Investigation for finding appropriate methods of wastewater pretreatment of a welding electrode manufacturing plant before use of nanofiltration is the subject of the present paper. Based on some preliminary experimentation, it was concluded that the coagulation-flocculation and centrifuge processes in the separation of colloid particles were more effective. At first step, the effect of different concentrations of a mixture of alum (Al2 (SO4)3. 18H2O) and ferric chloride (FeCl3) on turbidity reduction, clarified wastewater percentage, and the rate of sedimentation were closely studied. The results show that a mixture of 2.5g/l comprising of 25% alum and 75% ferric chloride is an optimum dose and more economic. In the next step, the effect of various RPMs and different time periods on the turbidity reduction by the centrifugal method was studied. The centrifuge experiment results show that the colloidal particles are easily converted to dense granular form. Wastewater turbidity can be reduced up to 98% for 4000 RPM and a 30 minutes time periods. Time operations more than 30 minutes and above 3000 RPMs did not have a significant effect on the turbidity reduction. Regarding the operation costs and energy consumption, it can be conclude that 30 minutes time operation and 3000 RPM were optimal. As the centrifugal method is free from chemicals and wastewater recovery with this method is high, this method appears to be advantageous.

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