Removal of nitrate from aqueous solution by using direct contact membrane distillation Ali Boubakri a,*, Amor Hafiane a , Salah Al Tahar Bouguecha b a Laboratory of Wastewater Treatment, Center of Researches and Water Technologies, P.B 273, 8020 Soliman, Tunisia. b Department of Mechanical Engineering, Faculty of Engineer, King Abdul-Aziz University, P.B: 80204 Jeddah 21589, Kingdom of Saudi Arabia. Abstract 2- Effect of feed flow rate The removal of nitrate contaminant from aqueous solution by direct contact membrane distillation (DCMD) using flat sheets polypropylene (PP) and polyvinylidene fluoride (PVDF) membranes were studied. Effect of operating parameters, as feed temperature, hydrodynamic conditions, initial nitrate concentration, feed ionic strength and competing co-existing anions on permeate flux and nitrate rejection was investigated. In all DCMD experimental runs, an almost complete nitrate rejection was achieved (higher than 99.8%) and the permeate nitrate concentration was largely below the maximum permissible level in potable water. Under the same operating conditions, PVDF hydrophobic membrane showed the higher permeate flux of 12.67 L/m²h than the PP membrane with a permeate flux of 4.29 L/m²h. permeate flux increased when the feed flow rate increased from 5 to 25 L/h for both membranes. At high flow rate, Reynolds number was increased, which enhances the heat transfer coefficient and thus reduce the effect TPC and CPC. 3- Effect of initial nitrate concentration Permeate flux decreased slightly when increased feed nitrate concentration from 50 to 1000 mg/L. Materials and methods Table 1. Characteristics of used membranes Material PP PVDF Pore size, µm 0.064 0.22 Thickness, µm 25 125 Porosity, % 55 62 Manufacturer Hoechst-Celanese GVHP, Millipore Contact angle, ° 120 113 LEP, kPa 200 204 Membrane area, m2 4.2 10-3 4- Effect of ionic strength   DCMD setup: (1) Permeate tank; (2) Cooling element (3) peristaltic pumps; (4) thermocouples; (5) conductimeter; (6) flat sheet module; (7) Feed tank; (8) Heating element. Conclusion PP membrane and PVDF membrane can achieved a very high nitrate rejection above 99.90%. Permeate flux was increased exponentially with increasing feed temperature, under the same operating conditions, PVDF membrane showed the higher permeate flux of 27.47 L/m²h than the PP membrane with a permeate flux of 3 L/m²h. Permeate flux was also affected by feed flow rate. Increases feed flow rate is accompanied by an increasing of permeate flux. Initial nitrate concentration and feed ionic strength had a slight effect on permeate flux compared with the effect of feed temperature or feed flow rate. All experimental results confirm that DCMD could be operated most efficiently for nitrate removal from natural contaminated water.     Results and discussions 1- Effect of feed temperature   * Corresponding authors: Ali Boubakri Phone number: +216 99 155 999 E-mail: ali.boubakri@certe.rnrt.tn