## 3 Results and Discussion
### Opportunities for Process Scale-Up and Use of Biomass Produced  
The experimental results obtained under controlled laboratory conditions allow us to outline realistic scenarios for the scale-up of the twin-layer photobioreactor (TL-PBR) system, as well as to assess the potential applications and economic value of the algal biomass produced. From an operational standpoint, the TL-PBR demonstrated a low energy footprint, as the main power consumption was limited to the peristaltic pump, artificial illumination, and the sensor system. In a future pilot or full-scale configuration, artificial lighting could be replaced with natural sunlight, and the overall energy demand could be met using photovoltaic systems. A representative scale-up scenario might involve a cultivation surface of 100 m\({}^{2}\), achievable through the modular arrangement of ten 2 \(\times\) 5 m panels. Under the most favourable experimental conditions observed (Exp3), regression analysis suggested that a surface biomass density of 1 kg\({}_{\text{DW}}\)/m\({}^{2}\) could be reached in approximately 54 days, aligning with relevant literature that testifies to rates of 10-15 g m\({}^{-2}\) day\({}^{-1}\)[20]. This would correspond to a theoretical yield of 100 kg of dry, clean algal biomass in under two months. However, this projection should be interpreted with caution, as it is derived from short-term lab trials and does not account for limiting factors such as non-uniform light distribution, accumulation of metabolic byproducts, or fouling. Large-scale cultivation would require addressing additional challenges such as the mechanical durability of the support materials, biofilm overgrowth, and long-term system maintenance. Biofilm thickness must be controlled to ensure adequate light penetration and gas exchange, and periodic harvesting is necessary to maintain productivity [53]. Repeated cleaning and scraping procedures may introduce physical stress to the materials used for the support and substrate layers, which could affect performance over time [55]. One particularly critical consideration when envisioning real-world applications is the method of sterilising the culture medium. In laboratory tests, sterilisation was achieved via autoclaving; however, this approach is impractical and unsustainable on a larger scale. In scaled-up systems, sterilisation could instead be implemented using UV lamps placed along the fluid circuit. This method is compatible with continuous or semicontinuous wastewater recirculation and has the advantage of being energy-efficient and reagent-free [56]. Nevertheless, it requires careful material selection, since the acidic pH used here in the cultivation of _G. phlegrea_ (2.5) can cause corrosion in metallic components, especially in the collection tanks of standard UV units. For this reason, future configurations may evaluate the cultivation of _G. phlegrea_ at slightly higher pH values, such as 4 to 5, which remain within the physiological tolerance of the species while mitigating corrosive effects and reducing the chemical demand associated with pH adjustment of the influent. Another important factor to consider in scale-up scenarios is the variability of the wastewater composition. Unlike synthetic media, urban effluents exhibit seasonal and diurnal fluctuations in nutrient content, ionic strength, and the presence of potential inhibitors [24]. These variations can affect not only biomass growth rates but also biomass composition. The monitoring system implemented in our prototype--based on an IoT platform for real-time remote acquisition of environmental parameters such as temperature, pH, and light intensity--offers a useful foundation for adaptive system management [57; 58; 59]. Future iterations may benefit from the inclusion of additional sensors for nutrient concentrations or biofilm thickness, enabling closed-loop feedback control and predictive operation supported by artificial intelligence models [60; 61; 62].