## 8 Future Perspectives
Figure 15 illustrates key design strategies aimed at enabling the practical deployment of photocatalytic reactors across various treatment settings. It highlights modular solar units tailored for decentralized or rural use, immobilized photocatalyst systems engineered for durability and surface efficiency, and configurations that integrate with existing water treatment infrastructure, such as hybrid membrane-photocatalysis systems. Additionally, the visual underscores the role of process intensification and automation in supporting continuous, real-time operation, facilitating broader adoption of photocatalytic technologies in scalable water purification solutions. (c) Coupling with renewable energy and circular water systems  
Heterogeneous photocatalysis aligns well with global transitions toward carbon-neutral and circular water management strategies. Efforts to integrate photocatalysis with renewable energy systems, especially solar energy, are likely to expand. Additionally, coupling photocatalytic processes with water reuse frameworks, such as greywater recycling and potable reuse can enhance resource recovery and resilience. Greywater refers to relatively low-contaminated domestic wastewater generated from activities such as bathing, handwashing, and laundry, while excluding blackwater that contains fecal matter. It typically represents 50-80% of household wastewater volume and is considered suitable for reuse after appropriate treatment [127]. Photocatalytic processes, particularly those based on visible-light-active semiconductors, have been explored for greywater treatment, due to their efficiency in degrading organic pollutants and microbial contaminants [86; 99]. Figure 14: Key strategic priorities in the development of visible-light-responsive photocatalysts for sustainable water treatment, including material design, enhanced light absorption, charge separation, surface reactivity, and long-term stability under real environmental conditions. Treated greywater is commonly reused for non-potable purposes such as toilet flushing, landscape irrigation, or industrial wash water, while direct mixing with potable water is avoided, unless subject to advanced multi-barrier treatment and regulatory approval [127]. Figure 16 provides a comprehensive visual representation of how heterogeneous photocatalysis can be effectively integrated into broader sustainability frameworks through coupling with renewable energy sources and circular water management strategies. On the energy side, solar panels are depicted as the primary renewable power source driving the photocatalytic process, reflecting the technology's alignment with global efforts toward carbon neutrality and low-energy water treatment. The diagram also illustrates how photocatalytic systems can be embedded into water reuse loops, such as greywater recycling and potable reuse schemes. Although not typically  
Figure 16: Integration of photocatalysis with renewable energy and circular water systems for sustainable treatment and resource recovery. Treated greywater is reused in non-potable applications, such as irrigation, toilet flushing, or industrial processes, in accordance with safety and water quality regulations. Figure 15: Design strategies supporting the development of scalable, modular, and integrative photocatalytic reactors for advanced water treatment applications. The figure outlines key aspects such as reactor configuration, catalyst immobilization, light delivery systems, and process integration to enhance treatment efficiency and adaptability. mixed directly with potable water, advanced treatment technologies, such as photocatalysis, can produce effluents of sufficiently high quality to allow safe discharge or reuse, in contexts where water quality is strictly regulated [86; 99; 127]. These pathways demonstrate how treated water can be redirected into various applications, including household, industrial, or agricultural applications, thereby reducing freshwater demand and enhancing resilience in water-scarce environments.