1、 Overview

Major achievements such as desalination components for medium salinity bitter salt seawater and frequent reverse electrodialysis technology have entered the international advanced level. Saudi Arabia is the world's largest producer of desalinated seawater, accounting for approximately 21% of the world's total desalination capacity.

2、 Operation method

Due to its high salt content, seawater cannot be directly used. There are mainly two methods for desalination, namely distillation and reverse osmosis.

1. Distillation method

Distillation method is mainly used in large-scale seawater desalination treatment and places with abundant heat energy. The reverse osmosis membrane method is widely used due to its wide applicability and high desalination rate. The reverse osmosis membrane method first extracts seawater for preliminary treatment, reduces seawater turbidity, and prevents the growth of bacteria, algae, and other microorganisms. Then, a special high-pressure pump is used to increase the pressure and allow seawater to enter the reverse osmosis membrane. Due to the high salt content of seawater, the reverse osmosis membrane must have high desalination rate, corrosion resistance, high pressure resistance, and pollution resistance. After being treated by the reverse osmosis membrane, the salt content of seawater is greatly reduced, and the TDS content increases from 36000 mg/L to around 200 mg/L. The desalinated water quality is even better than tap water, which can be used for industry, commerce, residents, as well as ships and vessels.

2. Reverse osmosis method

The high salt content and hardness of seawater make it highly corrosive to equipment, and the seasonal fluctuations in water temperature make reverse osmosis seawater desalination systems much more complex than conventional brackish water desalination systems, with much higher engineering investment and energy consumption. Therefore, it is particularly important to reduce project investment and energy consumption through careful process design and reasonable equipment configuration, thereby reducing unit water production costs and ensuring stable system operation.

3、 Process flow

1. Preprocessing

Whether it is seawater desalination or brackish water desalination, water pretreatment is the key to ensuring the long-term stable operation of reverse osmosis systems. When formulating seawater pretreatment plans, full consideration should be given to the presence of a large number of microorganisms, bacteria, and algae in seawater. The proliferation of bacteria and algae, as well as the growth of microorganisms in seawater, not only brings many troubles to water intake facilities, but also directly affects the normal operation of seawater desalination equipment and process pipelines. Periodic high and low tides, with a large amount of sediment carried in the seawater and significant changes in turbidity, can easily cause unstable operation of the seawater pretreatment system. Seawater has a high degree of corrosiveness, and the materials used for equipment, valves, and pipeline components in the system need to be screened to ensure good corrosion resistance.

2. Sterilization and algae removal

Foreign seawater desalination projects often use chemical reagents such as liquid chlorine, NaClO, and CuSO4 to kill bacteria and algae. Considering various factors such as transportation, it is difficult to add chemical reagents to kill bacteria and algae. Therefore, a seawater sodium hypochlorite generator was specifically used in the equipment development process of this project. After the seawater intake pump, a small stream of pressurized seawater is separated and enters the sodium hypochlorite generator. Under the action of a direct current electric field, NaClO is generated and directly injected into the beach well by the position difference to kill bacteria, algae, and microorganisms in the seawater.

Due to the high hardness of seawater, direct electrolysis of seawater to produce NaClO must overcome the problem of electrode scaling. In the development process, the electrodialysis frequent reversal (EDR) technology was borrowed, which means changing the electrode polarity every 5-10 minutes, effectively solving the problem of scaling and precipitation in the sodium hypochlorite generator.

3. Coagulation filtration

Coagulation filtration aims to remove colloids and suspended impurities from seawater and reduce turbidity. In reverse osmosis membrane separation engineering, the pollution index (FI) is commonly used to measure, requiring the FI value of the feedwater entering the reverse osmosis equipment to be less than 4. Due to the high density and pH value of seawater, as well as the seasonal variation of water temperature, FeCl3 is selected as the coagulant in the system, which has the advantages of being unaffected by temperature, having large and solid alum flowers, and fast settling speed.

4. Chemical regulation

To prevent the formation of insoluble inorganic salts such as CaCO3 and CaSO4 due to seawater concentration during seawater desalination, which can scale and precipitate on the surface of reverse osmosis membranes and system piping components, anti scaling agents should be added to seawater before entering the reverse osmosis desalination system.

Adding H2SO4 to adjust the pH value of seawater and decompose HCO-3 in seawater to prevent CaCO3 precipitation is the most commonly used and economical method in seawater desalination. Adding (NaPO3) 6 (SHMP) is an effective method to prevent CaSO4 precipitation, but the by-product phosphate produced by (NaPO3) 6 during scale inhibition can promote the growth of microorganisms, bacteria, and algae, and its use has certain limitations. The high price of specialized polymer scale inhibitors imported from Western countries directly affects the operating costs of seawater desalination projects. The final selection of H2SO4 as a scale inhibitor in this project is to control the pH value of the reverse osmosis system feedwater between 6.8 and 7.0, while also controlling the water recovery rate of the seawater desalination system to prevent the precipitation of CaSO4.

Considering the poor oxidation resistance of composite membrane elements using aromatic polyamide as membrane material in reverse osmosis seawater desalination system, which requires residual chlorine content in the influent to be below 0.1mg/L for reducing agent removal, NaHSO3 is added to seawater before entering the membrane system to control the oxidation-reduction potential (ORP) of seawater before entering the reverse osmosis device, so that the ORP of seawater before entering the reverse osmosis device is between 280-320mV. The amount of NaHSO3 added is three times the amount of residual chlorine in seawater.

5. Removing odors

The seawater in the sea area around the island is greatly affected by the surrounding environment, with a chemical oxygen demand (COD) of 1.7-2.5 mg/L. Especially in summer and autumn, the seawater sometimes has a strong odor. Therefore, in addition to adding NaClO for oxidation, adding an activated carbon filter and selecting fruit shaped granular activated carbon with high mechanical strength can effectively adsorb organic matter and odors, improve the quality of reverse osmosis produced water, and reduce surface pollution of the reverse osmosis membrane, thereby extending its service life.

6. Security filtering

The security filter adopts 316L filter, 5µ M filter element, filters seawater before entering the high-pressure pump, blocking seawater with a diameter greater than 5 μ m; M particle impurities ensure the safety and long-term operation of high-pressure pumps, energy recovery devices, and reverse osmosis membrane components.

7. High pressure pump and energy recovery device

High pressure pumps and energy recovery devices are important equipment for energy conversion and energy conservation in reverse osmosis seawater desalination. Based on the required flow rate and pressure for reverse osmosis seawater desalination, we have selected a single-stage centrifugal pump with a flow rate of 60m3/h and a head of 640psi; The energy recovery device is HTC-300 type, with a hydraulic turbine structure, which can use the pressure of concentrated seawater discharged by reverse osmosis to increase the inlet pressure of reverse osmosis by 30%, effectively reducing energy consumption.

4、 Reverse osmosis membrane components and devices

The reverse osmosis membrane element is the core component of reverse osmosis seawater desalination, which has good pressure resistance, oxidation resistance, and pollution resistance of over 99%. The reverse osmosis device adopts a combination structure of six membrane elements in series and six pressure main pipes in parallel, and is equipped with low-pressure automatic flushing and discharge and low-pressure automatic flushing and replacement of desalinated water. The unqualified produced water is automatically switched to the discharge device. In the system, high and low pressure protection, as well as high-pressure pumps and interlock for cleaning pumps, are also installed.

5、 Control of seawater desalination system

The entire reverse osmosis seawater desalination control system design adopts advanced computer program control at home and abroad, consisting of an industrial computer operation station programmable logic controller (PLC) to form a decentralized sampling control and centralized monitoring operation control system. Set high and low pressure protection switches and automatic switching devices according to process parameters. When there are abnormalities in conductivity, flow rate, and pressure, automatic switching, interlocking alarm, and shutdown can be achieved to protect high-pressure pumps and reverse osmosis membrane components. Variable frequency control is used to start and stop the high-pressure pump, achieving soft operation of the high-pressure pump, saving energy consumption, and preventing damage to the high-pressure pump and membrane components caused by water hammer or back pressure. The program design can achieve low-pressure automatic flushing before and after the start-up and shutdown of the reverse osmosis device. Especially during shutdown, the metastable state of concentrated seawater will transform into sedimentation, polluting the membrane surface. The low-pressure desalinated water automatic flushing can replace the concentrated seawater, protect the membrane surface from pollution, and extend the service life of the membrane. The system's temperature, flow rate, water quality, production and other related parameters can be displayed, stored, counted, tabular and printed. The dynamic process flow in the monitoring operation is clear and intuitive, and the system control simplifies manual operation, ensuring that the system can operate automatically, safely, and reliably.