Ultrafiltration Membrane Fouling Alleviation Mechanism Based On The Membrnae Modules Rotation

In recent years,membrane technology has been widely used in drinking water treatment.However,membrane fouling is inevitable due to the pollutants adsorbed into membrane pores or arrived to the membrane surface.To effectively alleviate membrane fouling,traditional coagulation plays an important role because of the excellent removal efficiency of pollutants and loose flocs formed.Up to now,at least three pretreatment technologies have been investigated to combine coagulation and membrane filtration: pre-adsorption,direct-filtration and integrated filtration.Owing to the small land use and excellent membrane performance,most studies have been focused on integrated filtration in recent years.However,a key problem is that ultrafiltration(UF)membrane modules are always static in membrane tank,resulting in the gradual development of cake layer and serious membrane fouling.In this work,UF membrane module was rotated during filtration owing to the loose flocs.Based on the computational fluid dynamics model,the fouling alleviation mechanism was proposed with different rotation speed and frequencies.It has been shown that the transmembrane pressure(TMP)induced by 20 mg/L HA significantly increased to 63.8 kPa after running for 12 days.With alum-based or iron-based flocs injected into membrane tank,the chance of HA getting to the membrane surface dramatically decreased,resulting in the reduction of membrane fouling.The TMP was reduced to 38.2 kPa and 30.4 kPa,respectively.When membrane module was rotated at 24 rpm,turbulent was induced in membrane tank.In addition,the outer of cake layer was loose and was easily broken,leading to a thinner cake layer and slighter membrane fouling.The corresponding TMP was further reduced to 4.1 kPa and 17.2 kPa,respectively.Rotation speed and frequency played an important role in influencing membrane behavior due to the variation of cake layer thickness and floc properties.Although the larger of rotation speed and frequency,the thinner the cake layer,the slighter the membrane fouling,the rate of membrane fouling alleviation decreased with increasing the rotation speed and frequency.It has been found that the membrane behavior was almost the same with the rotation frequency of 6h/d and continuous rotation at 24 rpm.In comparison to alkaline,more HA molecules were adsorbed because of the smaller floc size and higher positive charge.As a result,membrane fouling was alleviated to a large extent.In the above mentioned,raw water taken from south-to-north water was used to further test the membrane behavior.The results showed serious UF membrane fouling was caused,and TMP significantly increased to 87.1 kPa.When the membrane model was rotated 6 h/d at 24 rpm,the membrane fouling was also dramatically alleviated with flocs injected into the membrane tank and solution pH adjusted to 6.0.The TMP was further reduced to 14.4 kPa and 34.2 kPa.Based on the outstanding UF membrane performance,membrane module rotation shows considerable potential for application in water treatment.