Based on the optical properties of WM, the UV-A dose was experimentally and mathematically calculated as described in . WM was irradiated at doses 0, 143, 279, 571, and 838 mJ/cm 2 as described by . Estimation of light attenuation parameters; reflection, absorption and scattering are crucial for calculation of average UV dose delivery under ideal mixing conditions (Collimated beam UV system). IAD method was used to characterize light attenuation properties of the WM. Computational fluid dynamics was conducted to understand the intensity gradients. Computational Fluid Dynamics (CFD) simulation was carried out with the commercial code ANSYS 2021 R1. Fluent software was used for drawing the geometry (2D) and generating the mesh while Ansys Fluent v19.1 (Ansys Inc, Canonsburg) was used for solving the irradiation fields. The mesh resolution on was: Nodes: 3643; Elements: 36000. Fig 1 . shows the schematic representation of the geometry of the system adopted for the numerical simulations. The technique used radiation DO model, the fluid in a 10 ml beaker was exposed to the UV incident radiation coming from a near collimated beam apparatus. Fig 1 . presents the schematic drawing of the fluid domain in a collimated beam apparatus, including the detailed dimensions used in the simulations. The average fluence rate was calculated as 2.18 mW.cm -2 and 2.24 mW.cm -2 for AFB1 and AFM1 respectively. The product of average fluence rate and exposure time is defined as UV dose. In the near collimated beam apparatus, the local disinfection depends on the local Fluence Rate, and the kinetic rate constants. Modeling the collimated beam photo-reactor therefore involves two sub-models: intensity of the radiation field and UV-reaction kinetics.