Study On The Membrane Fouling And Fouling Controlling In The Direct Contact Membrane Distillation

Membrane distillation (MD) is a relatively new separation technique which has being developed for desalination and concentration of thermally liable substances. It has a good prospect in water treatment. The formation of deposit on the membrane surface (fouling) is a problem to be solved urgently in membrane distillation, but it is restricted within narrow limits. Recently, in this field, membrane fouling and fouling controlling is the hot research.The first part of this study chose humic acid molecule as object of analyzing, and the membrane fouling was analyzed by direct contact membrane distillation (DCMD). The concept of "the total transfer resistance" was introduced to the developed model. Combining with Heat transfer and Mass transfer in direct contact membrane distillation (DCMD),"the total transfer resistance" was divided into four parts, and they were the resistance in the thermal boundary layer, the fouling resistance, the membrane resistance and the resistance in the thermal boundary layer respectively. Moreover, there was a expression about the fouling resistance. The flux decline with time was studied as the keystone in the concentration of the humic acid suspension by direct contact membrane distillation (DCMD) at different operating conditions and different membrane material. At the same time, based on the experiments, the analysis of fouling resistance with time was carried out. The obtained results indicate the growth of fouling resistance during concentrating is greatly affected by the feed temperature, the feed flow rate, the initial particle concentration of the feed and Calcium ions. Besides, in order to limit the membrane fouling, washing of the branch water after DCMD was investigated. It was found that longer washing time and larger washing flow rate contribute to greater membrane flux recovery.The second part of this study investigates the effect of gas-backwashing on fouling control in concentrating the extract of traditional Chinese medicine (TCM) by direct contact membrane distillation (DCMD). And the influences of gas flow rate, gas-backwashing duration and the intermission time of gas-backwashing on fouling limitation were observed. The results revealed that the gas-backwashing can control the membrane fouling whatever the particles deposition caused obviously. Longer gas-backwashing duration, higher gas flow rate and shorter intermission time cause better effect in fouling control; The gas backwashing could maintain the membrane flux up80%of tap water membrane flux at gas-backwashing duration2min, gas flowrate2L·min-1and the intermission time of gas-backwashing as20min over the concentrating process lasting250min. Besides, according to the production capacity and efficiency of membrane by backwashing, the optimization of gas-backwashing duration was rensoned. The efficiency of membrane by gas-backwashing was calculated. The different influence factors on result was discussed.