2 Department of Life Sciences, Faculty of Sciences and Technology, University of Coimbra, 3000-456 Coimbra, Portugal 2 National Higher School in Environment, Georesources and Sustainable Development Engineering (ENSEGID), 14 allee [PERSON], 33600 Pessac, France; [EMAIL_ADDRESS]  
[PERSON]  
1 Marine and Environmental Sciences Centre (MARE-UC), Incubadora de Empresas da Fagueira da Foz, Parque Industrial e Empresarial da Figueira da Foz (Laboratorio MARFOFO), University of Coimbra, Rua das Aicolas Lote 40A, 3090-380 Figueira da Foz, Portugal, [EMAIL_ADDRESS] (D.P. ); [EMAIL_ADDRESS] (A.C.S.R. ); [EMAIL_ADDRESS] (A.B. ); [EMAIL_ADDRESS] (T.V.) 2 Department of Life Sciences, Faculty of Sciences and Technology, University of Coimbra, 3000-456 Coimbra, Portugal 2 National Higher School in Environment, Georesources and Sustainable Development Engineering (ENSEGID), 14 allee [PERSON], 33600 Pessac, France; ana. of 80% (for total phosphorus) and 70-80% (for total nitrogen) reduction in relation to the influent load. Moreover, the Directive 98/15/EC [9] aims to protect the aquatic environment, through a set of measurements leading to the significant decrease of the organic and inorganic load discharged into the water bodies. In WWTP, the effluent must complete four processes, namely the preliminary (harrowing and degreasing) [10], primary (biological treatment with nitrification/denitrification bacteria) [11], secondary (settling and sedimentation) [12] and tertiary (pathogens elimination) treatments [13]. Furthermore, the originated sludge is treated with heating and gravitational thickening, resulting in a liquid fraction, corresponding to the sludge run-off or centrate, and a solid fraction, the sludge, which is often incinerated or can be valorized for agricultural purposes [14]. Nevertheless, the treated effluents and sludge run-off still often present considerable high nutrient loads. The need to reduce costs associated to microalgal biomass production led to a set of studies aiming at the evaluation of wastewaters application for microalgae cultivation. In fact, the coupling of both processes could be advantageous, since microalgae are able to reduce the organic and inorganic load of wastewaters and sequester atmospheric carbon dioxide [15; 16; 17; 18; 19]. Thus, microalgae provide a sustainable biological treatment for wastewaters and the microalgal biomass can be further valorized for energetic or agricultural purposes [20; 21; 22]. Microalgae cultivation is, however, challenging since growth can be affected by several biotic factors--light intensity [23], temperature, pH [21], aeration rate [24], nutrients and carbon dioxide availability [25]--and abiotic factors, such as the presence of pathogenic or predatory organisms [26; 27]. Moreover, there are several systems (open or closed) [28; 29; 30], and strategies (batch, continuous or semi-continuous) [21; 31; 32; 33] that can be employed in order to optimize microalgal culture conditions. Closed systems or photobioreactors are distinguished by the absence of exchange between the community of microalgae and the surrounding environment, offering various designs, such as tubular, plastic, flat-plate and bubble-column bags [34]. Some of the difficulties faced by open systems, such as low efficiency, nutrient evaporation and contamination, are solved by closed-culture systems [30]. Nevertheless, these systems present a high demand for electricity and their implementation is costly [34]. In terms of cultivation strategies, the batch strategy is a low-cost strategy because it does not need much control. This strategy is distinguished by no renewal of the culture medium, which means that the culture of microalgae grows until it reaches the decline period. Nevertheless, in practice, the microalgal culture may crash for several reasons, such as nutrient or oxygen depletion, self-shading, pH variation or harmful compounds concentration [21; 31].