China pure water equipment reverse osmosis system pretreatment and design(1)

Design of reverse osmosis pretreatment system for insoluble inorganic salts in water

  • Ion exchange softening: This process is often used when the system does not choose to add organic scale inhibitors and the raw water has a low hardness content and a certain barium and strontium ion content in the water source. Generally speaking, this process is currently used in the pretreatment system of small reverse osmosis units and the reverse osmosis purified water preparation system for drinking water purification.
  • Lime softening auxiliary adding magnesium agent: This process is often used in large reverse osmosis systems with high carbonate hardness and dissolved silica content in raw water. Generally speaking, this method can reduce the carbonate hardness of the raw water to about 100mg/l, and at the same time, the dissolved silica content in the raw water can also be removed by about 50-60%. This process is mostly used in the treatment of surface water and industrial circulating water with poor water quality.
  • Metered dosing of scale inhibitor in feed water: Because this process has strong applicability to raw water and site conditions, easy automatic control, and reliable device operation, it is used in large-scale reverse osmosis systems and systems with high content of insoluble inorganic substances in raw water. used widely. At present, in the newly built reverse osmosis system, the antiscalants added are mostly imported from abroad. The common feature of these antiscalants is that they are very convenient for dilution and dosing. With high dispersing ability, pharmaceutical manufacturers can even guarantee that no scaling will occur when the LSI or S&DSI index of the R/O concentrated water system is as high as +2.5 to +3.0. In addition, the saturation of CaSO4, SrSO4, BaSO4 and CaF2 can Expanded by 2.3, 8.0, 60, and 100 times respectively; and some scale inhibitors are compatible with the addition of flocculants in the pretreatment system (eg: MDC-150 scale inhibitors and MPT150 ​​flocculants are compatible with each other). In the past, sodium hexametaphosphate, which was often used as a scale inhibitor in China, is being gradually replaced due to its shortcomings such as inconvenient dissolution, temperature influence, instability, and poor dispersion ability. In addition, the phosphate ions and phosphate scale generated by the hydrolysis of sodium hexametaphosphate are likely to become nutrients for microorganisms contained in the raw water, thereby promoting the reproduction of microorganisms in the reverse osmosis system. This is also the reason why sodium hexametaphosphate is being used by users. One of the reasons for the gradual deprecation. No matter which antiscale agent is selected, special attention should be paid to the control of LSI and S&DSI values ​​in the concentrated water system to ensure the safe operation of the system.
  • Weak acid cation exchange dealkalization softening: This method is mostly used in large reverse osmosis systems with high salt content and high alkalinity components in raw water (accounting for more than 70% of anion content). However, after this process, the pH value of the water to be treated is low (4-5), which often results in a low desalination rate of the reverse osmosis system due to the increase in the amount of mineral acid permeation of the reverse osmosis system; even after decarburization The desalination rate cannot reach the original ideal level by adjusting the pH value of the treated water or adopting a process that does not remove carbon dioxide. Nevertheless, this process has been widely used in the water quality conditions of high salinity and high alkalinity.

Design of reverse osmosis pretreatment system with high content of dissolved silicon in raw water

For the design of reverse osmosis pretreatment system operating under such water source conditions, there are generally the following methods:
When the site conditions allow, the temperature of the feed water is adjusted to about 28-35°C through the heat exchanger set in the system, thereby improving the solubility of the silicic acid compound in the water, and combining with the process design of controlling the water recovery rate of the system, to Ensure that no silica scale is formed during the operation of the reverse osmosis system. This is a method often used in engineering. Under such conditions, it should be generally noted that the content of silica in the reverse osmosis concentrated water system should be controlled below 150 mg/l.
The silicon is removed by the method of lime pre-softening and adding magnesium agent (magnesia). This method can remove more than 60% of the silica dissolved in the raw water. In addition, this process is troublesome for users to actually operate, so this process is rarely used in small water treatment systems, but is widely used in large reverse osmosis systems. use.
Add silicon dispersant. At present, due to the superior properties of imported silicon dispersants, this method has been widely used in large-scale reverse osmosis projects in China. In such systems, . Judging from the technical documents and related information submitted by the pharmaceutical supplier, some even allow the content of silica in the reverse osmosis concentrated water system to reach about 240~290PPM in application. However, for a reverse osmosis system designer, the allowable high content of silica in the reverse osmosis concentrated water system in a specific project should be based on the technical indicators allowed by the specific dosing agent and the agent dosing calculation software that meets the site conditions. determined by the simulation results.

Design of reverse osmosis pretreatment system for raw water containing metal oxides

A pre-oxidation process for raw water is set up in the pretreatment system, and then iron, manganese ions and their compounds are removed from the raw water through coagulation, sedimentation, sand filtration or manganese sand filtration and other processes.
In the pretreatment system, adding lime pre-softening, coagulation, clarification, and sedimentation combined treatment process can generally remove most of the metal oxides in the raw water.
The combined pretreatment process of electrochemical coagulation, sedimentation and multi-media filtration can also remove most of the iron oxides in the water.
Add chemical dispersant. While it can effectively prevent the scaling of inorganic salts, it can also prevent the deposition of a certain amount of metal oxides in the reverse osmosis membrane system. Such as adding MDC-200 chemical dispersant from ARGO Company in the United States to the system.

Design of reverse osmosis pretreatment system for raw water containing natural organic matter

In the pretreatment system, the lime pre-softening, coagulation and clarification combined treatment process is set up, and then through the process of multi-media filtration and fine sand filtration to remove the adsorbed natural humic organic matter in the raw water. This process is widely used in large-scale reverse osmosis pretreatment systems for desalination and purification of surface water and circulating water.
The activated carbon adsorption filtration process is set up in the pretreatment system to remove the organic matter remaining in the raw water. This process is often used in small and medium-sized reverse osmosis pretreatment, especially in pure water production and drinking water purification systems.
An organic matter cleaner process is added to the pretreatment system to more thoroughly remove organic matter in the raw water. This process is mostly used in the ultra-pure water system of the electronics industry and the reverse osmosis pretreatment system where the raw water is river, river, and lake water, and the organic content in the water is relatively large or the composition is relatively complex.
Microfilters (0.2μm) and ultrafilters (molecular weight cut-off between 6,000 and 20,000) are used as pretreatment equipment to remove organic matter, and this process is often used in small reverse osmosis systems.
In the pretreatment system, the nanofiltration membrane separation equipment is used as the pretreatment equipment of the reverse osmosis system, which can remove organic substances, microorganisms, viruses and heat sources with a molecular weight above 200. It is widely used in secondary seawater desalination systems, ultrapure water preparation systems and drinking water purification systems using surface water as the water source.

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Further reading:

operation of the pretreatment system

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