In open raceway ponds, microalgal biomass concentrations are typically about 0.5 g L-1. The N content of this biomass is about 7 % and the P content 1 %. As a result, the minimal nutrient concentration in the culture medium to achieve a biomass concentration of 0.5 g L-1 should be around 5 mg P L-1 and 35 mg N L-1. In photobioreactors, microalgal biomass concentrations are higher (up to tenfold), and a higher nutrient concentration is needed in the medium to achieve the maximal biomass concentration. If nutrient concentrations in the wastewater are lower, microalgal biomass will be nutrient-limited and the biomass concentration and productivity will be lower than can be achieved under optimal conditions. If the biomass concentration in the medium is too low, this may in turn result in higher harvesting costs.

Concentrations of N and P vary considerably between different types of wastewaters. Domestic wastewater contains about 15-40 mg N L-1 (Rahman et al. 2012; Peccia et al. 2013), which is perhaps just sufficient to achieve the maximal productivity of microalgae in raceway ponds (Olguin 2012), but too low for pho­tobioreactors. If wastewater with a lower concentration of N and P is used as a source of nutrients, the retention time of the microalgae in the system can be increased relative to that of the nutrients. This can be achieved in several ways. One option is to grow microalgae on a fixed support rather than suspended in the culture medium (Hoffmann 1998; Mulbry et al. 2008; Zamalloa et al. 2013; Boelee et al. 2013; Kesaano and Sims 2014). The fixed microalgae can be unicellular or fila­mentous species and can be grown on a variety of supports. Another option, as summarized in Chap. 2, is to immobilize suspended microalgae in alginate beads or alginate mats (e. g., Mallick 2002; Ruiz-Marin et al. 2010; Eroglu et al. 2012). However, it may be difficult to separate the microalgae from the alginate and use the biomass. Also, membrane photobioreactors can be used that retain microalgae in the photobioreactors but allow a high throughput of wastewater (Bilad et al. 2014). Wastewater derived from animal manure or industrial effluents are often high- strength wastewaters and can contain up to several grams of N L-1. When nutrient concentrations are higher than the requirements for microalgal production, the microalgae will be light-limited due to self-shading before all nutrients are con­sumed (Huisman et al. 2002). Wastewater with high “N” and “P” concentration can be diluted with water to match the nutrient supply with the productivity of the microalgae, yet the use of pure water to dilute wastewater is unsustainable (Mar — cilhac et al. 2014). Concentrated wastewaters can be diluted with seawater to produce a growth medium for marine microalgae (e. g., Craggs et al. 1995; Zhang and Hu 2008; Jiang et al. 2011). Alternatively, low nutrient domestic wastewater could be used to dilute concentrated wastewater. If this is not possible, the culture medium may be repeatedly recycled until all nutrients have been consumed by the microalgae. Few studies, however, have attempted this to date.