## 1 Introduction
Among the advantages they offer are (_i_) faster degradation kinetics, resulting in time-efficient experimental campaigns--this also allows to use cheap low-voltage lamps or light-emitting diodes (LEDs) systems [26] more efficiently when photocatalytic systems are investigated; (_ii_) reduced size--in addition to compactness and intrinsic safety, these devices present lower spatial gradients of the physical variables, e.g., light intensity and concentrations, resulting in simplified models and more precise parameter estimation [12]; (_iii_) reduced volumes with economic advantages in terms of amount of required materials and reduced amount of produced waste [23]; (_iv_) continuous flow operations, with the possibility of automation and direct coupling with analytical instruments; this allows to save additional time and resources by carrying out experiments in a fully automated fashion [27; 28; 29]; (_v_) adoption of very small amounts of immobilized solid catalysts and efficient gas-liquid and liquid-liquid mass transfer, depending on the investigated system [12; 22]. In most papers in the field of microreactors for wastewater treatment, they have been used to rapidly screen different reactive systems, study the effect of operative parameters, and compare performances [30; 31; 32; 33; 34]. From a kinetic point of view, this is usually carried out by estimating pseudo-first order or apparent kinetic constants [35; 36; 37; 38; 39; 40; 41]. Although this approach enables a rapid screening of competing materials under the same adopted conditions, pseudo-constants are dependent on the specific adopted device and conditions and do not allow any knowledge transfer across scales for the simulation and design of larger plants [12]. Although some authors suggested the possibility of direct use of these devices for water treatment by numbering up units [25; 42], this seems a fairly unrealistic and anticonomic scenario, with very few possible niche applications in the treatment of small volumes of water for specific applications. Only a limited number of papers have addressed the use of microreactors for the estimation of intrinsic parameters and mechanism identification, which can be used for the scale-up of AOPs for wastewater treatment, which, in fact, is the most promising application [9; 11; 12; 23; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54]. In this sense, there is still extensive room for improvement and exploitation of this technology for the design and scale-up of such tertiary treatments. The aim of this paper is a critical review of the literature dealing with the adoption of microreactors for the estimation of intrinsic kinetic parameters of AOPs for wastewater treatment in order to give some perspectives in the field and guide future research efforts. Papers discussing the relative influence of different transport phenomena on the estimation of degradation conversion are also taken into account. All papers investigating adopted materials and fabrication techniques of microreactors, as well as the ones dealing with mere comparisons of reactive systems and the estimations of pseudo-kinetic constant, have already been extensively reviewed [25; 55; 56] and are out of the scope of the present paper.