Nanofiltration devices are widely used in the purification of drinking water and industrial water, wastewater purification, and concentration of valuable components in process fluids. Due to its high separation performance and low operating pressure, nanofiltration devices play an important role in environmental protection and water treatment.

Definition and Principle:
Nanofiltration (NF) equipment is a pressure-driven membrane separation device between ultrafiltration and reverse osmosis. Its core is the nanofiltration membrane, the pore size of which is generally between 1 and 10 nanometers. Its separation principle is based on the screening effect of the nanofiltration membrane on different solutes and the charge effect between the membrane and the solute.

During operation, when a solution containing solute molecules and ions of different sizes passes through the nanofiltration membrane under pressure, the nanofiltration membrane allows water and monovalent ions (such as sodium ions, chloride ions, etc.) and small molecular organic matter to selectively pass through, while having a high retention rate for divalent and higher ions (such as calcium, magnesium ions, sulfate ions, etc.), macromolecular organic matter and colloids. For example, in the process of water treatment, nanofiltration can remove hardness ions in water to a certain extent, while allowing some salt to pass through, thereby achieving partial softening of water quality and separation of specific solutes.

Main components:
Nanofiltration membrane assembly:
Rolled membrane assembly:

This is a common form of membrane assembly in nanofiltration equipment. It is made of multiple layers of nanofiltration membrane, flow guide net and central tube, etc., which are tightly wound. When working, the incoming water flows along one side of the membrane. Driven by pressure, some solutes and water pass through the membrane and are collected in the central tube through the flow guide net to become permeate, while the retained substances remain on the other side of the membrane. The roll membrane assembly has a high membrane packing density and can provide a large membrane area in a small space, thereby improving the filtration efficiency.

Hollow fiber membrane assembly:

It is composed of many hollow fiber nanofiltration membranes. Water enters from the outside of the fiber membrane. Under the action of pressure, small molecules and some ions pass through the membrane wall into the inside of the fiber membrane, while large molecules and multivalent ions are retained on the outside of the membrane. The advantage of the hollow fiber membrane assembly is that the membrane area per unit volume is large, but when treating solutions with high concentrations of suspended matter, it is easy to get blocked.

Shell and support structure:
It is mainly used to protect the nanofiltration membrane assembly and provide it with necessary mechanical support. The shell material is usually stainless steel, fiberglass or high-performance engineering plastics, etc. These materials have good corrosion resistance and sufficient strength to withstand the pressure generated during the nanofiltration process. The design of the shell should ensure that the inlet and outlet water can be evenly distributed in all parts of the membrane assembly to ensure the efficient operation of the equipment.

High-pressure pump and circulation pump:
High-pressure pump:

Provides power for the nanofiltration process, so that the inlet water can overcome the osmotic pressure and resistance of the nanofiltration membrane and pass through the membrane assembly for separation. The nanofiltration process usually requires a higher pressure, generally between 1-10MPa, and the specific pressure depends on the inlet water quality, the performance of the nanofiltration membrane and the required separation effect. The selection of the high-pressure pump needs to be determined according to the flow requirements and the required pressure to ensure that a stable pressure source can be provided.

Circulation pump (some equipment has):

In some nanofiltration systems, in order to improve the separation efficiency and reduce membrane fouling, a circulation pump is set. The circulation pump returns part of the concentrated water that has not passed through the membrane to the feed port, mixes with the newly entered inlet water, and then performs nanofiltration again. By increasing the flow rate on the membrane surface, the accumulation of solutes on the membrane surface can be reduced, reducing the risk of membrane fouling.

Valve and piping system:
Valves are used to control the flow direction, flow rate and pressure of liquids. It mainly includes water inlet valve, water outlet valve, concentrated water discharge valve, cleaning liquid injection valve, etc. By reasonably adjusting the opening of the valve, precise control of the nanofiltration process can be achieved. The pipeline system is responsible for transporting the inlet water, permeate and concentrated water between the various components of the equipment. Pipeline materials usually choose corrosion-resistant materials, such as stainless steel pipes, UPVC pipes (rigid polyvinyl chloride pipes) or PE pipes (polyethylene pipes).

Cleaning system:
Since the nanofiltration membrane will be contaminated during use, the cleaning system is an important part of the nanofiltration equipment. The cleaning system includes chemical cleaning devices and physical cleaning devices. Chemical cleaning is to remove dirt on the membrane surface by injecting specific chemical cleaning agents into the membrane assembly, such as acids (for removing inorganic substances such as scale), alkalis (for removing organic matter and microorganisms), chelating agents (for removing metal ion pollutants), etc. Physical cleaning includes water washing, backwashing and sponge ball scrubbing to remove loose dirt on the membrane surface.

Workflow:
Pretreatment:

Before nanofiltration, raw water usually needs to be pretreated, such as removing larger suspended particles, colloids and some organic matter in the water by filtration, sedimentation, coagulation and other methods. The pretreated water enters the feed port of the nanofiltration equipment.

Nanofiltration process:

The feed pump delivers the pretreated water to the nanofiltration membrane assembly. Under the pressure provided by the high-pressure pump, water and some solutes pass through the nanofiltration membrane to form a permeate (product water), while the intercepted divalent and above ions, macromolecular organic matter and colloids form concentrated water. The permeate can be output as product water after being collected through the pipeline, and the concentrated water is discharged or further treated according to the specific situation.

Cleaning and maintenance:

As the nanofiltration process continues, the flux of the nanofiltration membrane will gradually decrease, which is due to the contamination of the membrane surface. When the membrane flux drops to a certain level, the nanofiltration membrane needs to be cleaned. First, rinse with clean water to remove loose dirt on the membrane surface, and then select a suitable chemical cleaning agent for chemical cleaning according to the type of contamination. After cleaning, the nanofiltration equipment can restore good filtration performance.

Performance characteristics:
Selective separation:

A notable feature of nanofiltration equipment is its selective separation ability. It can remove most of the harmful divalent and higher ions (such as heavy metal ions, hardness ions, etc.) and macromolecular organic matter in water while allowing some monovalent ions and small molecular organic matter to pass through, thereby achieving fine adjustment of water quality. For example, in drinking water treatment, some hardness and trace organic matter in water can be removed while retaining minerals that are beneficial to the human body.

Higher retention rate and flux:

For target interception substances, nanofiltration membranes usually have a higher retention rate, generally up to 90% - 99% for divalent ions, and a higher retention rate for macromolecular organic matter. At the same time, nanofiltration equipment has a good flux under appropriate operating conditions, and the flux range is generally between 10 - 100L/(m²・h), which can process a certain amount of liquid per unit time.

Anti-pollution ability:

The surface of the nanofiltration membrane has been specially treated and has a certain anti-pollution ability. However, due to the complex interaction between solutes and membranes during the nanofiltration process, the membrane is still susceptible to contamination by organic matter, colloids, and microorganisms. Therefore, reasonable pretreatment and regular cleaning are essential to maintain the high performance of nanofiltration equipment.

Compact equipment and high degree of automation:

The structure of nanofiltration equipment is relatively compact, especially the equipment using roll membrane components, which can achieve efficient separation functions in a smaller space. At the same time, the equipment has a high degree of automation, and can achieve precise control of processes such as feeding, filtration, and cleaning through an automatic control system, reducing the complexity and errors of manual operations.

Application areas:
Drinking water treatment:
Used to deeply purify raw water, remove some hardness ions (such as calcium and magnesium ions), trace organic matter (such as pesticide residues, disinfection by-products, etc.) and heavy metal ions (such as lead, mercury, etc.) in water, while retaining minerals that are beneficial to the human body. For example, in some high-end drinking water production, nanofiltration equipment can process tap water or natural water sources into products that meet high-quality drinking water standards.

Industrial wastewater treatment:
Dyeing wastewater treatment:

In the dyeing industry, nanofiltration equipment can be used to recover dyes and auxiliaries in dyeing wastewater, while removing salts and some organic matter in wastewater to achieve wastewater resource utilization and reduction. Through nanofiltration separation, dyes can be recycled and reused, and the permeated water can be reused or discharged after further treatment.

Electroplating wastewater treatment:

It can effectively remove heavy metal ions and some additives in electroplating wastewater, while recovering valuable metals and water resources. Nanofiltration membranes have a high retention rate for heavy metal ions (such as chromium, nickel, copper, etc.) in electroplating wastewater, and the treated wastewater can meet the discharge standard or reuse standard.

Food industry:
Juice concentration and clarification:

During the juice processing process, nanofiltration equipment can be used to remove part of the water in the juice to achieve juice concentration, while retaining small molecules such as sugar, organic acid and flavor components in the juice. In addition, nanofiltration can also remove macromolecular impurities such as pectin and protein in the juice, making the juice clearer and more transparent.

Dairy product processing:

In milk processing, nanofiltration can be used to partially desalinate and remove lactose to produce low-lactose or low-salt dairy products. At the same time, nanofiltration can also remove bacteria and viruses in milk to improve the safety of dairy products.