Seawater desalination equipment

2023-08-08

Seawater desalination equipment

Seawater desalination equipment refers to equipment specially used to desalinate seawater. Seawater desalination mainly uses two methods to desalinate seawater, namely, distillation and reverse osmosis to desalinate seawater.

process flow

preprocessing

Whether it is seawater desalination or brackish water desalination, feedwater pretreatment is the key to ensure long-term stable operation of reverse osmosis systems. When formulating a seawater pretreatment plan, it should be fully considered that there are a large number of microorganisms, bacteria and algae in seawater. The reproduction of bacteria and algae and the growth of microorganisms in seawater will not only bring many troubles to water intake facilities, but also directly affect the normal operation of seawater desalination equipment and process pipelines. Periodic tide rises and ebbs, a large amount of sediment is entrained in the seawater, and the turbidity changes greatly, which may easily cause the operation of the seawater pretreatment system to be unstable. Seawater is highly corrosive, and the materials of equipment, valves, and pipe fittings used in the system must be screened to ensure good corrosion resistance.

Sterilization

Foreign seawater desalination projects mostly use chemical reagents such as liquid chlorine, NaClO and CuSO4 to kill bacteria and algae. Considering various factors such as traffic, it is difficult to add chemical reagents to sterilize and kill algae, so seawater sodium hypochlorite generator is specially used in the equipment development process of this project. After the sea water is pumped, a small stream of pressurized sea water is separated, enters the sodium hypochlorite generator, generates NaClO under the action of a DC electric field, and is directly injected into the beach caisson by the potential difference to kill bacteria, algae and microorganisms in the sea water.

Due to the high hardness of seawater, the direct electrolysis of seawater to produce NaClO must overcome the problem of electrode fouling. During the development process, the electrodialysis frequent electrode reversal (EDR) technology was used for reference, that is, the electrode polarity was inverted every 5 to 10 minutes, which effectively solved the problem of scaling and precipitation of the sodium hypochlorite generator.

coagulation filtration

Coagulation filtration aims to remove colloid and suspended impurities in seawater and reduce turbidity. In the reverse osmosis membrane separation project, the pollution index (FI) is usually used to measure, and the FI value of the feed water entering the reverse osmosis equipment is required to be <4. Due to the large proportion of seawater, high pH value, and large seasonal changes in water temperature, the system uses FeCl3 as the coagulant, which has the advantages of not being affected by temperature, large and strong alum flowers, and fast sedimentation speed.

chemical regulation

In order to prevent insoluble inorganic salts, such as CaCO3 and CaSO4, from forming insoluble inorganic salts due to the concentration of seawater in the process of seawater desalination, scaling and precipitation on the surface of the reverse osmosis membrane and system piping parts, seawater should be added with an anti-scaling agent before entering the reverse osmosis desalination system.

Adding H2SO4 to adjust the pH value of seawater to decompose HCO-3 in seawater to prevent CaCO3 precipitation is the most commonly used and most economical method in seawater desalination. Dosing (NaPO3)6 (SHMP) is an effective method to prevent CaSO4 precipitation, but (NaPO3)6 produces phosphate as a by-product during scale inhibition, which will promote the growth of microorganisms, bacteria and algae, and its use has certain limitations. However, the price of special polymer scale inhibitors imported from western countries is relatively high, which will directly affect the operating cost of seawater desalination projects. In this project, H2SO4 is finally selected as the scale inhibitor, and the pH value of the reverse osmosis system feed water is controlled between 6.8 and 7.0. At the same time, the water recovery rate of the seawater desalination system is controlled to prevent the precipitation of CaSO4.

Considering that the composite membrane element with aromatic polyamide as the membrane material used in the reverse osmosis seawater desalination system has poor oxidation resistance and requires the residual chlorine content in the influent water to be below 0.1m g/L to remove the reducing agent, so the seawater is cast into the membrane system before it enters the membrane system. Add NaHSO3 to control the oxidation-reduction potential (ORP) of seawater before entering the reverse osmosis device, so that the seawater oxidation-reduction potential (ORP) before entering the reverse osmosis device is 280-320mV. The dosage of NaHSO3 is 3 times the amount of residual chlorine in seawater .

Deodorization

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