The consequences of highly saline freshwater on the ecosystem and humans are quite alarming and have gained little attention in recent times. Progressive advances in pervaporation have helped to unlock its potentials in the desalination of salty streams. In this study, desalination of lagoon-water using cellulose acetate membrane (CAM) and its copper-doped nanocomposite (CA-CuNP) membrane was investigated. A newly developed model was used in estimating salt diffusion coefficients in steady and unsteady state situations. At the experimental phase, permeate fluxes increased with temperature but dropped when the critical fluxes (5.11-6.01 L/m 2 h and 5.29-7.56 L/m 2 h) were exceeded for the CAM and CA-CuNP membranes respectively. At steady state, the critical permeate volumes for the pristine and nanocomposite membranes were 0.2273 and 0.1826 L with corresponding fluxes of 0.034 and 0.031 L/m 2 h after 10 and 9 h, respectively. The estimated steady and unsteady diffusivities for the membranes are: 1.