| As is one of the most important branches of membrane science, ultrafiltration technology is now widely used in fields of medicine, food, metallurgy, seawater desalination, wastewater recycling,etc. with low energy consumption, high efficiency, simple operation, no phase transition during separation process and other advantages, which make it one of the most promising water treatment technologies. However, its applications are significantly limited by membrane fouling inevitably occurred in the process. Therefore, the study of ultrafiltration membrane cleaning technology is of tremendous importance for the broader application. Nowadays physical and chemical cleaning methods are the most commonly used approaches against ultrafiltration membrane fouling. The latter is effective in removal of variety of pollutants, but may also damage the membrane and cause secondary pollution due to the use of chemical reagents while the former is mostly applicable to the early prevention of pollution. Thus, a gas/liquid two-phase flow cleaning method was put forward and its performance on cleaning membrane modules fouled in the complexion-ultrafiltration process was systematically investigated in this study.With the development of industry, the pollution caused by waste water containing heavy metal is getting increasingly worse. The heavy metals are usually difficult to degrade and will end up with bio-accumulation, which eventually bring great harm to creatures and human beings. Conventional technologies for the treatment of wastewater containing heavy metal are chemical treatment method, ion-exchange process, adsorption method and so on. The complexation-ultrafiltration technique is a new method developed in recent years to remove heavy metal in wastewater. However, the membrane fouling issue also exists in the process. Removal of Ni2+ in simulative wastewater by complexation-ultrafiltration technique and effects of operation parameters on complexation-ultrafiltration were studied. Taken the ultrafiltration membrane modules fouled in the above process as research subjects, this study also makes a series of comparisons of flux recovery among different cleaning methods, including gas flushing, water backflushing, chemical backflushing and gas-liquid two-phase flow with either water or chemical as cleaning solution.Firstly, complexation-ultrafiltration process was carried out to remove Ni2+ in simulated wastewater with PAAS as the complexing reagent. Effects of metal concentration, feed velocity, operation pressure and operation time on flux, Ni2+ retention and resistance distribution were studied respectively. The results showed that Ni2+ retention remained almost the same when feed concentration increased and increased slowly when operation pressure increased. Flux at steady state decreased when feed concentration increased and increased with operation pressure increasing. Total resistance (RT), fouling resistance (Rf) and polarization resistance (RP) all increased obviously with both feed concentration and operation pressure increasing, among which, Rf increased most significantly at high feed concentration and RT increased most significantly at high operation pressure. Compared to RT and Rp, Rf increased much slower when the operation pressure increased up to a much higher range (Δp=0.2MPa) and it showed similar trends at a higher feed concentration. RT, Rf and RP values all decreased when feed velocity increased, which showed that the increasing of feed velocity probably have decreased polarization and membrane fouling.Secondly, cleaning performance of different cleaning methods, including gas flushing, water backflushing, chemical backflushing and gas-liquid two-phase flow with either water or chemical as cleaning solution were compared. The results showed that flux recovery rate of the four cleaning methods all increased, among which the gas/liquid two-phase flow method increased the most and the fastest (15min-25min). This indicated that increasing cleaning time was more effective to enhance cleaning performance of gas/liquid two-phase flow method. Furthermore, effects of gas pulse time (tc), gas pulse interval (ti), velocity ratio of gas to liquid (Rg/l) on gas/liquid two-phase flow performance were investigated. The results showed tc had similar impacts on gas/liquid two-phase flow method with either water or chemical as cleaning solution, that is, the increase of tc at a certain range could promote cleaning performance of this method, but also would cause a decrease of flux recovery when tc was too long. When Rg/l was 20:1, the flux was recovered most with ti at 20s for gas/liquid two-phase flow method with either water or chemical as cleaning solution. Meanwhile, flux recovery rate decreased when cleaning frequency increased. Effects of gas/liquid ratio on cleaning performance with either water or chemical as cleaning solution were different. When tc was 15s and ti was 10s, the two-phase flow cleaning method with water as cleaning solution could recover the flux most with Rg/l=80:1 and the two-phase flow cleaning method with HCl as cleaning solution could recover the flux most with Rg/l=40:1. Results also indicated that it would not promote cleaning performance with too high or too low gas/liquid ratio.Finally, Ni2+ removal performance of complexation-ultrafiltration was studied with simulated wastewater containing Ni2+, in which the background water was collected from rivers flowing through Shanghai Jiaotong University, Minhang Campus. Cleaning performance of four different cleaning methods (gas flushing, water backflushing, chemical backflushing and gas-liquid two-phase flow with either water or chemical as cleaning solution) was compared. The results showed that Ni2+ retention was higher and flux at steady state was much lower compared to simulated wastewater prepared with ionic water. For all the cleaning methods, flux recovered notably at the initial time and the flux recovery rate decreased when cleaning cycles increased. With the condition of the same cycle of cleanings, the cleaning performance of gas/liquid two phase flow method was best and the two-phase flow cleaning method with water as cleaning solution recovered the flux most. When temperature was 25°C, the operation pressure was 0.1 MPa, cleaning time was 15min, tc was 15s and ti was 20s, increasing gas/liquid ratio could promote flux recovery rate of two-phase flow cleaning method with water as cleaning solution significantly with the maximum rate was up to 92%, which was better than that of two-phase flow cleaning method with HCl as cleaning solution. Flux recovery rate increased remarkably and could reach up to 92.8% when NaOH was used as cleaning solution, which indicated alkali condition was better for the removal of pollutants.Compared with cleaning methods of gas flushing, water backflushing, chemical backflushing, the gas/liquid two-phase flow cleaning method can effectively remove ultrafiltration membrane foulants in the complexion -ultrafiltration process and could be called a green membrane cleaning method because of less chemical used. |
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