Coupling of membrane processes for brackish water desalination

In the recent years, membrane processes have drawn more attention because of their strong separation capabilities and exhibiting a great potential for the treatment of waters and wastewaters worldwide. However, these membrane separation processes have some problems due to the formation of polarisation films and by-products which may generate bacteria and fouling. These problems may be overcome by combining two or more processes in the desalination units of the treatment process. The specificity of each process makes their integration more efficient. In this work, we report on the concept and the realisation of an experimental pilot plant which is composed of the following compartments: nanofiltration (NF), reverse osmosis (RO), cartridges filter, pump and measuring instruments. Samples of brackish water were filtered through filtrate cartridges in order to get rid of the suspended matter. A pilot plant equipped with composite RO and NF membranes was operated at 6 bar, ambient temperature and neutral pH with relatively good water recovery. This pilot plant shows the ease with which the combination of both processes is carried out. The main objective is to evaluate the effect of NF and RO membranes as well as their coupling on water quality. The effects of divalent and monovalent cations on operating parameters were studied. The recovery (Y) in NF/RO coupling was improved compared to that obtained in RO alone. This may be explained by a decrease in the input osmotic pressure of the RO membrane. This coupling also improves salt rejection and thus leads to a decrease in the salinity of water product. The results show that the coupling is efficient as it highly reduces water salinity.