Characterization of reverse osmosis and nanofiltration membranes: effects of operating conditions and specific ion rejection

Reverse osmosis (RO) and nanofiltration (NF) are two of the most commonly used technologies for desalinating brackish and saline waters to provide potable water. However, there is still lack of a thorough comparison between these two methods providing the better option in different conditions. Therefore, in this paper, salt rejection and the effects of operation conditions on the performance of RO and NF systems are compared. Inlet water conductivity, inlet pH, permeate flow rate, temperature, and recovery rate are used as variable operating conditions, and permeate conductivity is considered as the target of comparison as it reflects the level of salts in product water. Five combinations of inland brackish water, drawn from wells located at the experimental site, were applied as different feed waters and five distinct types of membranes, three RO and two NF, were studied using pilot-scale equipment. To allow meaningful comparison among RO and NF membrane performances, identical hydrodynamic operating conditions and feed water chemistries were employed during tests. In both systems, negative rejection occurred for specific ions. The results suggest that the performances of RO and NF membranes (i.e. the amount of total dissolved salts remaining in produced water) are quite different. Based on the experimental data, new insights can be reached regarding the best choice of membrane, based on the minimization of electrical conductivity and the ability to reject specific ions in different operating conditions.