Permeate recycle to improve the performance of a spiral-wound RO plant Original Research Article

This paper presents the results obtained from a simulation study on the effect of permeate recycle on the performance of a spiral-wound RO plant. The model used was developed for an actual small scale plant consisting of three stages. The first stage consists of two parallel vessels whereas the second and third stages contain a single vessel each. Each vessel houses three 8-inch spiral-wound brackish water FilmTec membranes. Part of the product water is recycled and mixed with the fresh feed. The model incorporates the solution diffusion mass transport theory, concentration polarization, along with the characteristics of the spacer for the estimation of the mass transfer coefficient and pressure drop. The use of these concepts together with the recycling of a portion of the permeate resulted in a model composed of a highly coupled system of nonlinear equations. The results of the simulation showed that recycling part of the permeate and mixing it with the feed was beneficial in terms of reducing the concentration polarization and improving the quality of the final product. However, as expected, this resulted in reducing the production rate. By using a recycle ratio of 25%, the concentration of the product was reduced by 15% compared to the case of no recycle, at the expense of 22% reduction in the production rate.