Photocatalytic reactor design for environmental applications is a major challenging task in heterogeneous photocatalysis because all the three components involved in the process are in different phases: pollutant- fluid, catalyst - solid and light photons - massless particles. Heterogeneity of photocatalysis makes the reactor design highly inter-disciplinary involving the knowledge of chemical, mechanical and environmental engineering concepts. So far literatures have focused on optimizing process parameters for specific pollutants, but the holistic approach in designing a photocatalytic reactor has been unclear. This article reviews about all types of photocatalytic reactors and compares them with calculated five benchmarks: Apparent reaction rate constant (k), Space time yield (STY), Photocatalytic space time yield (PSTY), Specific removal rate (SRR), Electrical energy consumption (EEC). It is interpreted that in designing a photocatalytic reactor, four important aspects are to be considered: Throughput, Performance, Energy efficiency, Cost. This review article calculates the above said 5 benchmarks for 24 reactor designs and highlights the important parameters to build an effective photocatalytic reactor. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.