Tomato wastewater is characterized by a large amount of suspended solids, red color, and bad smell. In this paper, the effect of transmembrane pressure (10, 15, 20 bar), pH (4, 5, 6) and temperature (30, 40, 50 oС) on the permeate flux, total hydraulic resistance, and COD rejection of tomato wastewater during nanofiltration treatment was investigated. The flux was found to be approximately constant during long period operation. The flux was reduced as the pH declined, while it was increased as the temperature and transmembrane pressure increased. The hydraulic resistance (at pH 5 & TMP 1.5MPa) increased from 5.79×1013 1/m to 7.25×1013 1/m when the temperature increased from 30 oС to 50 oС. The COD rejection was ranged from 75.65% to 85.35%, in which the highest value was obtained at pH 6, TMP 2.0 MPa and 30 °C. The Response surface methodology (RSM) results demonstrated that the quadratic, the two factor interaction (2.Fi) and the linear polynomial models are highly significant for modelling the flux (R2=0.98), hydraulic resistance (R2=0.96), and COD rejection (R2=0.90), respectively. Numerical optimization determined the optimum conditions based on the highest flux and rejection and the lowest fouling with transmembrane pressure of 20 bars, temperature of 30 C, and pH of 6, respectively

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