Reverse osmosis system engineering combines reverse osmosis units, pipelines, valves, and other equipment into a complete system. Besides the core membrane element, the system also includes high-pressure pumps, pressure vessels, security filters, scale inhibitor metering pumps, valves, instruments, and other related equipment.
1. High-pressure Pump
The high-pressure pump is one of the core components of a reverse osmosis system. It provides sufficient pressure to the raw water entering the reverse osmosis membrane elements to overcome osmotic pressure and operating resistance, ensuring the system reaches its rated flow rate.
The performance of the high-pressure pump directly affects the desalination effect and economy of the reverse osmosis system. A high-pressure pump with stable performance, high efficiency, low noise and low wear should be selected.
To ensure the safe operation of the high-pressure pump, a check valve and high/low pressure protection switch should be installed at its outlet to prevent the high-pressure pump from running dry or under pressure.
Controlling the outlet pressure of the high-pressure pump can maintain the designed water production rate without exceeding the maximum allowable inlet pressure of the membrane element. The maximum limits are: no more than 41 bar for JEC, JRW, JBW, and JNF elements; and no more than 80 bar for JSW elements.
2. Pressure Vessels
Pressure vessels come in various diameters, lengths, and pressure ratings. When selecting a pressure vessel, the chosen pressure rating must be higher than the maximum pressure under normal circumstances (generally, it must be 10% higher than the system operating pressure design value three years later). When dynamic pressure occurs on the product water side during operation, the strength of the product water outlet of some pressure vessels can become a limiting factor.
For specific selection, see section 5.3.2.2, “Selection of Membrane Shell”.
3. Security Filter
A security filter is placed at the inlet of the entire system to prevent particulate impurities that may be carried in the pretreated water from causing mechanical damage to the high-pressure pump and membrane elements. Typically, security filters use filter cartridges with a nominal filtration accuracy of 5 microns, usually made of polyethylene or polypropylene. To avoid corrosion, the filter body should be made of stainless steel or plastic.
Security filters not only trap particulate impurities but also remove turbidity and colloidal iron to a certain extent, reducing SDI. In system design, security filters only serve a safety function, preventing impurities that have leaked through pretreatment from entering the high-pressure pump and membrane element. They cannot be used as filters to reduce SDI or remove certain types of impurities. Therefore, the raw water entering the security filter must already meet the feed water specifications of the membrane element.
4. Dosing Pump
Pretreatment systems require dosing pumps to add chemicals such as acids, scale inhibitors, bactericides, reducing agents, and flocculants. Plunger pumps and diaphragm pumps are generally used to meet the requirements of low flow rate, accurate metering, and adjustable flow rate. Regardless of the type used, the material of the flow-through parts of the dosing pump must meet the corrosive requirements of the transported medium.
5. Valves
The following types of valves are typically used in the system:
- The system’s inlet valve serves as a reliable shut-off valve when the system needs maintenance or preservation.
- The regulating valve at the outlet of the centrifugal pump or on the bypass of the positive displacement pump should be able to control the operating pressure and the system pressurization rate.
- A check valve should be installed at the pump outlet.
- The product water pipeline should be equipped with a check valve and a pressure relief valve to prevent the product water pressure from exceeding the inlet water pressure;
- A concentrate flow control valve for controlling the recovery rate should be installed on the concentrate pipeline (a back pressure valve should not be used);
- The product water pipeline should be equipped with a drain valve for cleaning or discharging substandard product water during startup;
- Valves connecting to the cleaning circuit should be installed on the inlet and concentrate pipelines.
6. Monitoring Instruments
To ensure the safe and economical operation of the reverse osmosis system and to facilitate its monitoring and timely detection of problems and malfunctions, necessary instruments and control equipment should be installed.
(1) Preprocessing system
The most important monitoring item for pretreated effluent (RO feed water) is SDI (Self-Dissolved Water). It is a crucial parameter for determining whether the feed water quality meets the standards.
Other items that need to be monitored include the amount of coagulant added, the operating pressure difference of the multi-media filter and activated carbon filter, the concentration of COD and free chlorine, the amount of scale inhibitor added, the pH value of the acid addition system, the pressure and pressure drop of the security filter, and the liquid level of each water tank.
(2) Membrane system
Flow rate, pressure, temperature, and conductivity should all be monitored by online instruments.
Flow rate includes the flow rate of membrane feed water, concentrate water, and product water. Since the ratio of these flow rates determines the recovery rate of the reverse osmosis unit, a recovery rate that is too high or too low compared to the design value will have adverse consequences for operation. Therefore, ensuring the accuracy of flow monitoring instruments is particularly important.
Pressure includes the pressure of total feed water, feed water from each stage, concentrate water, and product water. A decrease in product water flow rate under constant pressure and temperature indicates that the membrane is becoming fouled.
Conductivity includes the conductivity of the membrane feed water and the product water. The magnitude of conductivity reflects the salt content in the water, and thus reflects the change in the quality of the product water.
Temperature, pressure, and flow rate are three interrelated parameters. Combined with pH value and conductivity, and after standardization, they can be used to determine whether the membrane system is operating normally, whether there is fouling or scaling, and whether cleaning is required. These are important parameters for judging the operating status of the membrane system.
7. Water Tank
Water tanks are generally divided into raw water tanks, product water tanks, intermediate water tanks, and cleaning water tanks, etc. The size and number of water tanks are determined according to the system requirements.
The water level in the tank should be maintained above the minimum level, and the system’s start-up and shutdown are interlocked with the tank’s high and low water levels. The inlet and outlet pipelines of the water tank must be installed appropriately to avoid dead zones within the tank, and measures must be taken to prevent dust and microbial contamination. When the water tank is installed outdoors, secondary pollution issues must be considered, and shading materials are generally used. For large water tanks, structural strength must be considered.
8. Dosing tank
When treating water with chemicals, a dosing tank must be set up, and the solvent is generally the amount of chemicals used per day. The number of dosing tanks is determined by the type of chemical being added, and the size of the dosing tank is determined by the amount of chemical to be added. Chemicals generally include oxidants, scale inhibitors, reducing agents, bacteriostatic agents, flocculants, and pH adjusters, etc., and one or more should be selected based on the actual local water source conditions.
9. Selection of materials for pipe fittings and valves
Corrosion issues in all flow-through components must be considered, including filters, pumps, water tanks, pipes, valves, and instrument interfaces. Appropriate materials must be selected to avoid pollution caused by corrosion.
Water tanks and low-pressure components, including water pipes and valves, are generally made of corrosion-resistant materials such as PVC, U-PVC, ABS engineering plastics, fiberglass, or stainless steel.
Security filters, high-pressure pumps, and high-pressure pipes and valves should be made of stainless steel, and different types of stainless steel should be selected according to the salinity of the raw water.
The basic advantages of stainless steel are its excellent resistance to general corrosion and high pressure resistance. However, stainless steel is prone to pitting and crevice corrosion, especially in areas where the stainless steel material is uneven or welded. To avoid pitting and crevice corrosion, the following stainless steel materials are recommended:
- Under normal water source conditions, stainless steel with 0Cr18Ni9 material can be selected;
- When the salt content of the raw water is between 2000-5000ppm, it is recommended to use stainless steel with a carbon content of less than 0.08% made of 0Cr17Ni12Mo2.
- When the salt content of the raw water is between 5000-7000ppm, it is recommended to use stainless steel with a carbon content of less than 0.03% (00Cr17Ni12Mo2).
- When the salt content of the raw water is between 7000-30000ppm, it is recommended to use 904L stainless steel with a molybdenum content of 4.0-5.0%.
- When the raw water salinity is above 30,000 ppm, it is recommended to use 254 SMO stainless steel with a molybdenum content greater than 6.0%.
Table 5.3 Stainless Steel Composition
|
Material |
C% |
Cr% |
Ni% |
Mo% |
Cu% |
N% |
|
00Cr17Ni12Mo2 |
<0.08 |
16.0-18.0 |
10.0-14.0 |
2.0-3.0 |
— |
— |
|
00Cr17Ni12Mo2 |
<0.03 |
16.0-18.0 |
10.0-14.0 |
2.0-3.0 |
— |
— |
|
904L |
<0.02 |
19.0-23.0 |
23.0-28.0 |
4.0-5.0 |
1.0-2.0 |
— |
|
254 SMO |
<0.02 |
19.5-20.5 |
17.5-18.5 |
6.0-6.5 |
0.5-1.0 |
0.18-0.22 |
In addition, the following should be noted during the design and manufacturing process:
- To prevent stagnant water from forming in the pipes;
- Stainless steel pipes are welded using inert gas shielding.
- After the pipeline is processed, protective measures such as pickling and passivation should be adopted.
- Before shutting down, flush and replace the water inside the membrane with reverse osmosis permeate.
10. Control of Membrane Systems
Reverse osmosis systems generally employ automatic control, and their control functions include the following aspects:
(1). Control of high-pressure pumps
When the inlet pressure of the high-pressure pump is lower than the set value, the pump will automatically stop operating; when the outlet pressure of the high-pressure pump is higher than the set value, the pump will automatically stop operating. In large reverse osmosis systems, the high-pressure pump needs to be automatically controlled by a frequency converter. The control system automatically adjusts the pump’s operating frequency to maintain a constant outlet pressure.
(2). The system control program is started and stopped
The control program should automatically complete the sequential start-up and shutdown of pumps including the feed water pump, dosing pump, and high-pressure pump. The membrane system is interlocked with the product water tank level (high level stops, low level starts).
(3). Automatic low-pressure flushing is indicated when the system starts and stops
When the membrane system starts or stops, the electric flushing drain valve automatically opens to perform low-pressure flushing on the membrane surface and clean the concentrated water inside the membrane.
(4). Monitoring and alarming of abnormal operating conditions
During membrane system operation, the control system automatically monitors the operating status of various devices such as high-pressure pumps, dosing pumps, and electric valves, and outputs alarm signals. It automatically monitors operating parameters such as temperature, flow rate, pressure, liquid level, conductivity, redox potential, and pH value, alarming when abnormal operating conditions occur and determining whether to shut down the membrane system based on different situations. Below are some examples of abnormal operating conditions and their handling methods:
Table 5.4 Abnormal Operations and Handling Methods
|
Abnormal situation |
Processing method |
|
The inlet water pressure is too high |
Install a high-pressure protection switch on the outlet pipeline of the high-pressure pump. |
|
Insufficient water pressure |
Install a low-pressure protection switch on the high-pressure pump inlet pipeline |
|
High inlet water temperature |
High temperature protection switch installed in water inlet pipe |
|
High pressure in the water production pipeline |
Install pressure relief safety devices on the product water pipeline |
|
High concentration of particles and colloids in the influent |
The inlet water pipeline is equipped with an automatic turbidity and SDI control device. |
|
High concentration of sparingly soluble salts in the influent |
The dosing pump and high-pressure pump are interlocked, and a high pH protection switch for the inlet water is installed. |
|
Oxidizing agent present in influent |
Configure an ORP controller or automatic chlorine detection device that can automatically shut down the system. |
|
Oil in the water |
Oil monitoring equipment installed in the water inlet |
(5). Automatic control and adjustment of drug dosage
By using the current or pulse signals output by the measuring instruments on the reverse osmosis feed water pipeline that measure flow rate, oxidation-reduction potential, and pH value, the output of each dosing pump is automatically adjusted to regulate the amount of chemicals added, thus achieving automatic proportional adjustment of the dosage.
