1. Several basic concepts in water treatment systems: TDS, SDI, LSI, KSP
TDS: Total Dissolved Solids (generally similar to mineralization)
SDI: Silt Density Index is an indicator for measuring the effectiveness of system pretreatment. The reverse osmosis device requires an influent SDI of SDI < 5.
LSI: Langelier Saturation Index is a measure of the scaling tendency of a reverse osmosis device. LSI = 0 indicates no scaling or corrosion tendency; LSI > 0 indicates a scaling tendency; LSI < 0 indicates a corrosion tendency.
For reverse osmosis systems, the LSI value should not exceed 0. The system's LSI value can be lowered by adding acid or by reducing the system's water recovery rate.
Ksp: Solubility product constant. Reverse osmosis devices selectively permeate solvents and solutes in the raw water. Concentration occurs on the concentrate side due to solvent reduction. When the concentration of dissolved solids on the concentrate side exceeds the solubility product constant, crystallization will occur, which is harmful to the reverse osmosis device.
The solubility product constant of the system can be increased by adding scale inhibitors. Scale inhibitors can increase the solubility of dissolved solids.
2. How can the LSI index be effectively controlled?
The system's LSI index can be effectively controlled through the following aspects:
(1) The system LSI index can be lowered by reducing the system's water recovery rate.
(2) The system LSI index can be lowered by adding acid.
(3) The solubility of dissolved salts in the system can be increased by adding appropriate chemicals, such as adding TRISPE1000 scale inhibitor.
(4) Ions that are easily structured in water can be reduced or pre-removed, such as softening the system influent through a softener.
3. What reverse osmosis pretreatment equipment do you know?
Pretreatment equipment includes: mechanical filter, high-efficiency fiber filter, activated carbon filter, precision filter, ultrafiltration, microfiltration, sodium ion softener, iron and manganese removal filter, dosing device, raw water tank, aeration tank.
4. What desalination equipment do you know?
Desalination equipment includes: electrodialysis device, reverse osmosis device, anion exchanger, cation exchanger, mixed ion exchanger, distillation device, EDI device.
5. How are precision filters selected? What are the types of filter cartridges?
The selection of precision filters is matched with the total influent volume. The diameter of the precision filter is selected according to the total influent volume. For a 40" 5um precision filter cartridge with a small flow rate, the single water production is approximately 2m 3 /h. For a 40" 5um precision filter cartridge with a large flow rate, the single water production is approximately 30-60m 3 /h.
Filter cartridge types generally include polypropylene filter cartridges, honeycomb filter cartridges, melt-blown filter cartridges, pleated filter cartridges, etc.
6. How is iron in water removed?
Answer: Iron in groundwater is generally ferrous iron, so ferrous iron must be oxidized to ferric iron. The oxidation process is completed through aeration. The aeration device allows the water to fully contact with oxygen to produce natural oxidation; the aerated water then undergoes an iron removal process through an iron and manganese removal filter.
If most of the iron in the water is ferric iron, aeration is not necessary, and it can be directly removed through an iron and manganese removal filter.
7. Why is a carbon dioxide remover necessary after a cation exchanger for some water types?
Answer: The exchange of metal ions in water with H+ ions on the cation resin results in H+ ions entering the water, so the effluent from the cation exchanger is acidic, causing most of the HCO3- in the water to be converted to H2CO3 and further converted to CO2 gas.
Because the solubility of CO2 gas is low, it provides good conditions for degassing. If degassing is not performed, H2CO3 will exchange with the anion exchange resin, increasing the burden on the anion exchange resin and shortening the water production cycle of the anion exchange resin.
Usually, the carbon dioxide remover is placed after the cation exchanger and before the anion exchanger. It can also be placed before reverse osmosis and other pre-desalination systems. Some places do not use a carbon dioxide remover. All of these depend on the user's water quality and type.
8. What are the different methods of corrosion prevention?
Corrosion prevention methods include rubber lining, epoxy, plastic lining, enamel, and other methods.
9. What equipment mainly constitutes a reverse osmosis device?
A reverse osmosis device mainly consists of a high-pressure pump, high-pressure pump outlet valve (manual or electric), high and low-pressure protection switch, influent flow meter (can also be omitted), product water flow meter, concentrate flow meter, product water conductivity meter, membrane assembly (pressure vessel, reverse osmosis membrane element), concentrate electric valve, concentrate shut-off valve, influent pressure gauge, inter-stage pressure gauge, concentrate pressure gauge, product water pressure gauge, reverse osmosis support, reverse osmosis control panel, reverse osmosis sampling panel, burst membrane, and corresponding pipes, clamps, elbows, etc.
10. What instruments and meters are necessary in a reverse osmosis system?
Several necessary instruments and meters in a reverse osmosis system include:
Silt Density Index (SDI) meter: Used to measure the SDI index of system pretreatment.
Concentrate flow meter: Used to measure the flow rate of the system concentrate, and used with the product water flow meter to determine the system recovery rate.
Product water flow meter: Used to measure the flow rate of the system product water.
Product water conductivity meter: Used to measure the water quality (conductivity) of the system product water.
Pressure gauges: Measure system inlet pressure, inter-stage pressure, concentrate pressure, and product water pressure.
Inlet flow meter: Used to measure the total inlet flow rate of the system.
Thermometer: Used to measure the operating temperature of the system.
Inlet pH meter: Used to measure changes in the pH of the system's inlet water.
Inlet conductivity meter: Used to measure the conductivity of the system's inlet water, and used in conjunction with the product water conductivity to determine the system's desalination rate.
Redox meter: Used to measure the amount of oxidizing substances in the system's inlet water to determine the level of threat to system safety.
High and low pressure protection switch: Used to protect the system from operating under low pressure (insufficient water supply) and high pressure conditions.
A reverse osmosis system is relatively complex, and the instruments and meters used are determined by process requirements and user investment.
A normal reverse osmosis system only needs a product water flow meter, concentrate flow meter, product water conductivity meter, pressure gauge, and high and low pressure protection.
11. What are the components of an electrodialysis device? What are the characteristics and functions of each part?
An electrodialysis device consists of several parts: anion membrane, cation membrane, diaphragm, electrode, clamping device, leak-proof rubber plate, acid washing system, flow meter, pressure gauge, ABS pipes and fittings, valves, and thyristor rectifier cabinet.
Anion membrane, cation membrane The selective permeability to ions in the water allows for the separation of concentrate, fresh water, and polar water, which is the desalination part of the device.
Diaphragm The main material is polypropylene, which supports the anion and cation membranes and forms the concentrate and fresh water chambers.
Electrode Mainly forms the electric field required for ion exchange membranes. The electrode consists of a water distribution head, a porous plate, and a PVC frame.
Clamping device Mainly used to fix the anion and cation exchange membranes, electrodes, and diaphragms to form a whole.
Leak-proof rubber plate It is located between the electrode and the diaphragm, preventing water leakage at the electrode edge.
Acid washing system It is an indispensable part of the entire device. When the electrodialysis device exhibits abnormal phenomena such as decreased desalination rate, decreased water production, and increased operating pressure, it is necessary to determine the cause, such as scaling, inorganic fouling, or organic fouling, and take corresponding chemical cleaning measures using chemical reagents.
Thyristor rectifier cabinet It is the energy input part of the device, which rectifies the industrial frequency AC power into a DC voltage with adjustable voltage through thyristor rectifier devices, applied to the electrodes to form a DC electric field in the membrane stack to drive the anions and cations in the solution to move directionally.
The main parameters of the thyristor rectifier cabinet are : Rectifier voltage, operating current, and rectifier power. Flow meters, pressure gauges, ABS pipes and fittings, and valves are auxiliary accessories of electrodialysis, which display various operating parameters of the electrodialysis device, connect the water chambers, and switch the water flow direction.
12. What are the advantages and disadvantages of electrodialysis?
Advantages of electrodialysis: Low energy consumption, small footprint; simple operation, low noise; stable effluent water quality, no phase change during desalination; small environmental pollution; wide application range of 200-40000mg/h.
Disadvantages of electrodialysis: Complex installation; desalination effect is not thorough, generally 75%; low water recovery rate, generally 50%.
13. What is the desalination principle of electrodialysis?
The anion and cation exchange membranes in the electrodialysis device have selective permeability. When ions in the solution move directionally under the action of an electric field, the selective permeability of the anion and cation exchange membranes is used to allow or prevent the corresponding ions from passing through the exchange membranes, forming concentrate or fresh water in different water chambers.
14. What is the approximate distribution ratio of concentrate, fresh water, and polar water in electrodialysis?
The distribution ratio of concentrate, fresh water, and polar water in the electrodialysis device is approximately 4 : 4:2 , therefore, saving polar water in the electrodialysis desalination system is very meaningful;
Common measures to save polar water include using part of the concentrate as polar water before discharge or using polar water circulation; the specific method of the polar water circulation system is to use softened water or desalted water + NaCl solution as the polar water circulation.
15. How to choose good anion and cation exchange membranes
High-quality heterogeneous ion exchange membranes must have the following characteristics:
(1) Strong selective permeability. Selective permeability is the main indicator to measure membrane performance, which directly affects the current efficiency and desalination effect of the electrodialyzer, and its selective permeability is greater than 85%.
(2) Low membrane resistance. An electrodialyzer consists of hundreds of pairs of ion exchange membranes, so the membrane resistance accounts for a large proportion of the total resistance. If the resistance is low, the operating voltage is low and the current efficiency is high.
(3) Strong chemical stability. During the migration of anions and cations, a high-concentration ion solution will be formed in the concentrate chamber; during polarization, the pH value of the stagnant layer on both sides of the membrane will also change, especially the polar water participating in the chemical reaction will produce highly oxidizing oxygen and chlorine, so the membrane must have strong chemical stability to extend the service life of the electrodialyzer.
(4) High mechanical strength and dimensional stability.
(5) Low diffusion performance.
(6) High removal effect on strong electrolytes.
16. What materials are the electrodes of electrodialysis made of? What are the specifications? What are the advantages and disadvantages of each?
Electrodialysis electrodes are divided into several types: Titanium-plated platinum electrode, titanium-coated ruthenium electrode, graphite electrode, stainless steel electrode ；
The electrode size varies depending on the size of the electrodialysis unit. Common engineering electrode specifications include: 800×1600mm, 400×1600mm, 400×800mm, 340×640mm, etc.
Different electrode materials have different characteristics:
Titanium-plated platinum electrode: It has excellent corrosion resistance and can be used under very harsh conditions, but the high price and limited resources of platinum restrict its promotion in China.
Titanium-coated ruthenium electrode: It is a compound of ruthenium (Ru), iridium (Ir), and titanium (Ti) coated on a titanium substrate. After high-temperature treatment, it forms a mixed oxide with excellent corrosion resistance, making it very suitable as an electrode material.
Graphite electrode: Graphite electrodes are easily corroded, mainly due to chemical corrosion and mechanical wear; when graphite is used as the anode, due to anodic oxidation, graphite is oxidized to CO2 or CO, destroying its crystal structure and causing damage; in electrodialysis devices, graphite electrode loss is mainly caused by mechanical action, the high-velocity brine has a strong scouring effect on graphite, and the gas generated by the electrode reaction has an impact on graphite. Coupled with electrochemical corrosion, graphite particles often peel off, polluting the water quality and even blocking the brine channel; with the advent of titanium-coated ruthenium electrodes, graphite electrodes have gradually been eliminated.
Stainless steel electrode: Generally speaking, stainless steel can only be used as a cathode and not as an anode, otherwise, because natural water contains many chloride ions, it will cause the anodic dissolution of stainless steel to generate divalent iron, nickel, and chromium ions.
The correct selection of electrode materials is of great significance for extending the service life of the electrodes and reducing system investment and operating costs. Different electrode materials can be selected for different water qualities:
For natural water with chloride as the main component, titanium-coated ruthenium electrodes are preferred.
For natural water with sulfate as the main component, lead plates, stainless steel, and titanium-coated ruthenium electrodes are preferred.
For natural water with calcium bicarbonate as the main component, stainless steel and titanium-coated ruthenium electrodes are preferred.
For natural water with mixed ions, titanium-coated ruthenium, graphite, and titanium-plated platinum electrodes are preferred.
17. What is the concentration polarization phenomenon of electrodialysis? What are the harms of concentration polarization?
When the working current of electrodialysis exceeds the limiting current, water electrolysis occurs at the interface between the anion exchange membrane and the freshwater, generating H+ and OH- ions. When these ions participate in charge transfer, polarization occurs.
The harm of polarization is that it consumes electrical energy on water electrolysis unrelated to desalination, resulting in energy waste. Moreover, OH- ions entering the concentrate chamber react with CO32- and CaCO3 to form scale, reducing the performance of the membrane and electrodialysis.
During polarization, the concentration of electrolyte ions on the membrane surface of the desalination chamber is much lower than that of the main solution, causing a high polarization potential, while the concentration on the membrane surface of the concentrate chamber is much higher than that of the main solution, causing ions that are easy to precipitate in the water to precipitate on the membrane surface. As a result, the apparent resistance of the membrane increases significantly, the current density decreases, and the desalination rate decreases.
Current efficiency decreases because a large part of the current is consumed in water electrolysis to generate H+ and OH- ions to transfer charges instead of counterions.
If the anion membrane is polarized first, the H+ ions generated by water dissociation in the desalination chamber pass through the cation membrane into the concentrate chamber, making the membrane surface of the desalination chamber alkaline, easily causing Ca2+, Mg2+ ions and CO32- to form CaCO3 precipitation. If the cation membrane is polarized first, the OH- ions generated by water hydrolysis in the desalination chamber pass through the anion membrane into the concentrate chamber, making it easy for Ca2+ and Mg2+ ions blocked by the anion membrane to form scaling.
In addition to increasing membrane resistance, significantly increasing the unit water production energy consumption, and increasing water flow resistance, the precipitation formed on the membrane surface also corrodes the ion exchange membrane due to changes in solution pH, shortening its service life.
18. How are the freshwater chamber, concentrate chamber, and electrode chamber distinguished?
A cation membrane, a diaphragm, and an anion membrane form a membrane pair. The space between the cation membrane and the anion membrane constitutes a water chamber. Under the action of an electric field, the ions in the water chamber move directionally. When the ions in the water chamber leave the water chamber due to the pulling force and the selective permeability of the membrane, This water chamber is called the freshwater chamber. ；
Conversely, when ions enter the water chamber due to the pulling force and the selective permeability of the membrane, This water chamber is called the concentrate chamber. ; the water chamber formed between the cation membrane, anion membrane, or diaphragm and the electrode is called the electrode chamber. 。
19. How is the frequent automatic polarity reversal system of concentrate water circulation implemented? What is its significance?
In the current water treatment industry, the frequent automatic polarity reversal system for concentrate circulation uses a programmable logic controller (PLC) as the control core, and the system's water production process runtime as the control function. It utilizes electric or pneumatic direct-through valves and three-way valves to periodically switch the flow direction of the concentrate and dilute water, ensuring that the fresh water always flows into the water production tank, while the concentrate is fixedly discharged into the concentrate circulation tank.
The frequent automatic polarity reversal system for concentrate circulation has profound significance. First, this system has a high water recovery rate, reaching up to 80% (depending on the influent water quality). The water-saving effect is very significant in some large-scale water treatment systems. 。
Second, this system has a relatively low cost, relatively low requirements for the influent water quality, and is easy to promote. (It is more competitive in water treatment projects for enterprises or factories with high recovery rate requirements but limited budget).
20. What types of pumps are needed in water treatment system engineering? How to select pumps from different manufacturers?
Water treatment system engineering generally requires ordinary pumps, booster pumps, and corrosion-resistant pumps.
Ordinary pumps generally use IS type cast iron pumps;
Booster pumps generally use stainless steel pumps such as high-pressure pumps imported from Grundfos, Denmark (depending on the specific situation);
Corrosion-resistant pumps generally use IH type chemical pumps or engineering plastic pumps.
The models of pumps from different manufacturers vary. First, select the pump flow rate according to the system's process requirements; second, select the pump head according to the process requirements (1 kg is approximately equal to 10 meters of head, 1 MPa is approximately equal to 10 kg);
Then, select the pump material according to the process requirements (mainly referring to the material of the pump head);
Finally, select a pump that meets the process requirements and saves system energy consumption based on the power consumption of various pumps.
21. What is water hammer phenomenon? How to solve this problem?
“Water hammer” is caused by the presence of air in the pressure vessel. When the device is started, the necessary measures to remove the air from the vessel are not adopted, so that when the high-pressure water flow mixed with air moves into the vessel, violent vibrations occur. In severe cases, the membrane elements may be shattered, resulting in irreparable losses.
“Prevention first, prevention is the key”. How to prevent the occurrence of “water hammer” is very important. Generally adopted measures include:
(1) High-pressure pumps use soft start methods to avoid water hammer, such as reduced pressure start, variable frequency speed regulation start, and series resistance start with an automatic controller.
(2) Avoid water hammer in the operation method, such as closing or partially closing the inlet valve during startup, and then slowly opening the valve until the system operating pressure is reached.
(3) Use control to prevent water hammer, such as using a PLC to control an electric slow-opening door to open the valve within tens of seconds.
(4) Use installation technology to prevent water hammer, such as setting a return pipe at the concentrate discharge port, so that the highest point of the pipe exceeds the highest pressure vessel in the reverse osmosis device. In this way, the pressure vessel will be filled with water when the device stops running.
The above points are commonly used measures in engineering applications. All of them can be adopted, or several points can be adopted according to the actual situation. It is worth noting that point 4 is necessary for any project.
22. Why must the reverse osmosis concentrate discharge pipe be slightly higher than the device?
The concentrate discharge valve is always open during reverse osmosis operation. Therefore, when the reverse osmosis device stops running, if the highest point of the discharge pipe is lower than the highest point of the pressure vessel, it will cause a siphon phenomenon. ， The water in the pressure vessel will flow out of the reverse osmosis device through the concentrate discharge pipe due to its own weight.
If air is mixed into the pressure vessel, first, it is easy to cause water hammer, and second, the oxygen in the air will have more or less oxidation effects on the reverse osmosis membrane elements during shutdown, affecting the service life of the membrane elements. 。
23. Influent indicators for electrodialysis and reverse osmosis?
Influent indicators for electrodialysis include:
Iron and manganese content: Fe≤0.3mg/l, Mn≤0.1mg/l
Turbidity: less than 0.3mg/l (for 0.9mm thick diaphragm)
SDI approximately equals 0
Free chlorine: CL≤0.3mg-0.5mg/l
Influent indicators for reverse osmosis include:
Turbidity: less than 1NTU
24. Influent indicators for mechanical filter, iron remover, manganese remover, and carbon dioxide remover
The influent suspended solids of the mechanical filter ≤20mg/l, and the effluent suspended solids ≤5mg/l. The influent iron content of the iron and manganese remover ≤30mg/l, and the effluent iron content ≤0.3mg/l. The influent carbon dioxide content of the carbon dioxide remover ≤330mg/l, and the effluent carbon dioxide content ≤5mg/l.
25. How to control concentration polarization in electrodialysis?
(1) Strictly control the operating current, and operate the electrodialysis under the condition of less than the limiting current density.
(2) Strengthen the transfer process in the electrodialysis compartment, such as using a screen with good turbulent effect and high-temperature electrodialysis.
(3) Adopt measures such as regular acid washing, adding scale inhibitors, and reversing electrodes to eliminate the precipitation caused by concentration polarization.
(4) Appropriate pretreatment can be used to improve the quality of the system's influent water.
26. What are the factors affecting the effectiveness of ultraviolet sterilizers and the precautions to be taken?
Factors affecting the effectiveness of ultraviolet sterilization are: The intensity of ultraviolet light, the wavelength of the ultraviolet spectrum, and the irradiation time. 。
Precautions for using ultraviolet sterilizers are:
Installation location: The closer the ultraviolet installation is to the point of use, the better, but there should also be space for inserting or removing the quartz sleeve and replacing the lamp tube from one end.
Flow rate: In the same sterilizer, when the radiation energy of ultraviolet light is constant and the bacterial content in the water does not change significantly, the flow rate of water through the sterilizer has a significant impact on the sterilization effect.
Physicochemical properties of water: The chromaticity, turbidity, and total iron content of water all absorb ultraviolet light to varying degrees, resulting in a reduction in the sterilization effect.
Lamp power: The ignition power of the lamp tube has a great impact on the sterilization efficiency.
Ambient temperature of the lamp tube: The radiant spectral energy of the ultraviolet lamp tube is related to the temperature of the lamp tube wall.
Quartz sleeve: The quality and wall thickness of the quartz sleeve are related to the transmittance of ultraviolet light. The higher the purity of the quartz sleeve, the better the efficiency. Water layer thickness: The thickness of the water layer has a great relationship with the sterilization effect.
