China pure water equipment reverse osmosis system pretreatment and design(2)

Reverse osmosis pretreatment design for surface water containing particles and colloids

A lime pre-softening process is set up in the pretreatment system, and a small amount of sodium aluminate is added to the clarifier to increase the clarification effect.
Before the pretreatment process such as multi-media filtration or fine sand filtration, additional pretreatment combined processes such as adding coagulant/coagulant aid, sedimentation, and clarification are added.
Before the reverse osmosis membrane separation system, set up microfiltration or ultrafiltration pretreatment equipment to remove such pollutants in raw water.
The reverse osmosis membrane system does not allow particulate matter larger than 5 microns to enter, because the particulate matter in the reverse osmosis feedwater will slip the ultra-thin barrier layer of the reverse osmosis membrane during the operation of the system, which will lead to an increase in the salt permeability of the membrane system. And the desalination rate of the system decreased.

Design of reverse osmosis pretreatment system for raw water containing bacteria and microorganisms or existing microorganisms in the system

In the reverse osmosis water supply system, intermittently dosing permitted non-oxidizing chemical bactericides, such as adding MBC881 and MBC2881 of American ARGO Company or other non-oxidizing chemical bactericides with equivalent performance.
Add a UV disinfection process to the reverse osmosis pretreatment system.
Add a microfiltration or ultrafiltration process to the pretreatment system.
Optimize the structural design of the reverse osmosis device to reduce the breeding area of ​​microorganisms; for example, when designing the piping of the reverse osmosis device, try to reduce the dead angle of water flow; another example, when designing the system to connect the pipeline, try to reduce the local water accumulation in some pipelines during shutdown. Design and connection of high-low-high form; add discharge valve, etc. in the appropriate low section of the pipeline.
When treating lakes, rivers and seawater, copper sulfate (0.1PPM) process is designed in the pretreatment system to control the growth and pollution of microorganisms and algae.
Generally speaking, the reverse osmosis system requires the total number of bacteria in the raw water to be controlled below 10000cfu/ml.

Design of reverse osmosis pre-treatment system for urban tap water or self-provided water source with poor raw water

Consider setting up a reducing agent (sodium bisulfite) metering device or setting up an activated carbon adsorption filter in the pretreatment system to eliminate the free chlorine remaining in the feed water to prevent the long-term presence of oxidizing substances in the water. The performance of the surface active layer of the permeable membrane is degraded. Generally speaking, activated carbon filters are chosen to be installed in small reverse osmosis systems, while in large systems, the metering of reducing agents is generally considered in the pretreatment system.
When choosing the type of scale inhibitor to be added to the system, it should be considered whether the added scale inhibitor is compatible with the flocculant and coagulant added in the early stage of the water supply. Generally speaking, in order to ensure the coagulation effect, cationic flocculants have been selected in the waterworks and self-provided water source systems that used surface water as the water source. When the raw water is used in the pretreatment process, it is necessary to pay attention to the compatibility of the medicines. Once the raw water uses cationic flocculants in the pretreatment process, it is necessary to resolutely avoid the use of anionic scale inhibitors in the subsequent reverse osmosis system; Adding anionic scale inhibitors may react with the cationic flocculants remaining in the filtered water, and the reactants formed due to the addition of the chemicals will deposit on the membrane surface in the form of colloidal compounds, thereby causing pollution to the reverse osmosis membrane. At present, some foreign pharmaceutical manufacturers in the market can generally provide organic flocculants that are compatible with their scale inhibitors. The provided MPT150 ​​flocculants are compatible with their MDC-150 scale inhibitors, so the system is particularly safe to use.

Design of reverse osmosis pretreatment system that may contain trace amounts of oil and grease in raw water

Oil and grease cannot be contained in the reverse osmosis feed water, because the presence of oil and grease in the raw water may cause chemical degradation of the aromatic polyamide active layer of the reverse osmosis membrane during the application process, and cause degradation of membrane performance. The adhesion on the membrane surface makes it easier for other pollutants in the water to stay on the membrane surface, thereby causing other fouling of the reverse osmosis membrane.
When designing a reverse osmosis system, when the content of oil and grease in the feed water is above 0.1PPM, oil-water separation, chemical coagulation, activated carbon adsorption filtration or ultrafiltration membrane separation should be selected according to specific conditions to remove them.

Reverse osmosis pretreatment system design for raw water that is in a reduced state (anoxic) and contains ferrous iron, manganese, hydrogen sulfide and ammonia salts

When the reverse osmosis system treats the raw water in a reduced state and contains iron and manganese ions, the designer should pay more attention to preventing the membrane fouling formed by iron and manganese oxides. This is because after the raw water has been treated by the pretreatment oxidation process – that is, when the oxygen content in the water is above 5PPM, the ferrous iron and manganese ions will become sols of insoluble hydroxides, although generally through coagulation, sedimentation and medium Combination processes such as filtration can remove such contaminants. However, in the actual reverse osmosis water treatment project, there are often many cases of iron fouling in the reverse osmosis membrane system. Years of engineering practice have shown that when the PH value of raw water is above 7.7, even if the iron content in the reverse osmosis feed water is 0.1PPM and the SDI test value is less than 5, the problem of iron membrane fouling may occur. The oxidation rate is closely related to the iron content, the concentration of dissolved oxygen in the water and the pH value and other factors, so attention should be paid to the control of the iron ion content in the raw water in the pretreatment system. Engineering practice has proved that: under normal circumstances, when the pH value of raw water is low, the allowable content of iron ions in reverse osmosis feed water can be slightly higher: when the pH value of raw water is less than 6.0, the content of dissolved oxygen is less than 0.5ppm, and the iron content of raw water is below 4ppm, the reverse It is basically impossible for iron pollution to occur in the osmotic membrane system; when the dissolved oxygen content of the raw water is between 0.5-5ppm and the pH is 6.0-7.0, the safe allowable content of iron ions in the water should be below 0.5ppm; when the dissolved oxygen content of the raw water is 5ppm Above, and when PH > 7.7, the safe allowable concentration of iron ions in reverse osmosis feed water is only 0.05ppm. In addition, when treating iron-containing groundwater and oxidizing raw water, do not use chlorination process, because the colloidal iron formed when iron in water is chlorinated is difficult to remove, which will pollute the reverse osmosis membrane.
Hydrogen sulfide in groundwater can be removed by chlorination and oxidation, but the actual effect of this method is closely related to the pH value of the treated water. When the pH of the raw water is lower than 6.4, adding chlorine to the raw water can convert hydrogen sulfide into sulfuric acid and exist in the water; but when the pH of the raw water is higher than 6.4, a part of the hydrogen sulfide will be oxidized into colloids during the chlorination of the raw water. sulfur. Engineering practice has proved that when the pH is 7~10, the two reaction components each account for about 50%. However, once colloidal sulfur is formed in the raw water system, it is very difficult to remove, and its pollution to the reverse osmosis membrane is relatively large, so special care should be taken in actual reverse osmosis engineering applications.
In addition, before the raw water enters the reverse osmosis system, most of the hydrogen sulfide in the raw water can be removed by degassing or stripping.
At present, there has been great progress and development in the removal technology of hydrogen sulfide. It can effectively remove hydrogen sulfide in raw water, and it is simple and safe to use. This treatment method differs from other treatment methods in that instead of converting hydrogen sulfide to other sulfides, it extracts hydrogen sulfide from the filtered water stream and produces no by-products that dissolve in the water stream. Sulfaclean is not a chemical additive and does not require maintenance. Its consumption is only related to the hydrogen sulfide content in the water, and it can adapt to changes in the hydrogen sulfide content without adjustment. The removal effect is very good. No, so the process is very safe for reverse osmosis systems.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

operation of the pretreatment system

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