Study On Surface Modification Of Hydrophobic PVDF Hollow Fiber Membrane In MBfR

Membrane Biofilm Reactor (MBfR) is a novel type of sewage treatment technology, which combined membrane aeration with conventional biological wastewater treatment technology. The hollow fiber membrane, as the oxygen transfer media and microorganism attachment carrier in MBfR system, is the most important factor to influence the performance (oxygen mass transfer performance, antifouling properties and biological affinity) of MBfR. Besides, the shortage of special membrane for MBfR has become a limitation of the application and development of MBfR. In this paper, the surface modification technology was taken to change the intrinsic properties of PVDF hollow fiber membrane. The properties of this new composite membrane were analyzed, and the best coating condition was fixed. The further research about stability and antifouling properties of PVDF membranes was also taken.Aiming at the shortcomings of hydrophobic PVDF hollow fiber microporous membrane, such as insufficient oxygen supply capacity and poor antifouling properties, surface modified composite membrane was prepared used self-made hydrophobic PVDF hollow fiber membrane by the method of self-polymerization of L-DOPA in this thesis. The effects of L-DOPA concentration, polymerization time and heat treatment temperature on the performance of composite membranes were investigated, of which contact angle and oxygen,mass transfer performance were the main evaluation indexes. It showed that, under the conditions of L-DOPA concentration 1.54 g/L, polymerization time 3.0 h and heat treatment temperature 37.0℃, the modified membrane had similar mechanical strength compared with the original one, and its oxygen transfer coefficient improved from 9.54×10-3 min-1 to 1.61×10-2 min-1,1.70 times as much as the original; results of the contact angle and the morphology of membrane surface indicated that, the surface hydrophilic of the PVDF/pDOPA modified membrane was improved (contact angle decreased from 75.4° to 41.4°) and the roughness was increased from 128.5nm to 222.1nm.PVDF/pDOPA modified membrane was handled by ultrasound or soaked in acid and alkali solution, then the basic properties and oxygen mass transfer performance of composite membrane were investigated to characterize the stability of the membrane. The results showed that the composite membrane after ultrasounds still had the very good hydrophilic properties; ultrasound 10,20 and 30 min, the modified membrane KLa attenuation coefficients were 4.89%,9.76% and 12.20% respectively. The modified membrane after ultrasounds,KLa was still higher than that of the original, respectively 1.64,1.55 and 1.51 times, which showed that the PVDF/pDOPA composite membrane had good stability. Acid or alkaline analysis of modified membrane showed that the stability was weak under strong acid/alkali, but relative stable under mild condition. When pH 12 and 8, the modified membrane KLa attenuation rates were 27.39% and 6.70%, respectively.Bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) were selectedto do membrane fouling experiments. Extended DLVO theory was adopted to quantitatively evaluate interface forces between each type of organic matter and PVDF original membrane and PVDF/pDOPA modified membrane. Pollution behaviors of these three organic pollutants to permeable membranes in MBfR were investigated. The results showed that, the original membrane polluted by the same organic matter was more serious than the modified one, SA on membrane fouling was the most serious, BSA followed and HA the lightest under the same operation conditions. Electrostatic layer interaction appeared to be the weakest among the three basic interfacial interactions, thus contributing the least to membrane fouling. However, van der Waals interaction and polar interaction varied, Van der Waals force favored the membrane fouling, whereas polar force could prevent it, which indicated that the polar force of membrane surface could increase by surface modification methods. Comparative analysis between interfacial free energy and fouling extent revealed that XDLVO theory can reasonably predict fouling behaviors between the three main organic matters of EPS and permeable membranes in MBfR.