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Introduction of reverse osmosis membrane separation technology and pretreatment process
In the practical application of reverse osmosis membrane separation technology, membrane pollution is inevitable, and membrane pollution is the decisive factor affecting the stability of the technology. Therefore, it is important to investigate the formation mechanism of membrane pollution and clean the membrane pollution to ensure the normal operation of reverse osmosis system and prevent its failure.
1. Definition of membrane fouling
Membrane fouling refers to the physical and chemical interaction between the particles, colloidal particles or solute macromolecules in the feed solution contacting with the membrane, or the concentration of some solutes on the membrane surface exceeds its solubility due to concentration polarization, and the adsorption and deposition on the membrane surface or in the membrane pore caused by mechanical action cause the membrane pore size to become smaller or clogged, resulting in irreversible changes of membrane permeability and separation characteristics Phenomenon.
The main reason for the flux attenuation is that the adsorption of pollutants, especially proteins, on the membrane surface and in the membrane pores leads to the decrease of flux and separation ability. However, the flux attenuation caused by membrane fouling is often mixed with the reversible flux decrease caused by concentration polarization, which further reduces the membrane separation efficiency.
2. Causes of membrane fouling
During the operation of reverse osmosis system, metal ions, microorganisms, insoluble precipitates, organic pollutants, biological slime, colloid, grease and so on in the wastewater (sewage) contact with the membrane for a long time, which will cause membrane pollution, significantly reduce the membrane flux and separation performance, and increase the pressure drop. The reasons mainly include the following aspects:
1) Concentration polarization
In the reverse osmosis desalination system, the selective permeability of reverse osmosis membrane makes water molecules continuously pass through the membrane from the high pressure side, while the solute molecules remain in the original solution, resulting in a concentration difference between the feed liquid on the membrane surface and the imported feed liquid, and in serious cases, a high concentration gradient will be produced. This phenomenon is called concentration polarization. Concentration polarization increases the osmotic pressure of the feed and decreases the effective driving force, resulting in the decrease of water permeability and desalination rate.
2) Ion scaling
The salts with small solubility product such as CaCO3, CaSO4, BaSO4, SrSO4, CaF2 and SiO2 may precipitate due to concentration exceeding their solubility product, resulting in sediment remaining on the membrane surface and in the water inlet channel to form scale. For example, the solubility product of CaCO3 is 8.7 × 10-9 (25 ℃), that is, when [Ca2 +] · [CO32 -] is greater than 8.7 × 10-9, CaCO3 will precipitate. J. H. bruus et al. Found that when Ca2 + was extracted from sludge, the number of small particles and filtration resistance increased.
3) Metal oxide deposition
Generally, some well water sources containing low valent iron ions and manganese ions in the brackish water range have certain reducibility. The main reason of membrane fouling caused by such water sources is that iron, aluminum and manganese produce colloidal particles on the membrane surface. The pH value required for iron oxidation is low, which makes the iron fouling and blocking in reverse osmosis system more frequent. The related pollutants causing the deposition of soluble ferrous and trivalent iron on the membrane surface may be as follows: oxygen enters the influent containing ferrous iron; FeCO3 is formed from high alkalinity water source; iron reacts with silicon to form insoluble ferric silicate; the oxidation of iron reducing bacteria will aggravate biofilm breeding and iron scale deposition; colloidal iron caused by the transformation of ferric flocculant; iron, aluminum and manganese After metal pollution, the water production decreased and the pressure difference increased.
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