Domestic drinking water softening treatment method 2
On the one hand, the raw water is increasingly polluted, on the other hand, people have higher and higher requirements for drinking water quality. With the improvement of membrane separation technology, the membrane softening process is getting more and more attention. Although the construction and operation costs are higher than lime softening, membrane softening can produce high-quality drinking water. Even if the future water quality standards are further improved, the membrane method can still meet the requirements. The advantages of membrane softening are quite obvious. Membrane softening can process raw water of different water quality, and can produce drinking water of various qualities according to people’s requirements. Lime softening treatment cannot meet future needs. When constructing a water plant, membrane technology should be selected in the long run. But the price of the membrane is relatively expensive at present. As the price of the membrane decreases, membrane softening will be widely used. Some high-hardness waters also contain high chroma and high natural organic matter content. For these raw waters with poor water quality and softening, membrane softening should be considered. At the same time as the membrane softens, the prerequisites for color, total organic carbon, and trihalomethane (THMFP) ) Can also be effectively removed. Membrane softening is suitable for areas where the feed water dumplings are poor and the requirements for softening water quality are high. Membrane softening has the advantages of no regeneration, no sludge generation, complete removal of organic matter, simple operation, and small footprint, which is impossible for other softening processes. Comparable. Commonly used membrane technologies include electrodialysis, nanofiltration and reverse osmosis.
When reverse osmosis is softened, all calcium and magnesium ions in the water can be basically removed. Drinking completely softened water will be harmful to human health. When reverse osmosis softening is used, part of the water bypasses the reverse osmosis device, the other part is subjected to membrane treatment, and then the two parts are mixed. The ratio of the two is called the mixing rate. By adjusting the mixing ratio, the hardness of the effluent can reach the desired value. In two rural areas in Florida, USA, a hybrid method was used when the reverse osmosis membrane was used to soften well water. The two reverse osmosis membrane separation water plants mainly differ in their well water quality and effluent quality objectives. Site A uses aquifers with lower hardness and shallow total dissolved solids, and the effluent water quality requirements are lower. Therefore, larger water is allowed to bypass, the mixing rate is high, and the cost of water production is reduced. The two water plants have similar annual operating costs, but the A water plant has higher water production. The third case is the use of a low-pressure softening membrane, which is sufficient to soften water, remove disinfection by-products, reduce total dissolved solids, and remove all chromaticity to meet drinking water quality standards. Due to the lower pressure and the higher hardness of the effluent from the membrane, the mixing rate is also lower. The total processing cost is close to the first two. With a lower mixing rate, more raw water passes through the membrane treatment, which can better control the prerequisite of trihalomethane.
Electrodialysis occupies an important position in the field of membrane separation and is the main production method of drinking water in some places. When using electrodialysis to reduce the hardness of water, attention should be paid to the pretreatment process. First, alkaline chemicals should be added to remove part of the non-carbonate hardness in the water, so that the hardness of the water after pretreatment basically meets the requirements of drinking water to reduce membrane surface junctions. dirt. In the pretreatment, the turbidity and chromaticity of the water must be reduced to meet the water quality requirements of electrodialysis, reduce membrane pollution, and extend the life of the membrane. In electrodialysis, do not remove all calcium and magnesium ions in the water. The completely softened water is not suitable for drinking. Usually the hardness of drinking water is 170 degrees. It is the best for the human body. In order to prevent fouling on the electrolysis membrane, electrodialysis can be used in the mode of frequent pole reversal, and the discharged pole water can be neutralized by alkaline solution. Electrodialysis is a deep treatment technology, which can reduce the total solid content of water while reducing the hardness.
Nanofiltration membrane belongs to organic polymer nanotechnology and is most suitable for softening drinking water. The nanofiltration membrane has a loose surface layer structure. Due to the positive and negative groups of amino and carboxyl groups in the membrane, it has a high removal effect on low concentrations of salts, and can achieve a higher water flux at a lower pressure (0.5 ~ 1Mpa). The total salt removal rate of the nanofiltration membrane is 50-70%, and the removal rate of Ca2+, Mg2+ and SO42+ is particularly high. It is suitable for the softening of water with high SO2+ content in the purification of water. Nanofiltration retains the needs of the human body. Harmless sodium potassium and other salts. Nanofiltration inlet water requires almost no turbidity, and generally requires SDI ≤ 3 for inlet water, so it is more suitable for softening groundwater with high hardness. Since nanofiltration removes most of the hardness, the outlet water will cause certain corrosion to the pipe network . After the membrane treatment, attention should be paid to the subsequent treatment, including chlorine disinfection, removal of chlorine dioxide, hydrogen sulfide and other gases to control corrosion.
The membrane softening process should select different softening membrane components according to different influent water quality, product water quality and water quantity requirements, etc., and establish the operation process flow and parameters. The pretreatment requirements for membrane softening are strict, usually through multiple pretreatment processes such as pre-precipitation, coarse filtration and fine filtration. In view of the high hardness caused by CaCO3 over-foaming and the tendency to form scales during the operation of the nanofiltration membrane, due to the construction party, it is fully automatic and easy to manage. Compared with the limestone softening water plant, the membrane separation water plant can provide better Water quality, membrane can remove Giardia, Cryptosporidium, radionuclide, sulfur, nitrogen, metals, most organic carcinogens. As chlorinated disinfection by-products and Cryptosporidium and other water-borne bacteria appear in drinking water, the membrane softening method is more preferred. Membrane purification plants are mainly of small water volume, but currently the maximum water production of membrane softening can reach tens of thousands of tons per day.
Various softening treatment methods have their own characteristics. The softening treatment is affected by many factors. The pretreatment of the lime coagulation method is simple, but the effluent quality is not as good as the membrane softening and the management is more complicated. The ion exchange softening method and membrane softening require pretreatment Strict and costly. If the membrane softening method does not bypass and the effluent quality is the same as that of lime softening, the operation and maintenance costs are lower than the lime softening method.