Breaking through the problem of brackish water desalination, nanofiltration technology rewrites the water resources dilemma

As a large country with a population of 1.4 billion and a complex geographical structure, China is a country lacking fresh water resources. Therefore, the development and utilization of unconventional water resources is a strategic priority related to our country's national economy and people's livelihood. It is also a core measure to ensure coordinated regional development and maintain ecological security.

Under the vast land in northwest my country, there is a kind of water resource that cannot be directly drank - brackish water. Brackish water covers 16% of my country's land area, especially in Inner Mongolia, Xinjiang and other places. The average annual precipitation is less than 100mm, but the evaporation is dozens of times higher than the precipitation, causing the groundwater salinity to soar. This brackish water is like a hidden "underground cancer", plaguing the drinking water safety of more than 38 million people. This kind of water source with a mineralization degree of more than 1000 mg/L has high salt content and excessive harmful substances such as fluorine and arsenic. Not only does it taste bitter, but long-term drinking can also cause kidney stones, digestive system diseases and even cancer, seriously threatening the health of local residents.

Although traditional treatment methods such as reverse osmosis (RO) can desalinate, their high energy consumption of 2.5-4kWh/ton and high cost prohibit many regions. However, with the breakthrough of nanofiltration technology, this dilemma is being completely rewritten.

1. Nanofiltration technology: the “precision scalpel” to crack brackish water

As a membrane separation technology between reverse osmosis and ultrafiltration, nanofiltration technology has become a "precision scalpel" for brackish water purification with its unique advantages.

Selective separation is the core charm of nanofiltration technology. Compared with reverse osmosis, which removes almost all solutes, nanofiltration membranes only have a high removal rate for specific solutes. This film, with a pore size of about 1-2 nanometers, can intercept divalent ions (such as Ca²⁺, Mg²⁺, SO₄²⁻) like a sieve, while allowing monovalent ions (such as Na⁺, Cl⁻) to partially pass. For example, in a brackish water desalination project in the northwest, the removal rate of total dissolved solids (TDS) by nanofiltration membranes is as high as 96%, and the removal rate of calcium, magnesium ions and sulfates exceeds 99%. The produced water fully meets the drinking water standards, while the energy consumption per ton of water is only 1.17kWh, and the cost is as low as 0.92 yuan [1].

Compared with reverse osmosis, the advantages of nanofiltration are more significant:

▲Energy consumption halved: Its operating pressure is generally 0.5-2.0MPa, which is much lower than reverse osmosis technology. The energy consumption of nanofiltration is about 50%-70% of that of reverse osmosis. For example, when treating brackish water, the electricity consumption of nanofiltration per ton of water is 1.2-2.5kWh, while reverse osmosis requires 2.5-4kWh.

▲Retain beneficial minerals: While desalting, nanofiltration can selectively retain monovalent ions such as potassium and sodium that are beneficial to the human body, making the water quality closer to natural mineral water and producing drinking water that is more in line with health needs.

▲ Stronger anti-pollution ability: The surface of the new nanofiltration membrane uses nano-coating technology to enhance hydrophilicity, reduce organic matter and colloid adsorption, and extend the membrane life by more than 30%.

2. Challenges and breakthroughs: from “usable” to “easy to use”

Although nanofiltration technology has significant advantages, large-scale application still faces three core challenges under complex working conditions such as high salt and high pollution. However, technological innovation in recent years has achieved key breakthroughs:

Membrane fouling problem: From "passive cleaning" to "active defense": Colloids, humic acid, microorganisms and other pollutants in high-salinity brackish water easily adhere to the membrane surface to form a "fouling layer", resulting in a 30%-50% decrease in membrane flux, seriously affecting operational stability. In response to this problem, the industry has formed a dual solution of "pretreatment optimization + anti-pollution membrane materials": using activated carbon adsorption, ultrafiltration and other pretreatments to remove macromolecular organic matter; new anti-pollution membrane materials can also be used to reduce organic matter and colloid adsorption, and improve anti-pollution performance and operational stability.

High magnesium-lithium ratio separation: from "difficult to separate" to "efficient purification": The magnesium-lithium ratio (Mg²⁺/Li⁺) in salt lake brine is often as high as 1000:1, making it difficult to separate efficiently with traditional processes. As a core material for new energy, lithium has an urgent need for purification. The latest research shows that the EDTA-assisted loose nanofiltration (EALNF) process can achieve a lithium-magnesium separation coefficient as high as 679 through chelation, with a lithium ion recovery rate exceeding 90%. At the same time, nanoscale magnesium hydroxide is produced as a by-product, achieving added value of resources [2].

Cost control: Domestic substitution promotes "universal application": the large-scale production of domestic nanofiltration membranes has significantly reduced costs. For example, Rinland Environmental Technology (Beijing) Corp., Ltd.uses nanofiltration-level low-energy consumption special membranes for fluoride removal in its drinking water advanced treatment project in Baoshan, Yunnan. The construction cost of equipment per ton of water is 50% lower than the cost of imported membrane construction, and the treatment scale reaches 40,000 tons/day.

3. Future Outlook: From brackish water purification to a new era of water resources recycling

With the development of materials science and intelligent technology, nanofiltration technology is extending from a single desalination of brackish water to a broader field of resource recycling, opening a new chapter in multi-scenario applications:

In the treatment of acidic waste liquids in electroplating, chemical and other industries, the nanofiltration-electrodialysis combined process can achieve precise recovery of salt. Nanofiltration membrane separates heavy metal ions and salts, electrodialysis concentrates the salt to 18%-20%, and directly produces industrial refined salt.

Intelligent integrated system: making water treatment "smarter"

The integration of the Internet of Things, big data technology and nanofiltration systems promotes the transformation of equipment from "passive operation" to "intelligent control". Real-time monitoring of 12 key parameters such as membrane flux, transmembrane pressure difference, TDS, and pH is used to build a water quality-energy consumption correlation model through machine learning algorithms, and automatically adjust operating pressure, cleaning cycles and chemical dosage to achieve dynamic optimization.

Driven by both policy and market: accelerating technology popularization

The national level continues to promote the innovative development of membrane technology and incorporate it into the national emerging industry layout to support the construction of water resources security system. The central government has allocated 47.1 billion yuan in water conservancy development funds in 2025, and part of the funds has been clearly used to support the construction of rural water supply projects and water-saving projects, creating favorable conditions for the large-scale application of membrane technology.

In specific applications, nanofiltration technology has demonstrated the dual value of resource recovery and water conservation: in the fields of textiles, chemicals, etc., nanofiltration membranes have been successfully used to efficiently recover raw materials such as caustic soda and indigo dyes, significantly reducing wastewater discharge and consumption of new raw materials; the MBR-nanofiltration combined process has also been used for domestic sewage regeneration and reuse, and the effluent water quality can reach domestic drinking water standards, providing a reliable water source solution for water-scarce areas.

Rinland Environmental Protection deeply complies with the policy guidance and popularizes the two-wheel drive technology of "independent innovation + scene implementation", demonstrating its unique industry competitiveness. At the same time, Zhongke Rinland has built a full industry chain layout of "membrane material research and development-equipment integration-engineering services" and deeply integrated independent membrane products with intelligent control systems, which can not only reduce the overall cost of the project, but also ensure the stability of the processing effect through a standardized operation and maintenance system, providing replicable and popularizable practical samples for policy implementation.

In the future, with the continuous technological breakthroughs of new membrane materials (such as MOFs-based materials, highly selective polyamide membranes) and the continuous integration of low-carbon driving technologies (such as photovoltaic-adsorption desalination systems), nanofiltration technology is expected to play a greater role in scenarios such as the development of unconventional water sources and recycling of industrial resources, further promoting the sustainable recycling and utilization of water resources.

reference:

[1] Li Dongyang, Yan Peng, Wang Shenghui, Huang Pengfei, Wang Ruihao. Application of nanofiltration technology in brackish water desalination engineering [J]. Salt Science and Chemical Engineering, 2021, (Issue 2).

[2] The Paper "Nat Sustain: Li Zhikao/Zhang Xiwang team achieves new breakthrough in extracting lithium and magnesium resources from salt lakes"