Comparative tests of the four different insert designs, namely columns, hollow sphere, spiral and grid revealed the grid design as the most efficient one (see Figure ). This is remarkable since the grid design possesses the smallest irradiated surface (7.15 cm²) of all designs. Two aspects might be relevant for these observations. First, the calculated irradiated area considers irradiation with a collimated light source, but the used LED has a conical emission pattern (see SI, Figure ). Therewith a larger area is irradiated as estimated from the calculations, for instance the walls of the inserts. Furthermore, spiral walls and the columns shade reactor parts, effectively reducing the irradiated area. Second, the flow patterns of the different inserts vary (see discussion above), and the grid design shows less pronounced dead zones or short cut flows, which could cause a better homogenization of the concentration of MB at the photocatalyst. Furthermore, the hollow sphere and the spiral design possess a directed, laminar flow, while the grid design features a more intense convection and thus an accelerated mass transport.