Reverse osmosis equipment is currently a widely used desalination technology to produce pure water. The separation objects of reverse osmosis equipment are mainly ions and organics in solution. At present, reverse osmosis equipment is an advanced membrane separation technology in today's society. The principle of reverse osmosis equipment is that under the effect of the osmotic pressure of the solution, other substances cannot pass through the semi-permeable membrane, so that these substances are separated from water. Because the membrane pore size of the reverse osmosis membrane is relatively small (only about 10A), it can remove inorganic salts, bacteria, microorganisms, viruses, colloids, organic matter, etc. in water.
The main causes of reverse osmosis membrane pollution include physical pollution, chemical pollution, and organic pollution. Physical pollution is metal (such as metal oxides such as iron and manganese), non-metals (such as less soluble non-metal salts such as CaCO3, CaSO4, BaSO4) in the original (waste, sewage) water, colloidal pollutants (such as silicon oxide Etc.) Deposition on the membrane surface, clogging the membrane pores: Chemical pollution refers to the harmful chemical components adsorbed by the membrane pores; organic pollutants are formed by the adhesion of bacterial slime, fungi, mold, organic matter, etc. on the membrane surface and in the membrane pores Pollution. Flux attenuation and separation ability reduction caused by the adsorption of pollutants, especially organic substances, on the membrane surface and in the membrane pores of substances that can cause pollution of reverse osmosis membranes are the main reasons for membrane flux attenuation. However, the flux attenuation caused by membrane fouling is often mixed with the reversible flux decrease caused by the concentration polarization phenomenon, which further reduces the membrane separation effect.
Classification of pollutants in reverse osmosis equipment: 1. Suspended solids
Suspended solid pollutants are commonly found in surface water and wastewater sources, and their particle diameters are often greater than 1 micron. Such impurities can be completely deposited when the water flow is in an unstirred state, and it can be easily filtered out by the fine sand filter and the multi-media filter provided by the reverse osmosis equipment. When a flocculant or agglomeration is set in the pretreatment system of the reverse osmosis equipment, In the dosing process of the agent, such impurities can also be adsorbed by the alum flower formed by the flocculant, and then filtered out by the multi-media filter fine or the sand filter.
Colloidal pollutants are also commonly found in surface water and wastewater sources, and their particle diameters are often less than 1 micron. Such impurities will not settle freely even when they are not agitated by the water flow, and will always remain in suspension. Such impurities may be monomer compounds composed of organic or inorganic components, or composite compounds composed of multiple types of compounds. Such as silicic acid compounds, iron-aluminum oxides, sulfides, tannins, humic substances and so on.
Third, biological pollutants
Biological pollutants are also found in surface water or wastewater. When dealing with this type of water source, pollution often starts to be reflected on the membrane element at the front of the reverse osmosis equipment. When such a phenomenon occurs, the pressure in the front section of the reverse osmosis equipment rises quickly, and the desalination rate of the reverse osmosis equipment will also It has improved, but with the continuous operation of the membrane system, biological pollution will gradually spread to the entire reverse osmosis equipment, thereby forming a large area of membrane pollution. When membrane fouling occurs in the membrane system, the operating pressure of the system rises significantly and the water yield decreases. These pollutants are usually bacteria, biofilms, algae and fungi. In general, when designing a reverse osmosis process system, we must pay attention to controlling the activity of raw water. When the bacteria content of raw water is more than 1000cfu / 100mg, removal measures must be considered during design.
Fourth, organic pollutants
The composition of organic matter in raw water is complex, and its pollution and influence on reverse osmosis membrane elements is also very difficult to predict. This index is the main pollutant that needs to be carefully considered when designing reverse osmosis equipment. The sources of organic pollutants are generally divided into two types: one is the natural humic organic matter formed by the decay of animals and plants, and the other is the organic matter formed by the pollution of industrial waste. When organic matter pollutes the reverse osmosis membrane, it is often that the organic matter is relatively strongly adsorbed on the membrane surface, which is more difficult to clean. Generally speaking, it is difficult to predict the pollution of organic membranes on the membrane system: some organic substances in water have almost no damaging effect on the reverse osmosis membrane, but there are individual trace organic substances that can not only pollute the reverse osmosis membrane once they are adsorbed on the membrane surface, It can even cause degradation and degradation of the membrane. Therefore, we must attach great importance to the organic matter present in the raw water. So in principle, when we design a reverse osmosis equipment, when the TOC content of raw water reaches 3mg / l, we must consider the corresponding removal measures in the system. Generally speaking, in the reverse osmosis pretreatment system for treating surface water and wastewater, most of the organic pollutants should be removed or decomposed and transformed during the pretreatment processes such as flocculation, clarification and oxidation. If the water inflow requirements of the reverse osmosis equipment cannot be met after the above treatment, you can consider further removing them through the settings of activated carbon adsorption filters, organic matter cleaners or ultrafiltration equipment to meet the water ingress requirements of the reverse osmosis equipment.
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