## 5 Reactor Systems and Process Parameters
### Photocatalytic Reactor Designs  
Photocatalytic reactors can be broadly categorized based on how the catalyst is deployed, either as a suspended powder (slurry systems) or as immobilized on a support (fixed systems) (Figure 9) [128; 129; 130; 131; 132]. * Slurry reactors: These systems involve the dispersion of catalyst nanoparticles throughout the aqueous phase, offering high surface area and mass transfer. However, post-treatment separation of the catalyst is necessary, which can increase operational complexity and cost. * Immobilized systems: In these reactors, the catalyst is fixed on substrates such as glass, ceramic, polymer membranes, or stainless steel meshes. While they reduce the need for separation, mass transfer may be limited, and surface deactivation can occur over time. * Annular reactors: These consist of a cylindrical configuration with a light source at the center and photocatalyst-coated surfaces arranged around it. They are commonly used in lab-scale and pilot studies for their uniform light distribution. * Flat plate and falling film reactors: Suitable for solar applications, these designs maximize light exposure and are effective for shallow water layers, making them ideal for outdoor use. * Fluidized bed and membrane photocatalytic reactors: Emerging designs such as these integrate advanced features like enhanced catalyst suspension, reduced fouling, or combined separation and reaction zones. A comprehensive comparison of various photocatalytic reactor configurations commonly employed in environmental applications, particularly in water and wastewater treatment, is presented in Table 7. Each reactor type exhibits distinct design principles, operational advantages, and associated challenges that influence its suitability for specific applications and scales. These systems address limitations related to fouling and scalability, although they often involve complex engineering and higher initial costs. Overall, Table 7 underscores the trade-offs among catalyst efficiency, operational complexity, and scalability, guiding the selection of reactor configurations based on specific treatment goals and implementation contexts. Slurry reactors, characterized by the dispersion of catalyst nanoparticles within the aqueous phase, offer high photocatalytic efficiency due to enhanced surface area and mass transfer. [PERSON] et al. (2023) investigated the photocatalytic degradation of methylene blue dye using an anatase-titania slurry in a falling film reactor [133]. The results demonstrated enhanced degradation efficiency, highlighting the effectiveness of slurry systems in dye removal applications. In a study conducted by [PERSON] and [PERSON] (2018), a slurry photocatalytic reactor equipped with a 16 W UV lamp emitting at 254 nm was utilized to degrade phenanthrene (PHE), a polycyclic aromatic hydrocarbon known for its persistence in the environment [134]. The researchers explored various operational parameters, including initial PHE concentration (ranging from 1000 to 1500 ug/L), catalyst dosage (between 0.1 and 0.9 g/L), and pH levels (from 3.0 to 9.0), to determine their effects on degradation efficiency. Figure 9: Representative designs of reactors employed in photocatalytic processes, including batch, fixed-bed, and slurry configurations. These reactor types influence light distribution, catalyst contact, and scalability, thereby affecting the overall performance of photocatalytic water treatment systems.