Studies On The Influence Factors Of Ultrafiltration Process Of Constant Pressure In Wastewater Treatment And Evaluation Of Membrane Fouling Characteristic

The membrane separation technology has obtained rapid development in water and wastewater treatment. Concerning the insurance of the water quality, controlling membrane fouling is the key to stable and lasting permeability performance, which has become the core of membrane separation technology research. Only if the membrane fouling has been controlled effectively, can operation cost of membrane separation technology be reduced, and can a wide application be achieved. In this paper the influences of trans-membrane pressure,on-line cleaning frequency,backwashing pressure,surface washing pressure,recovery rate and coagulation pretreatment as well as operation mode(constant pressure mode and constant flux mode)on ultrafiltration process were investigated and different ultrafiltration process was evaluated by the membrane structure parameter model in the condition of treating organic wastewater. The main results were shown as:(1) By investigating the influence of operating conditions in constant pressure ultrafiltration process,it can be found that reducing trans-membrane pressure,increasing on-line cleaning frequency,increasing backwashing pressure and reducing recovery rate can slow down the membrane flux decline effectively and inhibit membrane pollution in a certain operation pressure range. However, the change of surface washing pressure played little role in alleviating membrane fouling.(2) Coagulation pretreatment could slow down the decrease of membrane flux and inhibit membrane pollution during constant pressure ultrafiltration process.After filtered by ultrafiltration membrane(MWCO100kDa), organics whose molecular weights were more than 100kDa were mainly removed, but removal rate of organics whose molecular weights were less than 100kDa was low. Membrane fouling was mainly surface fouling of organics whose molecular weights were more than 100kDa in ultrafiltration process.While coagulation pretreatment could remove part of organics whose molecular weights were more than 100kDa,so it could slow down the extent of membrane fouling.Coagulation pretreatment combined with ultrafiltration technology could not only increase the rejection of pollutants in wastewater, but also improve the water quality.(3) By comparison of constant pressure ultrafiltration mode with constant flux ultrafiltration mode with the same filtering time and same water production, the degree of membrane fouling was larger for constant pressure ultrafiltration process, and the larger trans-membrane pressure, the faster the membrane fouling of constant pressure ultrafiltration process which explained constant pressure ultrafiltration process was easily polluted. In the membrane filtration process, constant flux code could reduce membrane fouling in a certain degree and enhance water production.(4) Different ultrafiltration process was evaluated by the membrane structure parameter model.The model equations are(α₁ is membrane pore size impact factor, m^-1;α₂ is membrane pore density impact factor, m^-1):The results shows:â‘ By researching the influences of trans-membrane pressure,on-line cleaning frequency and backwashing pressure on ultrafiltration process evaluated by the membrane structure parameter model,membrane pore size impact factorα₁ and membrane pore density impact factorα₂ decreased gradually with minor trans-membrane pressure, large cleaning frequency and large backwashing pressure, which explained membrane pore narrowing and membrane surface covering and blocking weakened and the degree of membrane fouling was small.â‘¡By researching the influence of surface washing pressure on ultrafiltration process evaluated by the membrane structure parameter model,membrane pore size impact factorα₁ and membrane pore density impact factorα₂ were almost invariant,which explained the change of surface washing pressure played no role in alleviating membrane fouling.â‘¢By researching the influences of recovery rate and coagulation pretreatment on ultrafiltration process evaluated by the membrane structure parameter model, membrane pore density impact factorα₂ decreased but membrane pore size impact factorα₁ increased with small recovery rate and big dosing quantity of coagulant,which explained membrane surface covering and blocking weakened but membrane pore narrowing enhanced,membrane fouling was determined by surface fouling and the degree of membrane fouling was light.â‘£By researching the influences of constant pressure ultrafiltration mode and constant flux ultrafiltration mode on ultrafiltration process evaluated by the membrane structure parameter model,membrane pore size impact factorα₁ and membrane pore density impact factorα₂ of constant pressure mode were bigger,which explained membrane pore narrowing and membrane surface covering and blocking were more obvious and membrane fouling of constant pressure mode was serious.Membrane pore density impact factorα₂ of constant pressure mode was bigger with large trans-membrane pressure,which explained membrane fouling increased faster and constant pressure model was easily polluted.The conclusions of membrane structure parameter model were consistent to test conclusions, which indicated that the membrane structure parametric model was suitable for evaluating the process of membrane fouling.