Experiments Of Wastewater Containing Fluoride By Nanofiltration Method

In recent years in petrochemical industry used BF3as the catalyst forthe production process, resulting in the waste water contains a lot offluoride. Precipitation is easy to operate and the cost is low, but there aremug settlement slowly and dehydration difficultly. Nanofiltration methodis easy to operate and not produce the secondary pollution, widely used inunderground water and industrial wastewater treatment. Because lack ofexperimental research and application on treating a high concentration ofwaste water containing fluoride by nanofiltration, therefore, the researchfor studying membrane separation technology to deal with a highconcentration of waste water containing fluoride is necessary. In thispaper, we did the research on fluoride removal from wastewater by aself-developed small-scale equipment.Firstly, four membranes （NF-90, NF-70, DL, DK） which had largerpure water flux were selected by the determination of pure water flux onfive kinds of nanofiltration membranes （NF-90, NF-70, DL, DK, BW-30）.Then inspected the effects of pressure, pH, temperature and flow rate onpermeate flux and fluoride rejection of nanofiltration membranes. Theresults show that, the permeate flux increase liner with the increase of system pressure, and the fluoride rejection increase firstly, and thendecrease. The fluoride rejection of NF-90membrane could reach94%between0.8and1.0pressure, but DL only78%. The pH has no obviousinfluence on the permeate flux on both of NF-90and DL membranes, butwith the increase of pH the fluoride rejection increase. In the pH of10,the fluoride rejection of NF-90could reach95%, and DL only78%. Withthe increase of flux both the fluoride rejection and the permeate flux andincrease slightly. With the increase of temperature, the permeate fluxincrease largely and the fluoride rejection decrease slightly. The bestoptimal operating conditions of NF-90for pressure1.0MPa, pH10, flow30L/h, temperature20³0℃; the best operating conditions of DL forpressure0.8MPa, pH10, flow30L/h, temperature20²5℃.At last theeffects of the initial fluoride concentration on the permeate flux andfluoride ion rejection were studied, the results showed that both permeateflux and rejection were decreased with the increasing of initial fluorideconcentration, the permeate flux of NF-90was decreased more fast thanDL, but the rejection of DL membrane was decreased more quickly thanthat of NF-90membrane. In the range of test concentration, the fluoriderejection of NF-90dropped from94%to83%, but the permeability fluxdecreased about20L/m2h-1compared to began; to DL membrane, therejection of fluoride dropped from75%to44%, and the permeability fluxdecreased only10L/m2h-1. The result indicated that NF-90membrane was more suitable for the separation of high fluoride content in wastewater.Inspecting the effect of BF₄^-,B₄O₇^2-,Ca²⁺,Al³⁺concentrationon the permeate flux and the rejection of fluoride. The results showed thatwith the increase of BF₄^-,B₄O₇^2-,SO₄^2-,Ca²⁺Al³⁺concentration, boththe permeate flux and the rejection of fluoride decreased. Among themthe B₄O₇^2-,SO₄^2-concentration had a large effect on removing fluoride.A mathematical model was set up and compared the theoreticalvalue and experimental data. The test results show that the theoreticalcurve of the pure water flux, penetration flux and interception rate had thesame trend of the tested curve, the theoretical values were slightly biggerthan experimental values.