Upscaling continuous-flow wastewater treatment with carbon-based materials: Limitations and challenges

Carbon-based materials have been used in water treatment for over a century, from early activated carbon to the engineered nanostructures embedded in advanced oxidation and electrochemical systems. Their structural diversity allows them to function across a wide range of continuous-flow processes. Unlike previous reviews organized by material class or pollutant type, this review analyzes 135 studies since 2015 using an operational, energy-oriented strategy. The reviewed literature is profiled and critically examined across the dominant treatment pathways, namely adsorption, chemical oxidation, redox-mediated catalysis, electrochemical treatment, and photocatalysis, with emphasis on matrix complexity, operational longevity, regeneration, and energy demand. Levels of operational validation remain uneven: adsorption accounts for the largest share of studies, 56/135 (41.5%), yet only 16/56 (28.6%) of adsorption studies report operation beyond 24 h. Chemical oxidation is the only class tested more frequently in real matrices than in synthetic ones 21/39 (53.8%), and redox-mediated systems show the highest share of operations exceeding 24 h 6/9 (66.7%). Across all classes, experiments are typically conducted at low flow rates (1–10 mL min -1 ) and lack reporting of hydraulic residence time, bed volume, mineralization, and energy consumption, undermining realistic assessment of durability and scalability. The main barriers to implementation arise not only from material activity but also from system-level constraints, such as hydraulic clogging, catalyst precipitation, membrane fouling, and energy demand, which are rarely normalized beyond electrical parameters. Progress toward scale-up will depend less on identifying a better carbon material than on integrating reactor design, regeneration, and long-term validation under conditions representative of current treatment practice.