Characterization Of Nanofiltration And Forward Osmosis Filtration Of Dissolved Organics

Membrane technology is widely used in water treatment engineering.In this thesis,the pollutants in the model of compressed membrane and non-compressed membrane filtration and the membrane pollution of natural surface water are taken as the starting point,and the similarities and differences of the two membrane technologies in the water treatment process are compared through the analysis of the characteristics of dissolved organic matter.Molecular weight distribution is an important index to evaluate membrane fouling.HPSEC-UV-TOC combined technology can realize continuous measurement of molecular weight distribution,truly reflect the physical and chemical properties of dissolved organic matter,and can effectively characterize the molecular weight distribution of natural surface raw water and its components.This technology plays an important role in the study of organic components in surface sources of drinking water.HPSEC-UV-TOC combined with three-dimensional fluorescence spectrum and parallel factor analysis can describe the properties of dissolved organic compounds and their components more comprehensively and objectively.Pressurized membrane represented by nanofiltration membrane is widely used in drinking water treatment.This topic investigated the purification efficiency of nanofiltration membrane on model pollutants tannic acid and dextran(T2),and studied the physical and chemical factors affecting the flux and retention of nanofiltration membrane.Finally,the effect of nanofiltration membrane on the filtration of raw water was investigated.It was found that the initial flux increased with the increase of external pressure.At the same pressure,the membrane flux of dextran was greater than that of tannic acid.Under alkaline conditions,the membrane filtration effect of tannic acid and dextran(T2)is better.The presence of calcium ions can aggravate membrane fouling.Under the same concentration of calcium ions,the fouling of dextran is less than that of tannic acid.With the increase of initial pollutant concentration,membrane flux attenuation becomes more serious.With the increase of cross flow velocity,the membrane flux also increases,and the initial flux increases linearly with the increase of cross flow velocity.The effect of calcium ions on the filtration of tannic acid in nanofiltration membrane was greater than that of pH value,while the effect of dextran(T2)was the opposite.Nanofiltration membrane has a good effect on the removal of dissolved organic compounds in various molecular weight ranges of natural surface water.The above experimental results provide effective data for the treatment of surface water by nanofiltration membrane,which is beneficial to the further study of nanofiltration for the removal of dissolved organic matter.Forward osmosis membrane is a new technology for water treatment in recent years.This thesis investigated the purification efficiency of forward osmosis membrane on dextran(T2),and studied the physical and chemical factors affecting the flux and interception effect of forward osmosis membrane.Finally,it investigated the filtration effect of forward osmosis membrane and nanofiltration membrane,as well as the flux change of filtered raw water under NF-FO-NF mode,and the results were as follows: In AL-DS mode,increasing the cross-flow velocity can only clean the surface of the supporting layer of the film,but not the blockage of the film hole inside the supporting layer.Tannic acid mainly clogs the inside of the supporting layer,while dextran mostly adsorbed on the surface of the membrane to form a loose cake layer.The membrane fouling is more serious when the concentration of contaminants is high,acid conditions and calcium ions exist.Both nanofiltration and forward osmosis can effectively remove dissolved organic matter in raw water,and the pollution inside the supporting layer of forward osmosis membrane is more serious.Compared with nanofiltration,forward osmosis in Al-DS mode has a poor effect on the removal of small and medium-sized molecular weight organic compounds in raw water.In the NF-FO-NF mode,the shear force generated by cross-flow in the second stage(FO)has limited effect on removing the fouling layer.The above experimental results provide effective data for the treatment of natural surface water by nanofiltration membrane and forward osmosis membrane,and provide data support for future research on the removal of dissolved organic matter by nanofiltration and forward osmosis.In this thesis,XDLVO theory was introduced to analyze and predict the decreasing mechanism of membrane filtration flux.According to the calculated adhesion free energy and polymerization free energy,the rapid decrease of flux in nanofiltration membrane is mainly due to the relatively dense cake layer formed by pollutants on the membrane surface,while the rapid decrease of flux in forward osmosis membrane is mainly caused by the pollution inside the supporting layer.The influence of dissolved organic matter on nanofiltration and forward osmosis membrane fluxes in raw water is mainly affected by organic-membrane and acid-base interaction between organics and organics.