Guiding Scheme for the Design Principles of Reverse Osmosis Pretreatment Equipment

Guiding Scheme for the Design Principles of Reverse Osmosis Pretreatment Equipment

Reverse osmosis feed water cannot contain oil and grease, 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 application, leading to membrane performance degradation. At the same time, the adhesion of oil and grease on the membrane surface makes it easier for other pollutants in the water to remain on the membrane surface, causing other pollution of the reverse osmosis membrane.

I. Design of Reverse Osmosis Pretreatment System for Sparingly Soluble Inorganic Salts in Water

1. Ion exchange softening

This process is frequently used when organic antiscalants are not added to the system, the raw water hardness is low, and there is a certain content of barium and strontium ions in the water source. Generally speaking, this process is currently most widely used in the pretreatment systems of small reverse osmosis units and reverse osmosis pure water preparation systems used for drinking water purification.

2. Lime softening with auxiliary magnesium addition

This process is often used in large reverse osmosis systems with high carbonate hardness and dissolved silica content in the raw water. Generally speaking, this method can reduce the carbonate hardness of the raw water to about 100 mg/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 poor quality surface water and industrial circulating water.

3. Metering and adding antiscalants to the feed water

Because this process is highly adaptable to raw water and site conditions, easy to achieve automatic control, and reliable equipment operation, it is widely used in large reverse osmosis systems and systems with high content of sparingly soluble inorganic substances in the raw water.

Currently, in newly built reverse osmosis systems, the added antiscalants are mostly imported products. The common characteristics of these antiscalants are that they are very convenient to dilute and add. The agent has a high dispersion capacity for various sparingly soluble substances in water. The pharmaceutical manufacturer can even guarantee that scaling will not occur even when the LSI or S&DSI index of the R/O concentrate system is as high as +2.5～+3.0. In addition, the saturation of CaSO4, SrSO4, BaSO4, and CaF2 can be expanded by 2.3, 8.0, 60, and 100 times respectively; and some antiscalants are compatible with flocculants added in the pretreatment system.

In the past, sodium hexametaphosphate, which was frequently used as an antiscalant in China, is being gradually replaced due to its disadvantages such as inconvenient dissolution, temperature sensitivity, instability, and poor dispersion capacity. In addition, the phosphate ions and phosphate scale generated by the hydrolysis of sodium hexametaphosphate may become nutrients for microorganisms in the raw water, thus promoting the proliferation of microorganisms in the reverse osmosis system. This is also one of the reasons why sodium hexametaphosphate is being gradually abandoned by users. No matter which antiscalant is selected, special attention should be paid to the control of LSI and S&DSI values in the concentrate system during application to ensure safe operation of the system.

Weak acid cation exchange dealkalization softening: This method is mostly used in large reverse osmosis systems with high salinity and high alkalinity (accounting for more than 70% of the anion content) in the raw water. However, after treatment with this process, the treated water has a low pH value (4～5), which often leads to a lower desalination rate of the reverse osmosis system due to the increased inorganic acid permeability of the reverse osmosis system; even if the decarbonated treated water is further adjusted for pH value or a process that does not remove carbon dioxide is adopted, the desalination rate cannot reach the original ideal level. Nevertheless, this process has been widely used under the conditions of high salinity and high alkalinity.

II. Design of Reverse Osmosis Pretreatment System for Raw Water with High Dissolved Silica Content

The design of reverse osmosis pretreatment systems operating under such water source conditions generally includes the following methods:

If site conditions permit, the feed water temperature is adjusted to about 28～35℃ through a heat exchanger in the system to increase the solubility of silica compounds in the water, and combined 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 frequently used method in engineering. Under such conditions, attention should generally be paid to controlling the silica content in the reverse osmosis concentrate system to below 150 mg/l.

Using a combination of lime pre-softening and magnesium addition (magnesium hydroxide) to remove silica. This method can remove dissolved silica in raw water more than 60%. In addition, this process is more troublesome for users in actual operation, so this process is rarely used in small water treatment systems, but is widely used in large reverse osmosis systems.

Adding silica dispersants. At present, due to the superior performance of imported silica dispersants, this method has been widely used in large reverse osmosis projects recently started in China. According to the technical documents and relevant information submitted by the pharmaceutical suppliers, in application, some even allow the silica content in the reverse osmosis concentrate system to reach about 240～290 PPM.

However, for a reverse osmosis system designer, the maximum allowable silica content in the reverse osmosis concentrate system in a specific project should be finally determined based on the technical indicators allowed by the specific added agent and the simulation results of the agent addition calculation software that meets the site conditions.

III. Design of Reverse Osmosis Pretreatment System for Raw Water Containing Metal Oxides

Pre-oxidation of raw water is set in the pretreatment system, and then the iron and manganese ions and their compounds in the raw water are removed through coagulation, sedimentation, and sand filtration or manganese sand filtration.

In the pretreatment system, the combined treatment process of lime pre-softening and coagulation, clarification, and sedimentation 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 metal oxides in the water.

Add chemical dispersants. While effectively preventing inorganic salt scaling, it can also prevent a certain amount of metal oxide deposition in the reverse osmosis membrane system.

IV. Reverse Osmosis Pretreatment System Design for Raw Water Containing Natural Organic Matter

In the pretreatment system, a combination process of lime pre-softening, coagulation, and clarification is set up, followed by multi-media filtration and fine sand filtration to remove adsorbed natural humic organic matter from the raw water. This process is widely used in large-scale reverse osmosis pretreatment systems for surface water and circulating water desalination and purification.

An activated carbon adsorption filtration process is set up in the pretreatment system to remove remaining organic matter in the raw water. This process is frequently used in small and medium-sized reverse osmosis pretreatment, especially in pure water production and drinking water purification systems.

An organic matter scavenger process is added to the pretreatment system to more thoroughly remove organic matter from the raw water. This process is often used in ultra-pure water systems in the electronics industry and reverse osmosis pretreatment systems where the raw water is river, lake, or river water, and the organic matter content is relatively high or the composition is more complex.

Microfiltration (0.2μm) and ultrafiltration (molecular weight cut-off between 6000 and 20000) are used as pretreatment equipment for removing organic matter. This process is frequently used in small reverse osmosis systems.

In the pretreatment system, using a nanofiltration membrane separation device as the pretreatment equipment for the reverse osmosis system can remove organic matter, microorganisms, viruses, and pyrogens with a molecular weight of over 200. It is more commonly used in secondary seawater desalination systems and ultra-pure water preparation systems and drinking water purification systems using surface water as a water source.

V. Reverse Osmosis Pretreatment Design for Raw Water Containing Particles and Colloids (Surface Water)

Lime pre-softening is set up in the pretreatment system, and a small amount of sodium aluminate is added to the clarifier to enhance the clarification effect.

Before pretreatment processes such as multi-media filtration or fine sand filtration, a combination of pretreatment processes such as coagulant/coagulant aid addition, sedimentation, and clarification is added.

Microfiltration or ultrafiltration pretreatment equipment is installed before the reverse osmosis membrane separation system to remove this type of pollutant from the raw water.

Reverse osmosis membrane systems do not allow particles larger than 5 microns to enter, because particulate matter in reverse osmosis feed water will abrade the ultra-thin barrier layer of the reverse osmosis membrane during system operation, leading to increased membrane system salt passage and decreased system desalination rate.

VI. Reverse Osmosis Pretreatment System Design for Raw Water Containing Bacteria and Microorganisms or Systems with Microbial Growth

Intermittently add permitted non-oxidizing chemical bactericides to the reverse osmosis feed water system and add a UV disinfection process to the reverse osmosis pretreatment system.

Add microfiltration or ultrafiltration processes to the pretreatment system.

Optimize the structural design of the reverse osmosis device to reduce breeding grounds for microorganisms; for example, minimize stagnant water areas in the piping design of the reverse osmosis device; also, when designing system connecting pipes, minimize the high-low-high design and connection that causes partial pipe local water accumulation during shutdown; add discharge valves at appropriate low points in the pipes, etc.

When treating lakes, rivers, and seawater, design a process to add copper sulfate (0.1 PPM) to the pretreatment system to control the growth and pollution of microorganisms and algae.

Generally speaking, the reverse osmosis system requires that the total number of bacteria in the raw water be controlled below 10000 cfu/ml.

VII. Reverse Osmosis Treatment System Design for Raw Water That Is Slightly Inferior City Tap Water or Self-Supplied Water Source

In the pretreatment system, consider installing a reducing agent (sodium bisulfite) metering device or an activated carbon adsorption filter to remove residual free chlorine in the feed water to prevent the degradation of the surface active layer of the reverse osmosis membrane due to the long-term presence of oxidizing substances in the water. Generally speaking, a activated carbon filter is selected in small reverse osmosis systems, while in large systems, metering of reducing agents is generally considered in the pretreatment system.

When selecting the type of scale inhibitor to be added to the system, the compatibility of the added scale inhibitor with the flocculant and coagulant added in the early stage of water supply should be considered.

Generally speaking, historically, in order to ensure the coagulation effect, cationic flocculants have been used in water plants and self-supplied water systems that previously used surface water as a water source. Therefore, when selecting scale inhibitors for reverse osmosis systems, the compatibility of the chemicals must be considered. If cationic flocculants are used in the pretreatment process of the raw water, the use of anionic scale inhibitors must be avoided in the subsequent reverse osmosis system; if this cannot be avoided, the subsequent process of adding anionic scale inhibitors may react with the remaining cationic flocculants in the filtered water, and the reaction products formed due to the chemical addition will deposit on the membrane surface in the form of colloidal compounds, thus polluting the reverse osmosis membrane. Currently, some foreign chemical manufacturers on the market can generally provide organic flocculants compatible with their scale inhibitors, making the system particularly safe to use.

VIII. Reverse Osmosis Pretreatment System Design for Raw Water That Is in a Reduced State (Anoxic) and Contains Divalent Iron, Manganese, and Hydrogen Sulfide and Ammonium Salts

When treating raw water containing iron and manganese ions with a reverse osmosis system in the reduction state, designers should pay more attention to preventing membrane fouling caused by the formation of iron and manganese oxides. This is because after the raw water has undergone pretreatment oxidation - that is, when the oxygen content in the water is above 5 PPM, divalent iron and manganese ions will become insoluble hydroxide sols. Although in general, this type of pollutant can be removed through combined processes such as coagulation, sedimentation, and media filtration.

However, in actual reverse osmosis water treatment projects, there are often many cases of iron clogging in the reverse osmosis membrane system. Years of engineering practice have shown that when the raw water PH value is above 7.7, even if the iron content in the reverse osmosis feed water is 0.1 PPM and the SDI test value is less than 5, iron membrane fouling may occur. This is because the oxidation rate of iron is closely related to factors such as iron content, dissolved oxygen concentration in water, and PH value. Therefore, the control of iron ion content in raw water should be paid attention to in the pretreatment system.

Engineering practice has proved that: generally, when the raw water PH value is lower, the allowable content of iron ions in the reverse osmosis feed water can be slightly higher: when the raw water PH value <6.0, dissolved oxygen content <0.5ppm, and raw water iron content is below 4ppm, the reverse osmosis membrane system is basically impossible to have iron pollution; when the raw water dissolved oxygen content 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 raw water dissolved oxygen content is above 5ppm, and PH >7.7, the safe allowable concentration of iron ions in the reverse osmosis feed water is only 0.05ppm.

In addition, when oxidizing raw water to treat iron-containing groundwater, do not use chlorination, 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 source. When the raw water PH is below 6.4, adding chlorine to the raw water can cause hydrogen sulfide to exist in the water as sulfuric acid; however, when the raw water PH value is above 6.4, during the chlorination of the raw water, part of the hydrogen sulfide will be oxidized into colloidal sulfur.

Engineering practice has proved that: at PH 7~10, the two reaction components account for about 50% each. However, once colloidal sulfur is formed in the raw water system, it is very difficult to remove, and it pollutes the reverse osmosis membrane to a greater extent, so special caution should be exercised in actual reverse osmosis engineering applications.

Alternatively, most of the hydrogen sulfide in the raw water can be removed by degassing or stripping before the raw water enters the reverse osmosis system.

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

Reverse osmosis feed water cannot contain oil and grease, 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 application, leading to membrane performance degradation. At the same time, the adhesion of oil and grease on the membrane surface makes it easier for other pollutants in the water to remain on the membrane surface, causing other pollution of the reverse osmosis membrane.

When designing a reverse osmosis system, if the oil and grease content in the feed water is above 0.1 PPM, oil-water separation, chemical coagulation, activated carbon adsorption filtration, or ultrafiltration membrane separation should be selected according to the specific situation to remove it.