Many industries and human activities generate wastewater, and as a consequence, there are many different types of wastewaters, each with a different chemical composition and volumetric production over time. Table 5.1 gives an overview of different types of wastewaters and their content of N and P. The two major sources of wastewater are domestic wastewater and wastewater derived from animal manure. Each person supplies about 3 kg N per year through domestic wastewater, which translates to about 21 million ton of N per year for a global population of 7 billion (similar to the estimate of Smil 2002; Van Harmelen and Oonk 2006). The major livestock animals that produce manure are pigs, cattle, and chickens.

Chickens produce a relatively dry type of manure that is suitable for application as fertilizer on agricultural land and we do not consider the N supply from chicken manure for algae production. Pigs produce about 16 kg N animal 1 year 1, cattle 35 kg N animal-1 year-1, and dairy cattle 75 kg animal-1 year-1. With global population sizes of 1 billion pigs, 0.9 billion cattle and 0.25 billion dairy cattle, wastewater from pig and cattle manure can theoretically provide 65 million tons of N, or about 3 times more than domestic wastewater (similar to the estimate of Van Harmelen and Oonk 2006). If we assume a “N” content of microalgal biomass of 7 %, the total human, pig, and cattle wastewater N nutrient is enough to produce about 778 million ton dry microalgal biomass per year. This is in the same order of magnitude as the global production of wheat or of corn. Although this is a lot of biomass, it can produce only about 233 million ton of oil (assuming a 30 % lipid content in microalgae). This corresponds to 1800 million barrels of oil, or only about 5 % of the global oil consumption. Thus, wastewater alone cannot supply sufficient nutrients for microalgal biomass to make a large contribution to the world’s energy demand. This conclusion is in line with Peccia et al. (2013) or Chisti (2013), who estimated that nutrients from domestic wastewater of a typical large city can only produce enough microalgal biofuel to supply 3 % of the fuel demand of that city. If microalgal biofuels are ever to make a larger contribution to the global fuel demand, it will be essential to recycle the nutrients during conversion of microalgal biomass to biofuels (Venteris et al. 2014). Although animal manure is a potentially significant source of nutrients for microalgae production, it should be noted that a significant proportion of animal manure is already used as a fertilizer in conventional agricultural production (Bouwman and Van Der Hoek 1997). Because synthetic fertilizer prices are increasing, the value of nutrients in animal manure also increases. Therefore, the use of animal manure as fertilizer in conventional agriculture is likely to increase in the future (Shilton et al. 2012), and microalgae and conventional agriculture may compete for animal manure nutrients. However, the main limitation of the use of raw animal manure in conventional agriculture is the high transport cost resulting from the high water content of animal manure. In areas where livestock numbers are high and agricultural crop production is nutrient — limited, it will unlikely be economically feasible to transport animal manure to the field, and microalgae may become a more attractive option to process large volumes of manure on a relatively small land area.

In addition to manure, there are many other sources of wastewater that could be used for microalgae biomass production, such as wastewater from olive mills, wineries, breweries, vegetable processing, tanneries, or the paper industry (Cai et al.

2013) . Some emerging technologies also generate a lot of wastewater. The use of anaerobic digestion to convert organic waste streams into methane is growing worldwide and generates a nutrient-rich effluent that could be processed with microalgae (Uggetti et al. 2014). Aquaculture is also increasing worldwide and generates a similar nutrient-rich wastewater that may be suitable for treatment using microalgae (e. g., Van Den Hende et al. 2014). The volumes that are produced by these industries are relatively small compared to the volumes of domestic and animal manure wastewater. Nevertheless, microalgae may be a solution to treat some of these wastewaters as conventional water treatment technologies may be too expensive or ineffective.