Wastewater has a relatively low nutrient content, with usually less than 1 % N and less than 0.5 % P. Because of this low nutrient content, it is not cost-effective to transport wastewater over long distances to microalgal farms. Therefore, microalgal farms should be situated as close as possible to the wastewater sources. It is also costly to store wastewater during periods when microalgae production is low, and the production of microalgal biomass should ideally more or less match the gen­eration of wastewater. Such spatial and temporal mismatches between availability of wastewater and microalgal productivity may limit the potential to convert wastewater nutrients into microalgal biomass. Large cities generate enormous volumes of wastewater that could be used for microalgae biomass production. These cities, however, often lack sufficiently large areas of low-cost land nearby that can be used for microalgae production (Fortier and Sturm 2012). In addition, many of the world’s largest cities are situated in temperate climates where micro­algal productivity is low in winter and freezing temperatures may even require complete shutdown of microalgal farms for a few months (Chiu and Wu 2013). At high latitudes in winter, it may be impractical to use wastewater for microalgae production (Van Harmelen and Oonk 2006), and at low latitudes, high temperatures may also limit productivity during the warmest months.

Availability of land is probably less problematic when animal manure is used for microalgae production than when domestic wastewater is used. Livestock farms are generally situated away from cities. In general, agricultural land is available that can be converted in microalgae cultivation ponds. In many countries, however, there is an ongoing debate whether microalgae cultivation is allowed on agricultural land or not (Trentacoste et al. 2014). Due to economies of scale, we can assume that the minimum size of a microalgae farm would be about 10 ha (Lundquist et al. 2010). A 10-ha farm that produces 300 ton dry microalgal biomass ha-1 year-1 consumes 27 ton N year 1. This corresponds to the N output of a municipality of 9000 inhabitants, or a farm with at least 7000 pigs or 400 cattle. Wastewater from smaller farms or villages may be collected and transported to a microalgae farm, but this is only possible over relatively small distances due to the high cost for transporting wastewater. Therefore, smaller and isolated sources of wastewater will unlikely be practical for microalgae production.