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Daily maintenance precautions for nanofiltration

Published Time:

2022-08-08

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In recent years, with the increasing environmental protection requirements, various reclaimed water reuse and zero-discharge wastewater projects have been launched successively. Traditional ultrafiltration and reverse osmosis membrane treatment processes can no longer fully adapt to various complex water qualities. Therefore, nanofiltration technology, which is between the two, has been gradually applied to various water treatment projects. Although nanofiltration also uses polyamide composite membranes, it still has some differences from reverse osmosis. The following is a brief introduction to nanofiltration and daily maintenance precautions.

There was a popular saying in the market: nanofiltration is a kind of loose reverse osmosis. In fact, this is a technical conceptual misunderstanding. The following figure shows a comparison of the rejection rates of different membranes.

The true separation concept of nanofiltration is to meet Donnan effect , and a filter membrane with selective retention of ions, a membrane whose sodium chloride permeability is proportional to the sodium chloride concentration and the ratio is greater than 0.4. It is mainly used for desalination and concentration of various materials.

Therefore, simply put, nanofiltration can pass through monovalent salts and retain multivalent salts , and it also has a molecular weight cut-off of 150-300 Daltons , which is a selective separation membrane.

Due to the high content of sodium chloride, a high osmotic pressure monovalent salt, in various water qualities, and the low rejection rate of nanofiltration for monovalent salts, there is no need to offset the osmotic pressure of monovalent salts, allowing nanofiltration to operate at lower pressure.

Due to these special properties of nanofiltration membranes, it can filter out most easily scaling ions and other impurities under low pressure. The main component of the permeated water is monovalent salts that are not easy to scale. Further concentration of monovalent salts using reverse osmosis will greatly reduce the pollution risk of reverse osmosis.

Therefore, this NF+RO process has been gradually used by various environmental protection companies in the industry, and its applications are mainly in various high-difficulty water treatments such as garbage leachate treatment, coal chemical wastewater treatment, mine wastewater treatment, and desulfurization wastewater zero discharge.

Since it is applied to high-difficulty wastewater, daily maintenance is essential. However, due to some objective factors, many equipment operators do not understand the characteristics and maintenance methods of the membrane. This causes various system failures. In response to these failures, some information has been compiled for your reference:

1. Failure to replace the residual liquid in the nanofiltration system with reverse osmosis water when shutting down:

Since both the concentrate and permeate of nanofiltration contain a lot of salts, if the water source in the membrane system is not replaced when shutting down, it is equivalent to soaking the membrane in salt water, which easily causes permeate back-suction, leading to delamination of the membrane separation layer and irreversible failure of the membrane retention.

2. Failure to clean in time when the membrane pressure difference increases:

Increased pressure difference indicates that the membrane is fouled. For common 6-core membrane shells, the maximum pressure drop is 3.45 bar, and the maximum pressure drop for a single membrane element is 0.69 bar.

When the pressure drop exceeds the above limit, the membrane element may be damaged by mechanical stress, which may lead to cracking of the fiberglass shell of the membrane and extrusion of the inlet screen. Cracking of the fiberglass shell and extrusion of the inlet screen do not necessarily immediately affect the water quality of the membrane element permeate, but the normal working life of the membrane element will be shortened.

High pressure drop is often caused by blockage of the membrane element inlet screen. In addition, water hammer during system startup can also cause high pressure drop.

3. Debris and fragments in the pipeline are not flushed clean before entering the membrane:

PVC debris or even some metal debris in the pipeline, if entering the membrane, may scratch the membrane under the action of water pressure, leading to a decrease in the membrane's retention performance. Before installing the membrane, it is necessary to flush the pipeline to avoid unnecessary losses.

4. No ORP monitoring, using tap water to soak the membrane:

Municipal tap water generally adds chlorine for disinfection. The residual chlorine in the water has strong oxidizing properties. Nanofiltration membranes are high-molecular organic materials and do not allow residual chlorine to enter the membrane, otherwise it will cause irreversible damage to the membrane.

If tap water must be used for treatment, a 1% concentration of sodium bisulfite solution can be added for protection. Oxidizability can be monitored using an ORP oxidation-reduction potential meter. Generally speaking, the membrane system is relatively safe below 200 mV.