The design principles and operation control of the reverse osmosis system need to be tailored according to your raw water quality report. This post analyzes in detail the details and principles that need to be based.
01.
The meaning of manual regulating valve and electric slow opening valve behind the high-pressure pump
The designed water production volume of the membrane element should be less than the standard water production volume.
Suppose the water production volume is designed according to the standard. In that case, the reverse osmosis membrane element will soon be contaminated, causing damage to the membrane element.
The design suggestions provided by the membrane element manufacturer should select different design water production rates according to different influent water sources.
A feed pump that can guarantee the designed output after 3 years should be selected.
That is to say, a higher-pressure feed water pump must be designed, but the system does not require a high-pressure start-up to achieve the desired water output.
As a result, the feedwater pump’s pressure is high during the system’s initial operation.
The pressure richness gradually decreases over time.
A manual regulating valve should be installed behind the high-pressure pump to adjust the feedwater pressure.
In some cases, a frequency conversion adjustment device can be set for the feedwater pump. In this case, the frequency conversion method can be used to realize the adjustment of the feed water pressure.
The manual control valve behind the high-pressure pump generally does not need to be adjusted frequently after setting.
It is basically maintained at a constant position for some time.
This valve does not need to be opened and closed every time the system is started.
Suppose there is no other valve behind the high-pressure pump. In that case, the high-pressure water source of the high-pressure pump will directly impact the membrane element every time the system is started.
Especially when there is air in the system, the phenomenon of “water hammer” will occur, which will easily cause the membrane element to rupture.
To prevent the occurrence of the above phenomenon, an electric slow-opening valve should be installed behind the high-pressure pump.
After starting the high-pressure pump, slowly open the electric slow-open valve, that is, slowly load pressure on the reverse osmosis membrane of the system.
The electric slow-opening valve should be fully open and closed, and its fully open and closed time can be adjusted.
It is generally set to 45~60s, so an electric slow-opening valve should be installed from the perspective of the safety of reverse osmosis membrane elements.
02.
Automatic flushing function
After the feed water enters the reverse osmosis system, it is divided into two paths.
One way passes through the surface of the reverse osmosis membrane to become produced water. The other way moves parallel to the surface of the reverse osmosis membrane and gradually concentrates.
These concentrated water streams contain a lot of salt, and even organic matter, colloids, microorganisms, bacteria, and viruses.
In the normal operation of the reverse osmosis system, the feed water/concentrated water flows along the surface of the reverse osmosiss membrane at a certain flow rate, and these pollutants are difficult to deposit.
However, suppose the reverse osmosis system stops running. In that case, these pollutants will immediately deposit on the surface of the membrane, causing pollution to the membrane elements.
Therefore, it is necessary to set up an automatic flushing system in the reverse osmosis system. And use a clean water source to flush the surface of the membrane element to prevent the deposition of these pollutants.
03.
The system is flushed without pressure when starting up
These protective agents should be discharged for systems that have taken the deactivated protective agents. Then rinse these protective agents clean by flushing without pressure and start the system.
For systems that have not taken the deactivated protective agent, the system is generally full of water at this time.
However, the water may have been stored in the system for a certain period. At this time, it is best to drain the water by flushing without pressure before starting the machine.
Sometimes, the water in the system is not in a full state. At this time, the air must be flushed out without pressure.
If the air is not exhausted, the “water hammer” phenomenon is likely to occur, which may damage the membrane elements.
04.
Record of initial operating data
Due to the system’s operating conditions, such as changes in pressure, temperature, system recovery rate, and feed water concentration, the product water flow and quality will change. To effectively evaluate the system’s performance, comparing the product water flow and quality data under the same conditions is necessary.
Because it is impossible to obtain these data under the same conditions every time, it is necessary to “standardize” RO performance data under actual operating conditions to evaluate the RO membrane’s performance.
The standardized reference point is the operating data at the initial commissioning (steady operation or after 24h).
Or refer to the standard parameters of the reverse osmosis membrane element manufacturer.
The “standardization” of product water flow and the “standardization” of salt permeability are two examples.
If you want to know more specific design details, or you need a RO reverse osmosis solution for your special water quality, please contact us through the form below.
05.
The pretreatment PH value is too high, and the overall system desalination rate is too low
PH is a measure of the pH of water.
Changes in pH will affect the balance of various ions in the water, especially the balance of ions in the carbonic acid system.
It will also affect the content of hydrogen ions and hydroxide ions.
The desalination rate of reverse osmosis membrane for various ions is different; and its desalination rate will be obviously interfered by the pH value.
The salt rejection rate is the highest when the pH value is between 6 and 8.
When the pH value is too high or too low, the salt rejection rate will significantly reduce.
The pH value of the lime softening pretreatment process is often more than 10, which will cause the system’s desalination rate to be significantly reduced.
06.
The rupture of the membrane element water production pipe
The user used an improper lubricant during installation. The lubricant reacted with the centre tube of the membrane element made of polymer materials.
At the same time, due to the stress during installation, the central tube of the membrane element was ruptured.
According to the recommendations of the membrane element manufacturer, petroleum-based (such as chemical solvents, petroleum jelly, lubricating oil and grease, etc.) lubricants are not allowed to be used to lubricate O-rings, connecting pipes, joint seals and concentrated water seals at any time .
The only lubricants allowed are silicone-based glue, water or glycerol (glycerin).
07.
Damage to the FRP skin of the membrane element
The corresponding gasket was not installed during installation at the connection between the membrane element and the pressure vessel by the manufacturer’s requirements.
And there is no electric slow-opening door installed at the entrance of reverse osmosis in the system.
When the system is started, there is no low-pressure flushing and exhausting, which causes the high-pressure feed water to be instantly loaded on the membrane element, causing the “water hammer” phenomenon.
At the same time, when the system is started, there is no low-pressure flushing and exhaust, the remaining air cannot be discharged and is compressed at the outlet end of the pressure vessel.
Therefore, when the system is shut down, the membrane element is pushed back again, causing the membrane element to move back and forth in the system.
08.
Design principles of a reverse osmosis system
A complete and accurate raw water analysis report must be provided before designing the reverse osmosis system.
The water quality analysis report includes water quality type and leading component indicators. The required indicators include dissolved ions, silicon, colloids, and organic matter (TOC).
8.1 Typical dissolved anions
Bicarbonate (HC03-), Carbonate (CO32-), Hydroxide (OH-), Sulfate (SO42-), Chloride (Cl-), Fluoride (F-), Nitrate (NO3-) ), Sulfide (S2-), Phosphate (P044-)
8.2 Typical dissolved cations
Calcium ion (Ca2+), magnesium ion (Mg2+), sodium ion (Na+), potassium ion (K+), iron ion (Fe2+ or Fe3+), manganese ion (Mn2+), aluminum ion (Al3+), barium ion (Ba2+), Copper ions (Cu2+) and zinc ions (Zn2+).
8.3 Others
The insoluble salts often encountered in reverse osmosis systems are CaS04, CaC03 and silicon.
Other infrequently encountered scales are CaF2, BaS04 and SrS04.
Other ions that cause problems will be discussed below.
Sulphate exists in large amounts in most raw water. Its concentration is sometimes increased by artificially adding sulfuric acid to adjust the PH value.
In this case, Ba++ and Sr++ ions should be analyzed and accurate to the ppb and ppm levels.
Because the solubility of BaS04 and SrS04 is lower than that of CaS04, they are difficult to re-dissolve after scaling.
Alkalinity includes carbonate, bicarbonate, and hydroxide in negative ions. Alkalinity in natural water is mainly formed by HCO3-.
In water with a pH below 8.3, bicarbonate and carbon dioxide exist in equilibrium.
When the pH is higher than 8.3, HC03- will be converted to CO32- and exist.
If the pH of the raw water reaches above 11.3, there will be a form of 0H.
Carbon dioxide in the air will dissolve in water to form H2C03, acidic water will dissolve CaC03, and CaC03 may be brought by flowing through rocks containing CaC03.
The chemical dynamic balance of CaC03 and Ca(HC03)2 determined by PH in many natural water bodies is close to saturation.
The solubility of Ca(HC03)2 is more excellent than that of CaC03. If the raw water is concentrated in the RO system, CaC03 will easily precipitate.
Therefore, adding scale inhibitors or adding acid to lower the PH value will often be used in RO systems.
Nitrate:
It is easily dissolved in water, so it will not precipitate in the RO system.
Nitrate is more closely related to health. When mammals, including humans, ingest nitrate, they will turn into nitrite.
It interferes with the combination of hemoglobin and oxygen in the blood and can cause serious consequences.
Especially for fetuses and children, for this reason, the nitrate content in drinking water is required to be less than 40mg/l.
The typical salt rejection rate of reverse osmosis for nitrate is 90-96%.
Iron and manganese:
It usually exists in water as a divalent dissolved state or a trivalent insoluble hydroxide.
Fe2+ may come from the well water itself or from the corrosion of pumps, pipelines, and water tanks, especially from the addition of acid in the upstream system.
If the iron and manganese concentration in the raw water is greater than O.O5mg/L; and it is oxidized to Fe(OH)3 and Mn(OH)2 by air or oxidant, precipitation will form in the system when the pH value is high.
The analysis shows that the presence of iron and manganese will accelerate the oxidative degradation of the film, so iron and manganese must be removed in the pretreatment.
Aluminium:
Generally, it does not exist in dry natural water bodies. Trivalent aluminum will form insoluble Al(OH)3 in RO system like trivalent iron.
When the pH is 5.3 to 8.5, Al2(SO4)3 and NaAl02 can be used for the pretreatment of surface water, to remove negatively charged colloids due to the high price of aluminum.
Be careful not to add too much aluminum salt, as the residual aluminum ions will contaminate the membrane.
FeCl3 and Fe(S04)3 can also be used instead of aluminum salts as coagulants.
Copper and zinc:
It rarely exists in natural water bodies. Sometimes trace amounts of copper and zinc in water come from pipe materials.
In the pH range of 5.3 to 8.5, Cu(OH)2 and Zn(OH)2 are insoluble in water because they are generally lower in water. Therefore, only when the system is not cleaned for a long time and accumulates to a certain extent, will it cause pollution to the membrane system.
However, suppose copper and zinc and an oxidant (such as hydrogen peroxide) exist in the raw water simultaneously. In that case, it will cause severe degradation of the membrane material.
Sulphide:
It is dissolved in water in the form of H2S gas. Hydrogen sulphide can be removed by a degassing device, oxidized with chlorine, or becomes insoluble sulfur in contact with air, removed by multi-media filtration.
Phosphate:
It has a strong negative charge and can easily form insoluble salts with multivalent ions.
The solubility of calcium phosphate is very limited when the pH is neutral, and the solubility is low when the pH is high.
Adding scale inhibitors or lowering the pH (less than 7) in the feed water can prevent phosphate precipitation.
Silicon:
Exist in most natural water bodies, the concentration ranges from 1 to 100 mg/L.
And when the pH is lower than 9.0, it mainly exists as Si(OH)4.
When the pH is low, silicic acid can polymerize to form a silica gel.
When the pH is higher than 9.0, it will separate into SiO32-ion and will form a precipitate with calcium, magnesium, iron or lead.
Silica and silicate precipitates are difficult to dissolve.
Amine hydrogen fluoride solution is more effective for cleaning silicon scale, but the discharge of amine hydrogen fluoride solution will cause environmental pollution.
When the silicon content in the influent water exceeds 20mg/L, pay attention to the potential danger of silicon scaling.
Colloid (suspended particles)
The Silt Density Index (SDI), also known as the Pollution Index (FI), is an important indicator to measure the potential pollution of colloids (particulate matter) in the RO influent.
The colloids in RO inlet water are various, including bacteria, clay, silica gel and iron corrosion products.
In the pretreatment clarifier, some chemicals, such as Mingji, ferric chloride or cationic polymerization agent, will be used to remove colloidal pollution or be removed by subsequent media filters.
Total number of bacteria and organic content
There are two methods to determine the number of bacteria in the water, one is the culture method, and the other is the fluorescence staining method.
The latter is more commonly used because it is convenient and fast.
The organic matter in raw water is generally oil-surfactant, water-soluble polymer and humic acid.
The detection indicators are total organic carbon (TOC), biochemical oxygen demand (BOD) and chemical oxygen demand (COD).
Suppose you want to analyze the organic components more accurately. In that case, you need to use liquid chromatography and GC/MS instrument analysis.
Suppose the TOC content in the raw water is greater than 3mg/l. In that case, the pretreatment unit should consider the organic matter removal process.
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