Effects Of Pretreatments On Ultrafiltration Purification Efficiency And Membrane Fouling

In recent years,the consumption of waste paper has a growth,and the proportion of secondary fiber paper pulp has increased year by year.However,the pulp and paper industry is facing great challenges in treating and reusing secondary fiber wastewater.Membrane technology has received great attention due to its outstanding retention performance and easy operation.However,membrane fouling has limited the large-scale application of membrane technology in the field of water treatment.The use of suitable pretreatment technology can not only improve the removal efficiency of organic matter in the filtration process,but also control membrane fouling efficiently,prolong the membrane lifetime and reduce the operation and maintenance cost of the membrane process.In this paper,coagulation and Fenton were used as pretreatment processes.Then the effects of pretreatments on purification efficiency and ultrafiltration membrane fouling were systematically investigated,and the influence mechanisms were discussed.Firstly,taking the secondary fiber secondary effluent from a papermaking mill in Guangdong as the research object,the Taguchi design experiment method was used to carry out coagulation pretreatment and Fenton pretreatment respectively and to optimize the pretreatment operating conditions.The optimal dosage of coagulation reagent and Fenton reagent were determined by chemical oxygen demand?COD?removal efficiency,color removal efficiency and total suspended solid?TSS?removal efficiency and the pretreated water samples were treated by ultrafiltration subsequently.The results showed that the optimal operating conditions for coagulation pretreatment were:poly aluminum chloride?PAC?dosage 1000 mg/L,cationic polyacrylamides?CPAM?dosage 20 mg/L,pH 9 and temperature 25°C.At the optimal conditions,the COD of the ultrafiltration effluent was declined to 95mg/L,TSS was 2 mg/L and colority was 19 Units PtCo.The optimal operating conditions for Fenton pretreatment were:FeSO₄ dosage 8 mg/L,H₂O₂ dosage 6 mg/L and pH4.45.At the optimal conditions,the COD of the ultrafiltration effluent was 60mg/L,TSS was0 mg/L and colority was 12 Units PtCo.Secondary fiber wastewater recycling could be realized by using Fenton combined ultrafiltration treatment process.Then,the membrane fouling mechanism in the ultrafiltration process of water samples after different pretreatment processes was studied.At the beginning of ultrafiltration,the main fouling mechanism was membrane pore blocking mechanism,including complete pore blocking,standard pore blocking and intermediate pore blocking,and in the late stage of ultrafiltration,the main fouling mechanism was cake filtration mechanism.The results showed that compared with the direct ultrafiltration treatment of secondary biological effluent,the pretreatment-ultrafiltration synergistic process could alleviate the decrease of membrane flux and mitigate the trend of membrane fouling.The pretreatment prior to ultrafiltration retarded the transition of membrane fouling mechanism from pore blocking to cake filtration,and Fenton pretreatment had a better retardation effect than coagulation pretreatment.At the same time,the molecular weight?MW?of organic matter in the water samples after different treatments was investigated.The results showed that coagulation pretreatment only reduced the content of high MW organic matter in the ultrafiltration influent,and the removal efficiency on low MW organic matter was limited,while Fenton pretreatment could be simultaneously reduced the content of high and low MW organic matter,which indicated that Fenton pretreatment had superior performance on reducing the ultrafiltration influent pollution load and mitigating membrane fouling.Finally,based on the membrane fouling during the membrane treatment of secondary fiber wastewater,a shear-assisted membrane cleaning method was proposed to recover the membrane flux of the ultrafiltration membrane efficiently.The cleaning performances of ultrafiltration membrane fouling by water rinsing and chemical cleaning were evaluated.The MINITAB data analysis software was used to clarify and optimize the cleaning process and the effect of stirring rate and cleaning temperature on the cleaning performance were studied.The results showed that the optimal operating conditions for the water cleaning process were:stirring rate was 400 rpm,cleaning temperature was 35°C,and the flux recovery rate was53.43%.The optimal operating conditions for chemical cleaning were:sodium hypochlorite,stirring rate was 400 rpm,the cleaning temperature was 45°C,and the flux recovery rate was78.57%.For water cleaning,the stirring rate was the most important condition factor,while for chemical cleaning,temperature is the most important conditional factor,followed by the stirring rate.It was found that sodium hypochlorite was an effective chemical cleaning agent for it could effectively remove the contaminants deposited on the surface of the membrane,improve the hydrophilic properties of membrane surface,and retain the membrane structure under suitable conditions.