1. What 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, chemical dosing device, raw water tank, aeration tank.

 

2. What desalination pretreatment equipment do you know?

Desalination pretreatment equipment includes electrodialysis and reverse osmosis devices.

 

3. What deep desalination equipment do you know?

Deep desalination equipment includes anion exchangers, cation exchangers, mixed ion exchangers, distillation devices, and EDI devices.

 

4. How are mechanical filters selected? What is its working principle?

The selection of mechanical filters is based on the total system inflow to select the size and combination of filters (multiple filters can be used in parallel if one is insufficient, as well as the number of backups). The total system inflow is determined based on the reverse osmosis system water recovery rate and the ratio of system water production.

The filter media inside the mechanical filter is composed of many refined quartz sands with different particle sizes, strictly arranged from large to small, thus forming a good quartz sand gradation. When the filter is first put into use, the filtration effect is often not very good, because at the beginning, the filter has not formed a "bridge." The so-called "bridge" refers to an interception network formed by suspended substances in the water. This interception network intercepts suspended substances with similar particle sizes, and then intercepts smaller suspended substances, forming an inverse particle size filtration process that first intercepts large particles and then small particles.

Once the filter forms a "bridge," the filtration effect is very good. As the operation time increases, the filtration accuracy becomes higher, the interception network becomes thicker, and the inlet and outlet pressure difference becomes larger. When the pressure difference reaches 1 kg/cm2, the filter should be backwashed. During the backwashing process, it is best to use compressed air to scrub the quartz sand. General engineering experience is that mechanical filters with a diameter less than 2500 mm do not need compressed air; while mechanical filters with a diameter greater than 2500 mm must be scrubbed with compressed air to achieve satisfactory cleaning results; the backwashing flow rate is generally 3-4 times the design capacity of the filter.

Old mechanical filters mostly use large cobblestones as the base layer, and the bottom uses convex steel plates with evenly punched water-permeable holes, resulting in uneven water distribution, easily causing large central filtration rate and small edge filtration rate; when the filter is backwashed, quartz sand mixing will occur, which inevitably leads to filter material leakage into downstream pipelines and precision filters, posing a serious threat to precision filters and reverse osmosis devices.

Through continuous practice and experiments, many manufacturers have improved mechanical filters. The water distribution device uses a perforated plate with a special ABS water cap. This ABS water cap has the function of different output in two directions, that is, the output is smaller during operation, and the backwashing output can be increased several times, making the water distribution more uniform during normal washing and more thorough during backwashing, greatly improving the water quality.

To prevent fine sand from penetrating the filter during operation or backwashing, the gap of this ABS water cap is very small, generally around 0.1-0.2 mm. It is worth noting that during the filling of the filter media, a certain amount of water must be injected into the filter to prevent large quartz sand from crushing the ABS water cap; during the installation of the water cap, hard shoes cannot be worn to prevent crushing the ABS water cap. The mechanical filter is equipped with a backwash water inlet limit butterfly valve to control and adjust the backwash water flow rate. The backwashing intensity should make the filter layer expand by 15-25%, and the backwashing compressed air intensity is generally 10-18 L/S.m2. If there is no compressed air, a Roots blower can be considered.

 

5. How are precision filters selected? What are the types of filter elements?

The selection of precision filters is matched with the total inflow, and the diameter of the precision filter is selected according to the total inflow. For a 40" 5um filter accuracy filter element, the single water production is approximately 2 m3/h. The types of filter elements are mainly polypropylene filter elements, honeycomb filter elements, melt-blown filter elements, and pleated filter elements.

 

6. Iron in water

The 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 is then passed through an iron and manganese removal filter for iron removal. If most of the iron in the water is ferric iron, aeration is not necessary, and it can directly enter the iron and manganese removal filter for removal.

 

7. Why is a carbon dioxide remover necessary after the cation exchanger for some water types?

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, which is further converted to CO2 gas. Due to the low solubility of CO2 gas, it provides good conditions for degassing. Secondly, 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 some pretreatment systems such as reverse osmosis. However, some places do not need to add a carbon dioxide remover. All of these depend on the water quality and type of the user.

 

8. What equipment mainly constitutes a reverse osmosis device?

A reverse osmosis device mainly consists of a high-pressure pump, a high-pressure pump outlet gate valve (manual or electric), high and low-pressure protection switches, an inlet flow meter (can also be omitted), a product water flow meter, a concentrate flow meter, a product water conductivity meter, a membrane module (pressure vessel, reverse osmosis membrane element), an electric concentrate valve, a concentrate shut-off valve, an inlet pressure gauge, interstage pressure gauges, a concentrate pressure gauge, a product water pressure gauge, a reverse osmosis support, a reverse osmosis control panel, a reverse osmosis sampling panel, a burst membrane, and corresponding pipes, clamps, elbows, etc.

 

9. What are the types of corrosion protection methods?

Corrosion protection methods include rubber lining, epoxy, plastic lining, enamel, and other methods.

 

10. What are the characteristics of bipolar ion exchange membranes?

High-quality heterogeneous ion exchange membranes must have the following characteristics:

1. Strong selective permeability. Selective permeability is the main indicator for measuring membrane performance. It directly affects the current efficiency and desalination effect of the electrodialysis device, and its selective permeability is greater than 85%.

2. Low membrane resistance. An electrodialysis device consists of hundreds of pairs of ion exchange membranes; therefore, 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; when polarization occurs, the pH value of the stagnant layer on both sides of the membrane will also change, especially when the water participates in the chemical reaction, it will produce highly oxidizing oxygen and chlorine. Therefore, the membrane must have strong chemical stability to extend the service life of the electrodialysis device.

4. Strong mechanical strength and dimensional stability.

5. Low diffusion performance.

6. High removal effect on strong electrolytes.

 

11. What material are electrodialysis electrodes made of? What are their specifications, advantages, and disadvantages?

Electrodialysis electrodes are divided into several types: titanium-plated platinum electrodes, titanium-coated ruthenium electrodes, graphite electrodes, and stainless steel electrodes. 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 electrodes: They have excellent corrosion resistance and can be used under very harsh conditions, but the price of platinum is expensive, and resources are scarce, limiting its promotion in China.

Titanium-coated ruthenium electrodes: Ruthenium (Ru), iridium (Ir), and titanium (Ti) compounds are coated on a titanium substrate, and after high-temperature treatment, their mixed oxides are formed. Because the ionic radii of ruthenium (Ru), iridium (Ir), and titanium (Ti) are very close, and the lattice structure and space group belong to the same type, a solid solution of RuO2-IrO2-TiO2 can be formed during co-oxidation in heat treatment, which has excellent corrosion resistance and is very suitable as an electrode material.

Graphite electrodes: 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 due to mechanical action. High-velocity water has a strong scouring effect on graphite. On the other hand, the gas produced by the electrode reaction has an impact on graphite. Coupled with electrochemical corrosion, graphite particles often fall off, polluting the water quality or even blocking the water channel. With the advent of titanium-coated ruthenium electrodes, graphite electrodes have gradually been eliminated.

Stainless steel electrodes: 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 material is of great significance for extending the service life of the electrode and reducing system investment and operating costs. Different electrode materials can be selected for different water qualities:

1. For natural water with chloride as the main component, titanium-coated ruthenium electrodes are preferred.

2. For natural water with sulfate as the main component, lead plates, stainless steel, and titanium-coated ruthenium electrodes are preferred.

3. For natural water with calcium bicarbonate as the main component, stainless steel and titanium-coated ruthenium electrodes are preferred.

4. For natural water with mixed ions, titanium-coated ruthenium, graphite, and titanium-plated platinum electrodes are preferred.

 

12. How are the dilute water chamber, concentrate water chamber, and electrode water chamber distinguished?

One cation exchange membrane, one diaphragm, and one anion exchange membrane form a membrane pair. The water chamber is formed between the cation exchange membrane and the anion exchange membrane. Under the action of the electric field, the ions in the water chamber move directionally. When the ions in the water chamber leave the water chamber due to traction and the selective permeability of the membrane, the water chamber is called the dilute water chamber; conversely, when the ions enter the water chamber due to traction and the selective permeability of the membrane, the water chamber becomes the concentrate water chamber; the water chamber formed between the cation exchange membrane, anion exchange membrane, or diaphragm and the electrode is called the electrode water chamber.

 

13. What parts does an electrodialysis device consist of? What are the functions and characteristics of each part?

The electrodialysis device consists of several parts: anion exchange membrane, cation exchange 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.

The anion exchange membrane and cation exchange membrane have selective permeability to ions in the water, resulting in the separation of concentrate water, dilute water, and electrode water, which is the desalination part of the device.

The diaphragm is mainly made of polypropylene and supports the anion and cation exchange membranes to form the concentrate and dilute water chambers.

The electrode mainly forms the electric field required for the ion exchange membrane. The electrode consists of a water distribution head, a porous plate, and a PVC frame.

The clamping device mainly fixes the anion and cation exchange membranes, electrodes, and diaphragms to form an integrated unit.

The leak-proof rubber plate is between the electrode and the diaphragm, preventing water leakage at the edge of the electrode.

The acid washing system is an indispensable part of the entire device. When the electrodialysis device produces 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.

The thyristor rectifier cabinet is the energy input part of the device. It rectifies the industrial frequency AC power into a DC voltage that can be adjusted via thyristor rectifier components. This voltage is applied to the electrodes to form a DC electric field within the membrane stack, driving the movement of anions and cations in the solution. The main parameters of the thyristor rectifier cabinet are: rectification voltage, operating current, and rectification power. Flow meters, pressure gauges, ABS pipes, fittings, and valves are auxiliary accessories of electrodialysis. They display various operating parameters of the electrodialysis device, control the connection of the water chambers, and switch the water flow direction.

 

14. What is the approximate distribution ratio of concentrate, dilute, and electrode water in electrodialysis?

The approximate distribution ratio of concentrate water, dilute water, and electrode water in the electrodialysis device is 4:4:2. Therefore, saving electrode water in the electrodialysis desalination system is very meaningful. Common measures for saving electrode water include discharging part of the concentrate water as electrode water or using electrode water circulation. The specific method for electrode water circulation is to use softened water or desalted water + NaCl solution as the circulating electrode water.

 

15. What types of pumps are needed in water treatment system engineering? How should pumps be selected?

Water treatment system engineering generally requires ordinary pumps, booster pumps, and corrosion-resistant pumps. Ordinary pumps usually use IS type cast iron pumps; booster pumps usually use stainless steel pumps (depending on the specific situation); corrosion-resistant pumps usually use IH type chemical pumps or engineering plastic pumps.

Pump models vary among different manufacturers. 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); third, select the pump material according to the process requirements (mainly referring to the material of the pump head); finally, select the pump that meets the process requirements and saves system energy consumption based on the power consumption of various pumps.