This document discusses the use of TiO2 and ZnO as heterogeneous photocatalysts for wastewater treatment. It begins with an introduction stating that industrial effluents cause environmental pollution and dyes from textile industries are toxic and difficult to degrade. It then provides details on the Z-scheme and heterojunction mechanisms for combining multiple photocatalysts to more effectively treat wastewater using solar light. The key advantages of these photocatalytic advanced oxidation processes are that they can mineralize organic pollutants under mild conditions and use solar energy.