Koenraad Muylaert, Annelies Beuckels, Orily Depraetere, Imogen Foubert, Giorgos Markou and Dries Vandamme

Abstract Production of microalgal biomass requires large amounts of nitrogen (N) and phosphorus (P). The sustainability and economic viability of microalgae pro­duction could be significantly improved if N and P are not supplied by synthetic fertilizers but with wastewater. Microalgae already play an important role in wastewater treatment, yet several challenges remain to optimally convert waste­water nutrients into microalgal biomass. This book chapter aims to give an over­view of the potential of using wastewater for microalgae production, as well some challenges that should be taken into account. We also review the benefits of combining microalgal biomass production with wastewater treatment.

5.1 Introduction

Microalgae have a high areal productivity, do not require fertile land, and are seen as a promising new source of biomass that could complement production by conventional agricultural crops. Microalgae have attracted a lot of interest in recent [1] [2]

years as a novel feedstock for biofuels (Schenk et al. 2008). But microalgae also hold a lot of potential for production of food (Draaisma et al. 2013), animal feed (Benemann 2013), or feedstock for the chemical industry (Wijffels et al. 2010). When compared to conventional agricultural crops, microalgae have a high content of proteins and lipids, and a low content of structural carbohydrates such as cel­lulose (Lam and Lee 2011). This is an attractive property of microalgae, because it implies that most of the biomass can be valorized. On the contrary, with conven­tional crops, only a small fraction of the biomass is used (e. g., seeds or tubers) and a large proportion of the biomass is left on the field (i. e., the fraction that contains mostly cellulose and lignin). Because of the low content of structural carbohydrates such as lignin or cellulose, microalgal biomass has a high content of nitrogen (N) and phosphorus (P): about 10 % N and 1 % P per unit dry weight. This is almost three times higher than the N and P content of terrestrial plants (Elser et al. 2000). Because of the high content of N and P in microalgal biomass, production of microalgae requires vast quantities of inorganic fertilizer, much more than the production of terrestrial crops (Sialve et al. 2009). This high fertilizer demand is a challenge to the sustainability of microalgae biomass production, and several life cycle analyses studies have shown that the energy required for synthetic fertilizer production contributes significantly the total energy demand for microalgal biofuels (Lardon et al. 2009; Clarens et al. 2010; Benemann et al. 2012). Production of N fertilizers through the Haber-Bosch process is highly energy-intensive and is reliant on fossil fuels (Smil 2002; Pfromm et al. 2011). Extraction and processing of mineral phosphates for production of P fertilizer is also energy-intensive (Johnson et al. 2013). Moreover, mineral phosphates reserves are limited and are rapidly being depleted (Cordell et al. 2011). If microalgae are to be produced on a large scale, e. g., large enough to contribute significantly to fuel demand, consumption of synthetic fertilizers is expected to increase strongly above current levels (Venteris et al. 2014). Microalgae are often promoted as a biomass source that does not compete with agricultural biomass production, and thus avoids the food versus fuel discussion. However, as both microalgae and agricultural crops require mineral fertilizers, microalgae may indirectly compete with agricultural crops and thus indirectly impact food production through increases in fertilizer prices (Pate et al.

2011) . Many studies and opinion papers have in recent years suggested to use wastewater rather than synthetic fertilizers as a source of nutrients for microalgae production and showed that this could significantly improve the sustainability and economic feasibility of microalgae production (Lundquist et al. 2010; Clarens et al. 2010; Christenson and Sims 2011; Pittman et al. 2011; Park et al. 2011b; Olguin 2012; Prajapati et al. 2013). Because microalgae are already being used for wastewater treatment, replacing synthetic fertilizer with nutrients from wastewater is feasible. The goal of this book chapter is to discuss both the potential as well as limitations of using wastewater as a nutrient source for microalgae production.