## 3 Results and Discussion
### Opportunities for Process Scale-Up and Use of Biomass Produced
From an economic and environmental perspective, the TL-PBR system shows advantages when compared with conventional systems such as open raceway ponds or tubular photobioreactors. Open ponds are characterised by low capital costs and simple operation, but they suffer from low areal productivity, high water losses through evaporation, and high risks of contamination. Tubular PBRs offer better control over culture conditions and higher volumetric productivity, but they require significant energy inputs for mixing, pumping, and often cooling, in addition to high infrastructure costs. Based on literature data, the operational cost of biomass production in open raceway ponds ranges approximately from EUR 2 to EUR 15 per kg of dry biomass [63; 64; 65], while closed PBRs can exceed EUR 30/kg under suboptimal conditions [66; 67; 68]. In contrast, TL-PBRs operate without active mixing, with minimal water use, and allow direct biomass harvesting via scraping, thereby avoiding the need for flocculation, filtration, or centrifugation--steps that are energetically expensive and economically burdensome [69; 70]. Although no dedicated techno-economic study on TL-PBRs exists to date, these features suggest that operational costs could reasonably fall in the range of EUR 4 to EUR 10 per kg of dry biomass under realistic conditions, depending on scale, climate, and configuration. These data are inferential estimates based on significant overall cost reductions (over 50%) associated with lower harvesting costs, media production and mixing when using twin layers compared to other conventional cultivation systems [64; 71; 72; 73; 74]. The quality and purity of the biomass obtained represent another significant advantage [8; 75]. As the TL-PBR supports attached growth in a controlled and physically isolated environment, the harvested biofilm is free of suspended solids and exhibits low microbial contamination. This clean biomass can serve as a platform for the production of high-value compounds, and the microalgal biomass of Galdieriaceae is a rich reservoir of biotechnology relevant compounds, with an established market demand [1; 2; 33; 76]. Literature reports indicate that members of the _Galdieria_ genus can accumulate phycobiliproteins, particularly phycocyanin and allophycocyanin, with yields reaching up to 14% of dry weight [16]. Market prices for purified phycocyanin currently range from USD 500 to over 900 per kg, depending on purity and intended use [2; 76]. In addition, _G. phlegrea_ is known to produce carotenoids such as zeaxanthin and \(\beta\)-carotene, bioactive polysaccharides with antioxidant and immunomodulatory properties, sulphur- and nitrogen-rich amino acids relevant for feed applications, and polyunsaturated fatty acids including linolenic and arachidonic acid [1; 11; 33; 77]. These compounds are in growing demand across the nutraceutical, cosmetic, and pharmaceutical sectors [8; 74; 76]. Even after the extraction of these compounds, the residual biomass can be further valorised. It may serve as feedstock for hydrothermal liquefaction processes [78] aimed at producing biofuels or be used as a biosorbent in processes for recovering metals and rare earth elements from industrial effluents [10; 79; 80]. These cascading uses align well with the principles of circular bioeconomy and are of increasing relevance in the context of EU strategies for waste valorisation and sustainable resource use [81; 82]. Although the data presented here are based on short-term experiments conducted at laboratory scale, they provide an evidence-based foundation for future studies. The TL-PBR system, in its current configuration, combines low energy consumption, reduced water demand, simplified harvesting, and a capacity to produce high-purity biomass from low-cost inputs such as urban wastewater. Its modular nature and compatibility with smart monitoring infrastructures make it a promising candidate for deployment in decentralised contexts where resource efficiency and environmental sustainability are priorities.