Polymeric composite membranes have shown great potential in removing pollutants from water. In the current study, flat sheet functional membranes of microfibrillated cellulose (MFC) with mixed and layered architectures are produced using the up-scaled Dynamic Sheet Former (Formette) in a fully water-based-system, and their potential for the removal of charged impurities is evaluated. The processing of composite membranes is unique in terms of their size (1 m × 20 cm), assembled MFC architectures, controlled/tunable porosity, functional groups densities, and free-standing at high water pressure. Such properties could be difficult to achieve with a lab-scale processing setup. It is shown that the MFC assembly has a direct influence on the pollutant removal efficiency, and again the layered architecture turns out to be a more efficient scavenger of the charged pollutants due to the combined actions of electrostatic interactions, hydrogen bonding, and size exclusion, which are responsible for an ultrafast separation of the impurities through the flat sheets membranes. These experimental results are supported by reactive molecular dynamics simulations of representative model systems that provided possible realistic scenarios at the atomic/molecular scale. All the data confirm the scalability and tunability of the produced MFC-based water cleaning membranes, which show high adsorption capacity, flexibility, hydrolytic stability, and mechanical robustness.