# Optimization of Microalgae-Bacteria Consortium in the Treatment of Paper Pulp Wastewater
## 1 Introduction  
The contamination of aquatic systems is increased due to the increase in wastewater produced by population growth and the expansion of industrial processes [1, 2]. To avoid negative environmental impacts, biological wastewater processes are widely used for domestic and industrial wastewater treatment [3, 4]. Conventional wastewater treatmentprocesses, such as activated sludge, stabilization ponds, or percolating filters, present technical restrictions and high operating costs when it comes to nutrient removal and biomass harvesting [5]. According to [PERSON] et al. [6], biomass harvesting for the creation of goods with added value is identified as the stage with the greatest cost of the production process--20% to 30% of the total production. Therefore, new technologies for industrial wastewater treatment must be investigated. New and innovative systems that use the microalgae-bacteria consortium seem to be a sustainable and environmentally friendly technological alternative, requiring less area to operate [7] and promoting more efficient and less expensive wastewater treatment techniques [8] than the conventional processes. The production of photosynthetic oxygen, which is utilized by aerobic bacteria to oxidize organic materials and ammonia, and carbon dioxide (CO\({}_{2}\)) sequestering by algae, are major benefits of this system, since it eliminates the need for artificial oxygen and avoids the release of carbon to the atmosphere. Biological sequestration of CO\({}_{2}\) by algae is a future field of research since its photosynthetic capability can efficiently capture carbon [9]. Algae and bacteria also uptake nutrients for their synthesis and maintenance. In addition to the economic aspects and nutrient removal, systems that use microalgae and bacterial biofoculation, in the form of a microalgae-bacteria consortium (MABA), show the potential for resource recovery and valorization, promoting new added-values in the scope of circular economy [10]. Thus, wastewater treatment plants (WWTP) that use microalgae-bacteria aggregates are effective for biomass growth, allowing for more sedimentation and biomass extraction [11]. Despite MABA's wide use to domestic wastewater, this synergistic consortium can also be used to treat industrial effluents. However, no research has been found on the use of MABA for the treatment of wastewater from the paper pulp industry. However, satisfactory outcomes with aerobic systems for pulp and paper wastewater have already been reported in the literature [12; 13] as well as interesting results using microalgae species such as _Scenedesmus_ sp. and _Chlorella vulgaris_ to treat paper industry effluents [14; 15; 16; 17]. The symbiotic process between microalgae and bacteria is also good for removing pharmaceutical substances, and has been already used to remove antibiotics, antihypertensives, and psychiatric pharmaceuticals under appropriate operating conditions [18; 19]. The use of MABA for the treatment of pulp and paper wastewater may provide the following advantages: (i) biomass production with potential for byproducts commercialization such as animal feed, biopolymers, and biofuels; (ii) low operating costs because it is a clean and sustainable alternative; (iii) carbon capture by microalgae, which can generate additional revenue in the future around carbon credits; and (iv) generation of high quality algal biomass, which can be sold to power plants and converted into electricity [16; 20; 21; 22; 23]. Wastewater generated in the paper and pulp industries originates from activities such as debarking wood, manufacturing paper, recycling fibers, and generating cellulose, among others [24]. In general, the operational processes include chemical, mechanical, thermo-mechanical, and chemical-mechanical technologies, which need a large amount of water consumption. Furthermore, these processes generate a large volume of effluents containing a variety of pollutants, as well as high operating costs for their treatment with classical technologies [15; 25].