Existence of sulphur compounds especially mercaptans in natural gas stream causes corrosion problems in transfer lines and storage tanks. So, it is necessary to reduce the amount of these components to international standard levels. The Merox process developed by UOP is the most popular process used for this purpose. In this process, light mercaptans are converted to disulfides by using caustic solution with Merox catalyst. In the present work, recovery of sodium hydroxide from waste stream of prewash section of Merox tower (containing Na 2 S, Na 2 CO 3 and excess NaOH) by membrane processes of electrodialysis (ED) and electrodeionization (EDI) were studied and effects of operating parameters such as voltage, feed flow rate, feed concentration and cell structure have been experimentally investigated. The result show that by increasing the applied voltage, the NaOH recovery enhanced and the economy of process decreased, while the feed flow rate did not have sensible effect on NaOH recovery. On the other hand, for all values of feed flow rate, the current increased with increasing the applied voltage with similar slope. It can be concluded that in our study no concentration polarization phenomenon was apparent in any case in the range of the applied voltage used for the experiments. Also an exclusive comparison between performances of ED and EDI processes was made. Results show that the total energy consumption in EDI is higher than ED and this is due to considerable energy consumption for water dissociation. But, when specific energy consumption was determined, EDI process was found more economic because of higher ion separation rate in this process. In studying effect of feed concentration, the results indicated that in system with lower concentration of sodium ion in the feed, the NaOH recovery was higher, whereas the economy of system and sodium ion current efficiency were lower. A comparison between two implemented configurations of ED process revealed that in the two-compartment cell, competition between protons and sodium ions for transferring across the CEM reduces efficiency of sodium ions traort and with using three-compartment cell, a higher recovery for NaOH and current efficiency for Na + was observed. Generally, as it was forecasted the implemented systems proved that can be considered as promising methods to recover unused caustic from waste stream of Merox process as the NaOH recovery for ED(Two-Compartment cell), ED(Three-Compartment cell) and EDI(Three-Compartment cell) was obtained about 48, 75 and 85%, respectively. However, EDI process had noticeable relative advantage in comparison with ED
