## 1 Introduction  
Paper and pulp industries require large amounts of water during their manufacturing stages. For example, the production of 1 kg of paper requires 10 to 50 L of water [1]. At the same time, large amounts of effluents (about 2000 m\({}^{3}\) d\({}^{-1}\)) are generated, presenting as main features [1,2]: (i) high chemical oxygen demand (COD, 1000-13,000 mg\({}_{\text{O2}}\) L\({}^{-1}\)); (ii) high total suspended solids contents; (iii) non-biodegradable organic materials; (iv) adsorbable organic halogens (AOX): (v) color; (vi) phenolic compounds; (vii) high total phosphorus contents; and (viii) limiting nitrogen concentrations. Due to the large volumes involved and respective compositions, discharge of these effluents without any treatment can cause several environmental problems [1,3]: (i) colored effluents can affect aesthetics,water transparency and gas solubility in water bodies; (ii) increase in the concentration of toxic compounds, which can affect aquatic flora and fauna; and (iii) eutrophication with consequent decrease of dissolved oxygen concentration and pH oscillations, which can negatively impact aquatic ecosystems. Therefore, treatment of these effluents is necessary before their discharge. Among the contaminants present in these effluents, phosphorus is of particular concern, as it subsists in the effluents after the secondary treatment step and is one of the main contributors to the eutrophication phenomenon [4]. Currently applied methods to reduce phosphorus concentration in these effluents include physicochemical methods, such as precipitation using aluminum and iron salts. However, these techniques tend to be costly and to produce large amounts of sludge contaminated with the referred chemical compounds, requiring further treatment [5; 6]. Therefore, microalgal cultures have appeared as a feasible alternative to conventional physicochemical methods. These microorganisms have shown their ability to effectively remove color, nutrients, such as nitrogen and phosphorus, trace metals and other compounds from the culture medium [7; 8]. Microalgae are fast-growing photosynthetic microorganisms that have gained much attention in recent decades, due to their high potential in a wide variety of applications. During photosynthesis, microalgae uptake CO\({}_{2}\) from the atmosphere or flue gas emissions, contributing to the reduction of the atmospheric concentration of this greenhouse gas [9]. These microorganisms also require inorganic sources of nitrogen and phosphorus as macronutrients, enabling the use of microalgal cultures as a tertiary treatment stage (when significant concentrations of these nutrients persist after previous treatment processes) [10]. Finally, microalgal biomass presents a very rich composition in polysaccharides, lipids, proteins, vitamins, and other valuable compounds, which make microalgae a valuable resource for several applications [11; 12], such as the production of natural colorants or dyes, bioenergy, and biofertilizers. Also, effluent treatment with microalgae has the following advantages [10]: (i) reduction of nitrogen and phosphorus concentrations to levels below the legislated limits for effluent discharge (EU Directives 1991/271/EEC and 1998/15/EC); (ii) recovery/recycle of these nutrients, which production presents negative environmental impacts; (iii) increase of the oxygen concentration in the treated effluent; (iv) production of biomass that can be integrated into the value chain of the company; and (v) reduction of net carbon dioxide emissions. Despite the need to search for eco-friendly and cost-effective remediation strategies, only a few studies have reported the treatment of pulp and paper industry effluents using microalgae [1]. [PERSON] et al. [7] evaluated the removal of color, AOX, and COD from an effluent resulting from a wood-based pulp and paper Turkish company using a mixed microalgal culture (composed by _Chloerella_ and diatoms).