POTENTIAL OF MEMBRANE SEPARATION TECHNOLOGY FOR FLUORIDE REMOVAL FROM UNDERGROUND WATER

The fresh water resources are very limited in world and demands for water quality are becoming increasingly more stringent. So treatment of groundwater for drinking purposes is becoming more complex and interesting. Because of the number of components that are being considered for regulation and because of the diversity of their physical and chemical properties, their removal presents a promising challenge to water treatment engineer. Though the membrane separation processes have over the past two decades achieved a lot of industrial importance for wastewater treatment and seawater desalination, their potential for treating groundwater to remove fluoride and other undesirable ions remain unexploited. Because of their ability to reject high percentages of many dissolved components, membrane process offers a single treatment alternative for removing many of these contaminants. Now a days, about 20 nations of world including India are facing the problem of high fluoride levels in underground water which is the major source of drinking water supplies. The present study deals with removal of fluoride and other undesirable ions from underground water of rural areas of district Gurgaon.Both hyperfiltration and nanofiltration membranes were tested for removal of excess fluoride. The nanofiltration membrane was tested with different water matrices. Such membranes show high percentage rejection of bivalent ions as compared to monovalent ions. But fluoride behaved like bivalent ion, i.e. rejection percentage of fluoride was fairly good like bivalent ions. Thus excess fluoride present in water can be removed specifically without removing other useful ions like sodium, chloride etc. This exceptional behavior of fluoride can be explained due to low solubility product of calcium fluoride, produced as a result of compelxation of fluoride and calcium on the membrane surface leading to scale formation and the ion size of fluoride. So, solubility product of calcium fluoride and ion size of fluoride were determined using different water matrix.Then, a spiral wound module of RO membrane made up of cellulose acetate was used. Initially the experiments were done with synthetic samples prepared by adding known amounts of sodium fluoride and calcium chloride in distilled water. The effect of feed water composition, pH, temp. of feed water, operating pressure and feed water flow rate on separation efficiency of membrane was studied by varying one parameter at a time and keeping all other parameters constant. Thus the optimum operating conditions for the process were determined and after that, ground water samples collected from three villages of district Gurgaon (Farukhnagar, Wazirpur and Mevka) were treated under optimum operational conditions. The pH and temp. of the ground water was adjusted upto optimum level. The mass transfer coefficient and membrane parameters were estimated for each data point using two-parameter model (Film theory and Solution-diffusion model) to study the concentration polymerization on membrane surface. The results revealed that at high pressure RO membrane removes practically all the ions present in water, so at the end it might be necessary to re-mineralize the water to make it suitable for drinking purposes, whereas at low pressure, rejection percentage goes down to get permeate of required quality.