There are many kinds of seawater desalination technologies, such as automatic filter physical filtration method, membrane method (reverse osmosis, electrodialysis membrane evaporation, etc.), ion exchange method, freezing method, etc., but the method for large-scale desalination of sea water is only automatic filter. , multi-effect distillation (MED) and reverse osmosis (RO).
The automatic filter seawater desalination is compared with the distillation method. The membrane seawater desalination can only use electric energy, and the distillation method uses desalination to utilize thermal energy and electric energy. Therefore, the fully automatic filter is suitable for applications where there is a power source, and the distillation method is suitable for various occasions with a heat source or a power source. However, with the improvement of the performance of the automatic filter and the advent of the energy recovery device, the power consumption per ton of water is gradually reduced. The fully automatic filter desalination can produce desalinated water equivalent to the quality of tap water after one desalination. Although the seawater desalination water quality is higher, the automatic filter technology still has its own advantages. For example, the application range is wide, the scale can be large or small, and the construction period is short. It can be built not only on land but also on vehicles. , ships, offshore oil rigs, islands, wilderness, etc. The fully automatic filter system requires good pretreatment to ensure the effluent quality. In the field of seawater desalination, pretreatment is the key to ensuring long-term stable operation of the reverse osmosis system. Due to the high hardness, total solid dissolved matter and other impurities in seawater, the fully automatic filter system is sensitive to turbidity, pH, temperature, hardness and chemical substances during operation, so it is correct. The water requirement is relatively high, and if the influent water quality is poor, the water production rate is very low. Therefore, seawater must be pretreated before entering the fully automatic filter unit. The following is a brief description of the common processes for seawater desalination.
Seawater sterilization
Due to the large amount of microorganisms, bacteria and algae present in sea water. The growth of bacteria, algae and microbial growth in seawater not only causes many troubles for water intake facilities, but also directly affects the normal operation of desalination equipment and process pipelines. Therefore, the desalination project mostly uses liquid chlorine, sodium hypochlorite and copper sulfate. Chemical agents are used to sterilize algae.
Coagulation filtration
Because seawater has periodic high tide and low tide, the water often entrains a large amount of sediment, and the turbidity changes greatly. It is easy to cause the seawater pretreatment system to be unstable. Therefore, coagulation filtration should be added in the pretreatment to remove the colloid in seawater. Suspend impurities and reduce turbidity. In the reverse osmosis membrane separation project, the pollution index (SDI) is usually used to measure the SDI of the feed water entering the reverse osmosis equipment. Due to the large specific gravity of seawater, high pH value and large seasonal variation of water temperature, ferric chloride is often used as a coagulant in pretreatment system. It has the advantages of no temperature influence, large and firm sputum, and rapid sedimentation speed. .
Fully automatic filter filtration
In order to further improve the influent water quality of reverse osmosis and reduce the turbidity of the influent water, a backwashing filter is usually added after the coagulation filtration to further remove the microscopic suspended matter and particulate matter in the water to ensure the water quality. Further improve.
Scale inhibitor and reducing agent
The composition of seawater is very complicated, and the hardness and alkalinity are very high. In order to make the reverse osmosis system run better and keep the system running without scaling, it is necessary to add the corresponding scale inhibitor according to the specific water quality. . In addition, since the oxidant sterilization is added to the reverse osmosis pretreatment, it is necessary to add a reducing agent to reduce the reverse osmosis water, so that the residual chlorine of the reverse osmosis system is less than 0.1 ppm (or ORP < 200 mV), which satisfies the reverse The requirement of the osmotic system for the content of oxidizing substances in the influent.
Fully automatic filter
Because the salt content of seawater is very large, the material of the automatic filter should be 316L filter. The filter element diameter is usually 5μm. It is filtered into the seawater before the high pressure pump to block the particle impurities larger than 5μm in seawater to ensure high pressure pump and energy recovery. Safe and long-term stable operation of the device and reverse osmosis membrane elements.
High pressure pump and energy recovery device
The high-pressure pump and energy recovery device are important equipments for energy conversion and energy saving for reverse osmosis seawater desalination. According to the flow and pressure selection required for reverse osmosis desalination, the energy recovery device has a hydraulic turbine structure and can be concentrated by reverse osmosis. The pressure of seawater increases the pressure of reverse osmosis water by 30%, so that the energy of concentrated water can be effectively utilized while reducing energy consumption, thereby effectively reducing operating costs.
Reverse osmosis element and device
The reverse osmosis membrane element is the core component of reverse osmosis seawater desalination. The seawater desalination membrane corresponding to the seawater desalination system should be selected. According to the different design of the system, the desalination component has high salt rejection, good pressure resistance and oxidation resistance. And resistance to pollution and other properties. The material of the desalination system should be made of stainless steel of 316L or higher in the high pressure part to prevent the corrosion of the high pressure pipeline caused by the high water content of seawater.
System control
The automatic filter seawater desalination control system usually adopts a programmable controller PLC to form a distributed sampling control to centrally monitor the operation of the control system. According to the process parameters, the high and low voltage protection switch is set, the automatic switching device, when the conductance, flow and pressure are abnormal, automatic switching, automatic interlock alarm and shutdown can be realized to protect the high pressure pump and the reverse osmosis membrane component. The variable frequency control starts and stops the high pressure pump, realizes the soft operation of the high pressure pump, saves energy consumption, and prevents damage of the high pressure pump and the membrane element due to water hammer or back pressure. The program design can realize low-pressure automatic flushing before and after the reverse osmosis device is turned on and off. Especially in the case of shutdown, the metastable state of the concentrated seawater will be transformed into precipitation, contaminating the membrane surface, and the low-pressure desalinated water can automatically displace the concentrated seawater to protect it. The membrane surface is not contaminated and the membrane life is extended. Relevant parameters such as temperature, flow, water quality, and water production of the system can be displayed, stored, counted, tabulated, and printed. The dynamic process flow in the monitoring operation is clear and intuitive, and the system control simplifies manual operation to ensure that the system can operate automatically, safely and reliably.