This study was devoted to provide good understanding of the operating characteristics and their effects on NF membranes system, to utilize in the treatment of wastewater containing synthetic reactive dyes which are common in Ethiopia. The effectiveness of NF systems in rejection of reactive dyes was evaluated under different operating conditions, such as working pressure, feed flow rate and dye concentration keeping feed temperature and pH constant. All experiments were conducted on RO/NF pilot

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... scale plant including facility of feeding and product tank, on line measurement of flow rate and pressure. Wastewater containing reactive Dye was brought from Yirgalem Textile factory and its characteristics were analyzed. The result shows that dye-house wastewaters, containing reactive dyes, are hazardous to the environment since their COD, BOD, TS and pH values are higher than the free discharge limit values and also they are highly colored. It is found that all the three parameters affect the permeate flow and dye removal significantly. The increase of both inlet flow rate and working pressure has positive effect on the permeate flow rate. But increase in dye concentration generally shows negative impact. On the other hand, the increase of inlet flow rate, dye concentration and working pressure increases percent of dye removal. In this investigation, greater than 99% dye removal and maximum permeate flow rate within the minimum fouling rate was obtained at feed flow rate of 0.86m3/h, working pressure of 20 bars, and dye concentration 146.1mg/L. Keywords: Membrane separation, Synthetic dyes, Nano filtration, Wastewaterreuse