*4.1.4 Future prospects for photocatalysis*

There have been tremendous efforts dedicated to improving photocatalytic degradation efficiency, where some role of the aforementioned parameters on the AOPs performance have been studied with different types of wastewaters from the textile, oil refinery, slaughterhouses [46, 47, 59]. However, in relation to relative operational and energy utilisation costs, as well as the formation of unknown toxic intermediates from the parent compounds (semiconducts) and the pollutants remain unsolved. Although, this method is prone to shifting of hydroxyl radicals by non-target substances, but some are not suitable for certain varieties of toxic compounds that can counterattack the hydroxyl radicals rendering an increase in ecotoxicity when discharged into the environment. In response, there have been attempts to improve the semiconductor material performance by discovering the reaction mechanisms to develop commercial scale technology, but there are still some challenges which include mass transfer limitations, catalyst deactivation, generation of intermediate products and by-products, and the multi-complex optimisation of the materials and the reactor configuration [49, 55, 58]. Furthermore, due to the intrinsic nature of photocatalysis, it is very difficult to model photocatalytic reactors with a uniform light along the whole reactor volume [58, 59]. Therefore, a feasibility study on the irradiance distribution inside the reactor is required, especially with heterogeneous media such as TiO2 in suspension.
