Studies On Establishment And Evaluation Of Ultrafiltration Membrane Fouling Model Based On Membrane Structure Characteristics And Features Of Organic Matter In Water

This paper took membrane pore density and membrane pore size of UF membrane as membrane structure parameters, adopted a field emission scanning electron microscope (FESEM) to examine membrane structure parameters. In the experiment, a membrane structure model that revealed the changes of membrane structure parameters during UF water treatment was established and a systematic study was conducted to examine how the different pretreatment methods of ozonization, coagulation, powder activated carbon (PAC) adsorption, and solution environment conditions in water, the quality of membrane material, operational conditions and running modes (constant flux or constant pressure) influenced the UF water treatment process. Then the established model was used to evaluate the change characteristics of membrane structure parameters. The main results included:(1) FESEM and the professional image analysis software were adopted to analyze UF membranes. Then parameters of UF membrane density, the mean pore size and the porosity on membrane surface were figured out.(2) According to the basic rationale of hydrodynamics, the study took membrane pore density and the mean pore size as membrane structure parameters and introduced two evaluating factors in the case of filtration modes of constant flux and constant pressure: membrane pore size changing coefficien a₁, and membrane pore density changing coefficient a₂. Then the membrane structure parameter model used to evaluate UF water treatment process was established. The model equations were: (3) UF membrane structure parameter model was simulated and checked in the light of the experiment data; the results showed that the model could preferably simulate UF water treatment process. In the process of UF water quality purification, model parameters a₁ and a₂ could evaluate organic matter in water was dominated by membrane pore interior adsorption or membrane pore blocking on membrane surface, and the organic matter in water was mainly influenced by which water quality characteristics of molecular weight size or distribution of organic matter in water, the hydrophilicity or hydrophobicity of organic matter, or ionic strength. It provided a method to evaluate UF membrane fouling process when membrane structure characteristics were combined with raw water properties.(4) The molecular weight distribution of organic matter in secondary effluent was changed with preozonization, and PVDF-300 flat UF membrane was adopted to filtrate the secondary effluent with different molecular weight distribution of organic matter. The study found the relationship between molecular weight distribution of organic matter in water and UF membrane resistances in the UF process was: (i) when 1/3>d/φ>2/15 (d: molecular size of organic matter in water,φ: membrane pore diameter), the organic matter tended to be adsorbed inside membrane surface, when d/φ<2/15, the organic matter could easily permeate the membrane pore; (ii) when 1>d/φ>1/3, the organic matter mainly caused the blocking on membrane surface and inside the membrane pore; (iii) when d/φ>1, especially when d/φ>5/3, the organic matter (including dissolved and granule one) mainly formed membrane surface cake. Meanwhile, the experiment found among the UF membrane resistances, the pore block resistance had great influence on membrane fouling resistance. When the ozone contact time was 1 min, membrane pore block resistance obviously decreased and membrane fouling could be effectively slowed down. (5) Coagulation, powder activated carbon (PAC) adsorption, ozonization-PAC adsorption were respectively adopted to pretreat the secondary effluent, and the influence of different pretreatment methods on UF membrane permeability was examined: (i) consulting membrane structure parameter model, the study found during the UF process of the secondary effluent, a₁ and a₂ were high which indicated that membrane mean pore size and membrane pore density decreased quickly, namely, the secondary effluent contained a lot of matter with small and high molecular weight; (ii) after coagulation treatment, a₁ changed slightly, a₂ became smaller, which showed membrane mean pore size reduced quickly, while membrane density reduced slowly. That is to say, coagulation could remove an amount of high molecular matter, while it could remove a small amount of small molecular matter; (iii) after the treatment of PAC adsorption and ozonization-PAC adsorption, the adsorption of PAC to low molecular matter reduced the change of membrane mean pore size, and ozonization reduced the high molecular organic matter in the secondary effluent, the reduction of membrane pore density became gentler.(6) In the secondary effluent, the molecular weight distribution of hydrophilic and hydrophobic organic matter was similar, and there was a large amount of organic matter with molecular weight less than 2kDa. The UF experiment to 3 kinds of water samples of organic matter with different features showed hydrophilic organic matter was the main material causing membrane fouling. Moreover, the stronger the ionic strength in the secondary effluent, the quicker the membrane flux declined, and its decline degree increased with the increase of ionic strength.(7) In this experiment, 6 kinds of UF membranes with typical material were adopted to filtrate the secondary effluent and the influence of different membrane material properties on UF permeability was studied. The study found that membrane fouling was mainly caused by membrane surface block fouling which was produced by large molecular organic matter. Meanwhile, the wider the membrane pore size distributed, and the more regular the membrane pore shape was, the easier membrane surface fouling occurred. It resulted in the decrease of membrane pore density, at the initial stage of filtration membrane flux quickly declined. The thicker the membrane filtration layer, the more irregular the membrane pore shape was, the easier the inside membrane pore fouling occurred. It resulted in the narrowing of membrane pore size and membrane flux declined fast in the anaphase of filtration.(8) In the case of membrane fouling characteristics in UF water treatment process, the study found when conducting proper pretreatment on raw water, changing the molecular weight distribution of organic matter in water, meanwhile, adopting the filtrating modes of low pressure and constant flux and combining corresponding backflushing with fast membrane surface washing, then we could achieve the purposes of controlling membrane fouling, slowing the decline of membrane flux, improving the membrane permeability and prolonging membrane life-span.