Summary of common problems of Industrial reverse osmosis plant

How often should the reverse osmosis system be cleaned

In general, when the product water flow decreases by 10-15%, or the system desalination rate decreases by 10-15%, or the operating pressure and inter-stage pressure difference increase by 10-15%, the RO system should be cleaned.

The cleaning frequency is directly related to the degree of system pretreatment. When SDI<3, the cleaning frequency may be 4 times a year; when the SDI is around 5, the cleaning frequency may be doubled, but the cleaning frequency depends on each item actual situation on site.

What is SDI

At present, the best possible technique for evaluating colloidal pollution in RO/NF system feed water is to measure the sediment density index (SDI, also known as fouling index) of feed water, which is an important parameter that must be determined before RO design.

During the operation of RO/NF, measurements must be made periodically (2 to 3 times a day for surface water), and ASTM D4189-82 specifies the standard for this test.

The water inlet regulation of the membrane system is that the SDI15 value must be ≤5. Effective technologies for reducing SDI pretreatment include multi-media filter, ultrafiltration, microfiltration, etc. Adding polydielectrics prior to filtration can sometimes enhance the physical filtration described above, reducing the SDI value.

How many years can reverse osmosis membrane elements generally be used for

The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the components, the cleanability, the water source, pretreatment, cleaning frequency, operation management level, etc. According to economic analysis, it is usually more than 5 years.

What is the difference between reverse osmosis and nanofiltration

Nanofiltration is a membrane liquid separation technology between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solute with a molecular weight of less than 0.0001 micron, and nanofiltration can remove solutes with a molecular weight of about 0.001 micron.

Nanofiltration is essentially a low-pressure reverse osmosis, which is used in occasions where the purity of the water produced after treatment is not particularly strict. Nanofiltration is suitable for treating well water and surface water.

Nanofiltration is suitable for water treatment systems that do not need a high desalination rate like reverse osmosis, but has a high ability to remove hardness components, sometimes called "softening membrane". The operating pressure of the nanofiltration system is low and the energy consumption is lower than Corresponding reverse osmosis system.

What kind of desalinatiom ability does membrane technology have

Reverse osmosis is currently the membrane technology with the highest desalination ability. The reverse osmosis membrane intercepts inorganic molecules such as soluble salts and organic substances with a molecular weight greater than 100. On the other hand, water molecules can freely pass through the reverse osmosis membrane. Typical soluble The removal rate of salt is >95-99%. Operating pressures range from 7bar (100psi) for brackish water to 69bar (1,000psi) for seawater.

Nanofiltration can remove impurities with a particle size of 1nm (10 angstroms) and organic matter with a molecular weight greater than 200-400. The removal rate of dissolved solids is 20-98%, and the removal rate of salts containing monovalent anions (such as NaCl or CaCl2) is 20-80%, while the removal rate of salt containing divalent anions (such as MgSO4) is higher, which is 90-98%.

Ultrafiltration can separate macromolecules larger than 100-1,000 Angstroms (0.01-0.1 microns). All soluble salts and small molecules can pass through the ultrafiltration membrane, and the substances that can be removed include colloids, proteins, microorganisms and macromolecular organic substances. Most ultrafiltration membranes have a molecular weight cut-off of 1,000 to 100,000.

The range of microfiltration to remove particles is about 0.1 to 1 micron. Normally, suspended solids and large particle colloids can be intercepted while macromolecules and soluble salts can freely pass through microfiltration membranes. Microfiltration membranes are used to remove bacteria, micro Floc or total suspended solids TSS, typical pressure on both sides of the membrane is 1 to 3 bar.

What is the maximum allowable silica concentration in the reverse osmosis membrane

The maximum allowable concentration of silica depends on temperature, pH value and scale inhibitor. Usually, the maximum allowable concentration at the concentrated water end is 100ppm when no scale inhibitor is added. Some scale inhibitors can allow the highest concentration of silica in concentrated water 240ppm, please consult the antiscalant supplier.

How does chromium affect the RO membrane

Some heavy metals such as chromium will catalyze the oxidation of chlorine, which will cause the irreversible performance degradation of the diaphragm. This is because Cr6+ is less stable than Cr3+ in water. It seems that metal ions with high oxidation valence have stronger destructive effect. Therefore, the concentration of chromium should be reduced or at least Cr6+ should be reduced to Cr3+ in the pretreatment part.

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