Porous Membrane Contamination Study

In this paper, theories of porous membrane fouling and cleaning were explored, and a series of experiments were especially done to clean CaSO4 scale in porous membrane. The high-efficiency cleaning agent and optimum conditions were chosen and it has some guiding significance on practice. The main tasks include:By the contrastive experiments of sodium tripolyhosphate, sodium hexametaphosphate, Na2EDTA and Na4EDTA dissolving CaSO4, Na2EDTA and N^EDTA were chosen as cleaning agents in this experiment.Effects of Na2EDTA and Na4EDTA's concentration on dissolving CaSO4 were studied. The reaction rate was relatively easy to reach the balance when concentrations of cleaning agents were higher.The dissolving reaction rate was fitted under the experiment conditions when NNa2EDTA and NCaso4 is 0.029mol:where v is the reaction rate, mol/l-s; and t is the reaction time,s.The change of pH was examined in the process of Na4EDTA and Na2EDTA dissolving CaSO4. Some conclusions were obtained: 1. PH value of Na4EDTA solution was higher than that of Na4EDTA solution when the mol concentration of the both solutions is same.2. For Na4EDTA solution or for Na2EDTA solution, the higher the solution concentration, the higher pH value was, however, the Na2EDTA solution is adverse. 3. With the increasing of the chelating reactions, pH value decreased gradually.The effect of pH value on the chelating properties of Na4EDTA and Na2EDTA for CaSO4 was studied. For Na4EDTA, and the effect of ionic strength on the chelating properties of Na4EDTA and Na2EDTA for CaSO4 was reviewed. With the increasing of ionic strength, the chelating property increased.On the basis of above experiments, the different factors and levels were selected and thethree-factor-four-level orthogonal experiments of Na4EDTA and NaiEDTA were done. By range analysis and variance analysis, time factor and pH factor were considered that had high remarkable effects on the course of Na4EDTA dissolving CaSO4. In all selected levels, the dissolving effect is best when time=20rm'nand pH=ll. In the course of Na2EDTA dissolving CaSO4, time factor has high remarkable effect and pH has remarkable effect. The effect is best when time=25min and pH=5.At last, on the basis of above experiments, the five-factor-three-level orthogonal experiments on the relative flux recovery ratio after Na2EDTA and Na4EDTA cleaning PVDF microfiltration membrane that was fouled by CaSO4 were done. By range analysis and variance analysis, the effect of concentration, pH and pressure on the cleaning property of Na4EDTA to the fouled membrane were high remarkable, but, time and temperature did not affect much. The optimum condition was that Na4EDTA concentration is 4%wt, operating pressure is 0.1 Mpa and pH is 11. In the process of Na2EDTA cleaning the fouled membrane, concentration and time had high remarkable effects, pH and pressure had remarkable effects but temperature had not obvious effect. The optimum condition was that NaiEDTA is 4%wt, cleaning time is 40min, operating pressure is 0. IMpa and pH is 11.Integrating the dissolving experiments of Na2EDTA and Na4EDTA and the orthogonal experiments, we can conclude that the dissolving property of Na^EDTA is relatively more stable and higher than that of Na2EDTA in this experiment. And, the conclusion can be made that for the CaSO4 scale in membrane Na4EDTA has the best cleaning effect and the optimum condition is pH=11, operating pressure=0.1Mpa and concentration=4%wt.