A new experimental set-up permitting scale-up and serial internal numbering-up of meso- and micro photocatalytic systems, respectively, was developed to compare their photodegradation efficiency, by using of a commercially avaliable TiO2 (P25), against Rhodamine B (RB) and Methylene Blue (MB) under UV irradiation in water. A circular photocatalytic unit constituted of TiO2 embedded into PDMS was the basic unit of the meso and microchemical reactors. Different volumetric capacity, from 10 μL to 1000 μL, and different flow rates (0.05–1.0 mL min−1) were evaluated to understand the best approach to design a more efficient fixed bed photoreactor for photocatalytic degradation. The multi-photocatalytic microreactor system with the best performace, built-up by plugging six modular microreactor plates (M6), was tested for photooxidition of phenol under UV irradiation, as well. P25 catalyst showed different photodegradation efficiency depending on the photocatalytic meso- and microreactor design and flow parameters. The photocatalytic efficiency for the meso- and microdevices was evaluated by figure-of-merit electrical energy per order (EEO) and apparent quantum yield. The numbered-up M6 system had a lower electrical energy consumption compared with others. The microfluidic device M6 was able to achieve 97% of photodegradation of MB and RB (1.2 10−5 mol L−1) in 1 h compared to 96% conversion of the best mesoreactor in 16 h, while reached a total mineralization of phenol (1.2 10−4 mol L−1) to CO2 and H2O in 6 h.