Identification Of Specific Organic Foulants And Control For UF Membrane Fouling

The flux decline and the reversibility and irreversibility of membrane fouling caused by various foulants were great different.It is important to identify the foulants responsible for membrane fouling and to improve the performance of membrane filtration.Three-dimensional fluorescence excitation–emission matrix（EEM）was performed for detection foulants characteristic and physicochemical property of water taken from two lakes,one river and one wastewater treatment plant（WWTP）secondary effluent,with the purpose of identifying the major ultrafiltration（UF）membrane foulants in different water sources.Through the correlation analysis between the fluorescence components（Fmax）and the membrane fouling,reversible and irreversible membrane foulants were identified and the dynamic evolution of foulants was also studied to verify its seasonal variation.The results may provide basic formation for developing suitable fouling control strategies for sustainable UF processes.Because the molecular weight distribution of the pollutants may affect the membrane fouling,the NOM in natural water was fractionated into five components to study the effect on the fouling reversibility.Detection technology was performed on the fractions to identify the responsible substances.After recognition of the responsible membrane foulants,coagulation pretreatment was carried out before ultrafiltration to reduce the concentration of typical model foulants which caused membrane fouling.The fouling behavior was studied in the processing of Coagulation-UF to illuminate the influence of pH,Na⁺,Ca²⁺on the membrane fouling,and to explore the best control parameter.The study on foulants characteristic and physicochemical property of water revealed that NOM in natural water was mainly comprised by small-MW foulants and NOM in WWTP was mainly comprised by the high-MW,high-MW foulants contribute to most of membrane fouling.High-MW foulants in WWTP mainly include protein-like and humic-like substance,and in natural water mainly include protein-like substance.High-MW foulants was hydrophilic;Statistical analysis revealed that membrane fouing was not correlated with TOC and UV₂₅₄,TOC and UV₂₅₄which represent total organic matter in water could not use as the index of membrane fouling.Three-dimensional fluorescence excitation-emission matrix（EEM）coupled with parallel factor analysis（PARAFAC）was performed for water samples taken from four water sources（one river water,two lakes and one WWTP secondary effluent）,with the purpose to identify the major UF membrane foulants in different water sources.Three fluorescent components were identified,which represented terrestrially derived humic-like substances,microbial derived humic-like substances,and protein-like substances.The correlations between the different fluorescent components and UF membrane fouling were analyzed.It was shown that for the WWTP secondary effluent,all the three components made a considerable contribution to the irreversible and total fouling of UF membrane.However,for the two lakes and river water,only the protein-like substances exhibited a strong correlation with the membrane fouling,indicating the protein-like substances were the major membrane foulants in the lake and river waters.Significant attachment of C1,C2 and C3 to the UF membrane was also confirmed by mass balance analyses for the WWTP secondary effluent;while the attachment of C1 and C2 was shown to be negligible for the two lakes and river.Low concentration of humic-like substance in natural water has little impact on membrane fouling,but for the WWTP,Humic-like substance exist in the form of large molecules may cause serious membrane fouling.To investigate the influence of different molecular weight distribution to UF membrane fouling,NOM in natural water was fractionated into five components:<5kDa,5-10 kDa,10-50 kDa,50-100 kDa,100kDa-0.45 um based on their size and their potential to develop irreversible membrane fouling（IF）and reversible membrane fouling（RF）was conducted by unstirred cell test.The size fractionation combined with PARAFAC of three-dimensional fluorescence excitation-emission matrix（EEM）of fractions was performed to identify the responsible substances for IF and RF.Meanwhile mass balance analysis coupled with the correlation analysis was performed to acquire the information of organic matter fouling behavior.Based on fouling effect,scanning electron microscope（SEM）was used to mechanism analysis.In five fractions,the fraction of 100kDa-0.45μm which only account for 7.2%DOC in which exist protein-like substances recognized by EEM-PARAFAC contributed to the most membrane flux decline caused by both IF and RF.The fractions of<5kDa which account for 70%DOC and mainly contain humic-like substances and protein-like substances caused little flux decline,but the contribution is irreversible.Mechanism analysis made clear that small molecule substances（<5kDa,5-10kDa）caused irreversible flux decline of membrane due to pore blocking,and big molecule organic（50-100 kDa,100kDa-0.45μm）contribute irreversible and reversible membrane fouling due to pore blocking and cake layer forming.It is important to consider the contribution of<5kDa and 100kDa-0.45μm in choice of pretreatment technology to control membrane fouling.Control of ultrafiltration membrane fouling by pH enhanced coagulation showed that the pH value under 7 could better relieving membrane fouling,improve membrane flux and increase the retention rate of pollutants.Mechanism analysis showed that the membrane fouling control by pH enhanced coagulation was realized by controlling the electrostatic repulsion between pollutants,mechanism of membrane fouling control by pH enhanced coagulation was realized by controlling standard blocking,complete blocking,intermediate blocking and the cake filtration together;Control of ultrafiltration membrane fouling by Na⁺and Ca²⁺enhanced coagulation showed that the low concentration of Na⁺and Ca²⁺could better control membrane fouling,improve membrane flux and increase the retention rate of pollutants.High dosing Na⁺and Ca²⁺was not good for membrane fouling control.Mechanism analysis showed that the removal of pollutants by Na⁺and Ca²⁺enhanced coagulation was realized by control of the polymerization of small molecule organics,mechanism of membrane fouling control by Na⁺was realized by reducing the complete blocking,intermediate blocking and the cake filtration,as to the Ca²⁺,the membrane fouling control was realized by reducing the standard blocking.Control of ultrafiltration membrane fouling by adding PACl coagulant showed that the PACl under optimal dosing of 20 mg/L could better control membrane fouling,improve membrane flux and increase the retention rate of pollutants.By adding PACl,the membrane flux of humic acid was improved by reducing standard blocking,as to the bovine serum albumin,the flux was improved by complete blocking.