strains of microalgae for some specific purposes. Furthermore, most wastewaters are opaque, limiting light penetration, and thus are not suitable for photoautotrophic culture.

Types of wastewaters investigated for microalgae cultures include municipal, industrial, and agricultural wastewaters (Jiang et al. 2011). For example, poultry litter contains approximately 3.3 % nitrogen and 2.6 % phosphorus and cell growth promoters, such as glycine, are released from poultry manure on decomposition (Schefferle 1965). The composition of poultry manure depends on the type of feed used. For example, according to Magid et al. (1995), some common nutrients in poultry manure include (g/Kg) potassium 37.5, phosphate 25.5, and nitrogen 55.7. Nitrogen is normally in the form of uric acid, and about 66 % can be available on decomposition (Ruiz et al. 2009).

It has further been reported that various species of microalgae were cultivated in POME and biomass productivity varied from 2.9 to 8.0 mg/L/day and the oil content ranged from 21.34 to 30.83 % (Putri et al. 2011; Nwuche et al. 2014). One problem of POME as a medium for microalgae is the high COD content, dark color of tannic acid, and high impurity. This could be solved by anaerobic digestion to significantly reduce COD, TS, TSS, T-nitrogen, and orthophosphate (Habib et al.

2003) . Rubber Mill Effluent consists of latex washings and a solution containing proteins, sugars, lipids, and inorganic and organic salts. The high level of ammonium and other plant nutrients make it a good medium for algal growth (Azimatun-Nur and Hadiyanto 2013). Fermented cocoa bean mill effluent, also called cocoa-sweating effluent, contains several sugar residues and micronutrients (Syafila et al. 2010). The final lipid content of the culture with feeding of effluent from stably operated anaerobic continuous-flow stirred-tank reactor was 27 ± 1.11 % after 168-h cultivation in flasks, which was higher than the value obtained with glucose of the same COD concentration (Wen et al. 2013).