The process of reverse osmosis treatment of seawater desalination

Desalination refers to the technology and process of obtaining fresh water from seawater. There are many types of seawater desalination technologies, including distillation, membrane (reverse osmosis, electrodialysis membrane evaporation, etc.), ion exchange method, etc., but the only methods suitable for large-scale seawater desalination are distillation and reverse osmosis.

The following will give you a detailed introduction to the process of desalination using reverse osmosis.

1. Seawater sterilization and algae killing

Due to the presence of 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 the water intake facilities, but also directly affect the normal operation of seawater desalination equipment and process pipelines.

2. Coagulation filtration

In order to further improve the influent water quality of reverse osmosis and reduce the turbidity of the influent water, a multi-media filter is usually added after coagulation filtration, so that the fine suspended solids and particulate matter in the water can be further removed to ensure the further improvement of water quality.

3. Scale inhibitors and reducing agents

The composition of seawater is very complex, the hardness and alkalinity are very high, in order to make the reverse osmosis system run better and keep the system running without fouling, it is necessary to add the corresponding scale inhibitor according to the specific water quality. In addition, because the oxidant is added to the reverse osmosis pretreatment for sterilization, it is necessary to add a reducing agent to reduce the inlet water during reverse osmosis, so that the residual chlorine in the inlet water of the reverse osmosis system is less than 0.1ppm (or ORP<200mV), which meets the requirements of the reverse osmosis system for the content of oxidized substances in the influent water.

4. Security filter

Because the salt content of seawater is very high, the security filter requires 316L material, and the pore size of the filter element is usually 5μm, which filters the seawater before entering the high-pressure pump, blocks the particle impurities with a diameter greater than 5μm in the seawater, and ensures the safe, long-term and stable operation of the high-pressure pump, energy recovery device and reverse osmosis membrane element.

5. High-pressure pump and energy recovery device

High-pressure pump and energy recovery device is an important equipment for reverse osmosis desalination equipment to provide energy conversion and energy saving, according to the flow rate and pressure selection required by reverse osmosis desalination equipment, the energy recovery device has a hydraulic turbine structure, can use the pressure of reverse osmosis discharge concentrated seawater to increase the pressure of reverse osmosis inlet water by 30%, so that the energy of concentrated water can be effectively utilized, and reduce energy consumption at the same time, so as to effectively reduce operating costs.

6. Reverse osmosis elements and devices

Reverse osmosis membrane element is the core component of reverse osmosis desalination equipment, to choose the seawater desalination membrane corresponding to the desalination equipment system, according to the different design of the system, you can choose different membrane elements, reverse osmosis membrane element has high desalination rate, good pressure resistance, oxidation resistance and pollution resistance. The material of the seawater desalination reverse osmosis equipment system should be made of stainless steel above 316L in the high-pressure part to prevent the corrosion of the high-pressure pipeline by the high salt content of seawater.

7. System control

The control system of reverse osmosis desalination equipment usually adopts a programmable controller PLC to form a control system for decentralized sampling control and centralized monitoring of operation. High and low voltage protection switches are set according to the process parameters, and the automatic switching device can realize automatic switching, automatic interlocking alarm and shutdown when the conductivity, flow and pressure are abnormal, so as to protect the high-pressure pump and reverse osmosis membrane elements. The inverter controls the start and shutdown of the high-pressure pump, realizes the soft operation of the high-pressure pump, saves energy consumption, and prevents the damage of the high-pressure pump and membrane elements caused by water hammer or backpressure. The program design can realize low-pressure automatic flushing before and after the start and shutdown of the reverse osmosis device, especially when the operation is stopped, the substable state of the concentrated seawater will be transformed into precipitation, polluting the membrane surface, and the automatic flushing of low-pressure desalination water can replace the concentrated seawater, protect the membrane surface from pollution, and prolong the service life of the membrane. The temperature, flow, water quality, permeate and other related parameters of the system can be displayed, stored, counted, tabular and printed. The dynamic process screen 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.