1Brackish water desalinationDevice Overview

Water treatment equipment for desalination of brackish water: reverse osmosis system, pure water for the food industry, ultrafiltration pretreatment for boiler water in paper and power plants, desalination water for power plants, pure water for the pharmaceutical industry, integrated water purifier, ion exchange softening system, river water purification softening system, filtered water supply system, groundwater purification system.

2、 Brief description of desalination of brackish water

The desalination equipment for brackish water is suitable for high salt alkali areas and areas with seawater inflow. It uses reverse osmosis membrane components imported from the United States, high-quality domestic pressure vessels, and high-pressure pumps imported from Germany. With reasonable pretreatment equipment, it can effectively remove harmful components such as inorganic salts, heavy metal ions, organic matter, bacteria, and pathogens from water, and desalinate brackish water into high-quality water that meets national drinking water standards. Based on detailed analysis and special design of the local water quality of the customer, we strive to achieve high efficiency and durability, and ensure that the desalinated water quality fully meets the national drinking water quality standards (GB5749-85), completely solving the drinking water problem in water scarce areas.

Desalination of brackish water is a process of separating substances using a semi permeable membrane driven by pressure. The surface activity of reverse osmosis membrane only allows water to pass through, and 99% of soluble salts and all bacteria and acrobatics are intercepted. Moreover, there is no phase change in water, so reverse osmosis desalination is the most advanced and energy-saving desalination method.

3、 What is brackish water

When the fluoride content in water exceeds 1.2mg/L, it is called fluoride exceeding the standard, also known as high fluoride water. When the dissolved solid content is greater than 1500mg/L, it is called brackish water. Fluorine is a non-metallic chemical element, ranked ninth in the periodic table and belonging to the main group of elements in the seventh group of the second period. Its chemical properties are very active and it can hardly exist alone. It exists in various compound states in nature and is widely distributed in rocks, soils, and oceans. Its distribution on Earth is quite extensive. High levels of fluoride can easily lead to fluorosis.

Fluorine has a wide range of applications in daily life, including pharmaceuticals, chemicals, aerospace, atomic energy, and other fields. For example, sodium fluoride itself is a toxic substance, but it can also be used as a drug or insecticide. Endemic fluorosis is caused by people living in a high fluoride environment and long-term excessive intake of fluoride, which leads to chronic toxic changes in the body. It mainly affects the hard tissues of the human body, including teeth and bones, and also damages other soft tissues. Of course, the most obvious clinical manifestations are dental fluorosis and skeletal fluorosis. Fluoride not only affects the appearance of teeth, but also impacts chewing and digestive functions. Fluorosis is generally more common in adults, and children may also develop fluorosis. The earliest changes in fluorosis, as seen from clinical examination, include skeletal curvature such as inability to extend the arms, which affects daily life and labor. The harm of endemic fluorosis is significant, not only causing pain to patients, but also affecting local economic development. After fluoride enters the body, it causes excessive calcium deposition on blood vessels, leading to vascular calcification and arteriosclerosis. Drinking high fluoride water can easily cause fluorosis. In China, except for Shanghai, Hainan, and Taiwan, no cases of fluorosis have been found. Other provinces, cities, and autonomous regions have varying degrees of endemic fluorosis outbreaks. 77 out of 101 counties in Inner Mongolia currently suffer from drinking water fluorosis, with approximately 6 million people living in high fluoride areas and 1.7 million dental fluorosis patients. According to incomplete statistics, nearly 30 million people in China drink high fluoride water. Bitter and salty water in China is mainly distributed in the northern and eastern coastal areas. More than 38 million people in rural areas drink bitter and salty water. Bitter and salty water mainly has a bitter taste and is difficult to drink directly. Long term consumption can lead to gastrointestinal dysfunction and weakened immunity.

4、 Scope of application of brackish water desalination device

Due to its high content of sulfates and chlorides, brackish water has a bitter and salty taste. Long term consumption of brackish water can harm human health and even lead to disability. This device is suitable for water sources with sulfate and chloride content not exceeding 500mg/L, and is widely used for the treatment of brackish water in western regions. The product features a dedicated resin for desalination of brackish water. After adsorption and filtration, sulfate ions and chloride ions in the water are removed. The traditional brackish water equipment has been improved and enhanced in terms of materials and technology, achieving the goals of easy operation and low treatment cost. It is widely used in the treatment of brackish water in western regions.

5、 Methods for desalination of brackish water

1. Distillation method

Distillation is the process of heating brackish water or seawater to boil and evaporate, and then condensing the steam into fresh water. Distillation method is the earliest adopted desalination method, which has the main advantages of simple structure, easy operation, and good quality of fresh water obtained. There are many types of distillation methods, such as multi effect evaporation, multi-stage flash evaporation, pressure vapor distillation, membrane distillation, etc.

2. Electrodialysis method

A、 The basic principle, characteristics, and scope of application of electrodialysis in desalination of brackish water, abbreviated as ED, is a membrane separation technology that uses ion exchange membranes to separate anions and cations in saltwater under the action of an electric field, thereby reducing the salt concentration in the freshwater chamber and obtaining freshwater. The electrodialysis device utilizes the directional migration of ions under the action of an electric field, and achieves desalination by selecting a permeable ion exchange membrane. Under the action of an external direct current electric field, ions in water undergo directional migration (cation exchange membranes only allow cations to pass through, anion exchange membranes only allow anions to pass through), causing most ions in one type of water to migrate to another. This technology is relatively mature and has the main advantages of simple process, high desalination rate, low water production cost, easy operation, and no environmental pollution. However, it has disadvantages such as strict water quality requirements and the need for pre-treatment of raw water. In the 1950s, the United States and the United Kingdom began using this method for desalination of brackish water. In the 1980s, China used this method for brackish water desalination, industrial pure water, and ultrapure water production.

B、 Application characteristics of electrodialysis in desalination of brackish water

(1) Electrodialysis has a removal rate of 66% to 93% for soluble inorganic salts such as iron, magnesium, calcium, potassium, chloride, as well as toxicological indicators such as arsenic and fluoride, which can meet the needs of desalination of brackish water;

(2) The removal rates of oxygen consumption, NH32N, NO-32N, NO22N, and silicon by electrodialysis are relatively low, only 15% to 45%. However, due to the low content of these indicators in the raw water, although the removal rate is low, it can still meet the hygiene requirements of drinking water;

(3) The removal rate of SO2-4 by electrodialysis is 63.8%, which is difficult to meet the hygiene requirements of drinking water for desalination of SO-42Na type and SO4 · Cl2Na type water;

(4) The energy consumption of electrodialysis process is closely related to the salt content of the feed water. The higher the salt content of the feed water, the greater the energy consumption. Therefore, electrodialysis is more suitable for desalination of low salt brackish water. In addition, due to the inability of electrodialysis to remove organic matter and bacteria from water, as well as the high energy consumption of equipment operation, its application in brackish water desalination projects is limited. Therefore, the original electrodialysis devices are gradually being replaced by reverse osmosis devices in brackish water desalination.

3. Reverse osmosis method

The basic principle and characteristics of reverse osmosis method are to use a selective permeable membrane to divide a container into two halves. Pure water and saltwater are added to both sides of the membrane at the same time, so that the liquid level on both sides of the membrane is the same. After a certain period of time, the liquid level on the saltwater side will rise while the liquid level on the pure water side will fall. This is the result of water molecules migrating through the semi permeable membrane to the saltwater side, and this phenomenon is called permeation. A membrane that has selective permeability to water or solution is called a semi permeable membrane. If appropriate pressure is applied to the concentrated solution, permeation can be stopped. The pressure at which the permeation of dilute solution into concentrated solution stops is called osmotic pressure. Reverse osmosis is the process of applying a higher pressure than natural osmosis to one side of a concentrated solution, reversing the direction of natural osmosis, and pressing ions in the solution to the other side of a semi permeable membrane. This is opposite to the normal process of osmosis in nature, hence it is called "reverse osmosis". This device is called a reverse osmosis device. The reverse osmosis method can remove over 90% of soluble salts and over 99% of colloidal microorganisms and organic matter from water. Compared with other water treatment methods, it has the advantages of no phase change, normal temperature operation, simple equipment, high efficiency, less land occupation, convenient operation, less energy consumption, wide adaptability, high degree of automation, and good effluent quality. Especially the reverse osmosis purification device powered by wind and solar energy for brackish water is an economical and reliable way to solve the problem of water supply for people in areas without electricity and with conventional energy shortages. The reverse osmosis desalination method is not only suitable for desalination of brackish water, but also for desalination of brackish water. Among the existing desalination methods, reverse osmosis desalination is the most economical, even surpassing electrodialysis desalination. Due to the fact that the driving force of the reverse osmosis process is pressure, there is no phase change during the process, and the membrane only plays a "screening" role, so the energy consumption required for the reverse osmosis separation process is relatively low. Among the existing desalination methods for seawater and brackish water, reverse osmosis is the most energy-efficient. The characteristic of reverse osmosis membrane separation is its "broad-spectrum" separation, which means that it can not only remove various ions in water, but also remove particles larger than ions, such as most organic matter, colloids, viruses, bacteria, suspended solids, etc. Therefore, reverse osmosis separation method is also known as broad-spectrum separation method.

6、 Research on desalination of brackish water

China is a water scarce country, with over 300 cities experiencing water shortages, 110 cities severely lacking water, and 9 out of 14 coastal open cities severely lacking water. In the arid northwest regions of our country, the available freshwater resources are even more limited. In addition, there are thousands of inhabited islands along the coast of our country, but their freshwater resources are scarce, and drinking water problems can only be solved by accumulating rainwater and transporting water from land by boat, which is costly. In severe weather conditions, water supply is often not guaranteed.

In the arid northwest region of China, although freshwater resources are severely lacking, there are abundant underground brackish water or brackish water lakes. Coastal areas have abundant seawater resources. It can be seen that the research, development, and promotion of effective desalination technology for brackish water have significant practical significance. It is a feasible and fundamental measure to solve the water resource crisis in China's coastal areas, address the shortage of freshwater resources in some brackish water areas in western and northwestern China, and solve the problem of domestic water supply for many island residents along the coast of China.

The main research objective of this project is to explore a new reverse osmosis desalination process for brackish water in response to the shortcomings of traditional reverse osmosis pretreatment, reduce desalination costs, and provide a technical basis for safe drinking water in areas of China where freshwater is scarce but saltwater resources are abundant.

Based on research and data analysis, this project has identified a new reverse osmosis process for brackish water: the full membrane process. The main advantages of this full membrane process are: simple process, short process route, small equipment volume, easy integration, low investment and operating costs, and easy implementation of automated control of the entire machine.

A comparative analysis was conducted on polymer membrane materials, and the membranes used in each process were selected: the first stage microfiltration used polyethylene sintered microfiltration membrane with an accuracy of 5 μ m for coarse filtration; The second stage microfiltration uses polypropylene folded membrane with a precision of 0.2 μ m for security filtration. The ultrafiltration membrane is made of polysulfone material and spun into a hollow fiber membrane (30000 daltons). The reverse osmosis membrane material is selected from aromatic polyamide with high tensile strength and solvent resistance.

We have developed and assembled a mobile desalination device for brackish water, which can be made movable to meet the needs of different regions. The device adopts continuous online monitoring and over limit alarm control of operating pressure, inlet flow rate, desalinated water flow rate, and concentrated water flow rate. It is equipped with a shutdown and alarm system for reverse osmosis inlet underpressure and high-pressure pump outlet overpressure, and a safety protection device for reverse osmosis membrane cleaning and preventing membrane damage. Adopting frequency conversion control to control reasonable output power and achieve the goal of energy conservation and consumption reduction.

Based on the water quality of brackish water in various regions of China, a simulation experiment of brackish water desalination was conducted using the above-mentioned device. The experimental results showed that when the total dissolved solids (TDS) of the raw water reached 22g/L, the TDS of the fresh water was only 368mg/L, which can meet the drinking water quality standards (China's drinking water quality standards stipulate TDS<1000mg/L). For all simulated brackish water, the removal rate of TDS is over 95%, and the reduction rate of conductivity is also over 95%.

7、 Summary

The control system of the brackish water desalination device has been in operation for a period of time, and all indicators have met the design requirements. After further summary, valuable theoretical and practical experience has been accumulated for continuous improvement and promotion of application in the future.