Common problems in reverse osmosis in wastewater treatment

How often should a reverse osmosis system be cleaned?

　　Generally, when the standardized flux decreases by 10～15%, or the system desalination rate decreases by 10～15%, or the operating pressure and interstage 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 SDI15 <3, the cleaning frequency may be 4 times a year; when SDI15 is around 5, the cleaning frequency may need to be doubled, but the cleaning frequency depends on the actual situation of each project site.

　　What is SDI?

　　Currently effective evaluation The best technology for evaluating possible colloidal contamination in the feed water of RO/NF systems is to measure the silt density index (SDI) of the feed water. This is an important parameter that must be determined before RO design. During the operation of RO/NF, regular measurements must be carried out. ASTM D4189-82 specifies the standard for this test. The feed water specification for membrane systems is that the SDI15 value must be ≤5. Effective pretreatment technologies for reducing SDI include multimedia filters, ultrafiltration, and microfiltration. Adding polyelectrolytes before filtration can sometimes enhance the ability of the above physical filtration to reduce the SDI value.

　　Should reverse osmosis or ion exchange be used for general feed water?

　　Under many feed water conditions, the use of ion exchange resin or reverse osmosis is technically feasible. The choice of process should be determined by economic comparison. Generally, the higher the salt content, the more economical reverse osmosis is; the lower the salt content, the more economical ion exchange is.

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

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

What is the difference between reverse osmosis and nanofiltration?

　　Nanofiltration is a membrane-based liquid separation technology located between reverse osmosis and ultrafiltration. Reverse osmosis can remove smaller solutes with a molecular weight less than 0.0001 microns, while nanofiltration can remove solutes with a molecular weight of around 0.001 microns. Nanofiltration is essentially a low-pressure reverse osmosis, used in situations where the purity of the treated water is not particularly strict. Nanofiltration is suitable for treating well water and surface water. Nanofiltration is suitable for water treatment systems that do not require the high desalination rate of reverse osmosis, but it has a high ability to remove hardness components and is sometimes called a "softening membrane." Nanofiltration systems have low operating pressure and lower energy consumption than corresponding reverse osmosis systems.

　　What is the separation capacity of membrane technology?

　　Reverse osmosis is a precise liquid filtration technology. Reverse osmosis membranes retain soluble salts and other inorganic molecules and organic substances with a molecular weight greater than 100, while water molecules can freely pass through the reverse osmosis membrane. The typical removal rate of soluble salts is >95～99%. The operating pressure ranges from 7 bar for brackish water feed water to 69 bar for seawater. Nanofiltration can remove impurities with particles of 1 nm and organic substances with a molecular weight greater than 200～400. The removal rate of soluble solids is 20～98%, the removal rate of salts containing monovalent anions is 20～80%, and the removal rate of salts containing divalent anions is higher, at 90～98%. Ultrafiltration separates macromolecules larger than 100～1,000 Å. All soluble salts and small molecules can pass through the ultrafiltration membrane. Substances that can be removed include colloids, proteins, microorganisms, and macromolecular organic matter. The molecular weight cutoff of most ultrafiltration membranes is 1,000～100,000. Microfiltration removes particles in the range of approximately 0.1～1 micron. Typically, suspended solids and large colloidal particles are retained, while macromolecules and soluble salts can freely pass through the microfiltration membrane. Microfiltration membranes are used to remove bacteria, microflocs, or total suspended solids (TSS). The typical pressure difference across the membrane is 1～3 bar.

　　What is the effect of chromium on RO membranes?

　　Certain heavy metals, such as chromium, can catalyze the oxidation of chlorine, leading to irreversible performance degradation of the membrane. This is because in water, Cr6+ is less stable than Cr3+. It seems that the higher the oxidation state of the metal ion, the stronger this destructive effect is. Therefore, the concentration of chromium should be reduced in the pretreatment section, or at least Cr6+ should be reduced to Cr3+.

　　What kind of pretreatment is generally required for RO systems?

　　A typical pretreatment system consists of the following: coarse filtration to remove large particles; addition of an oxidizing agent such as sodium hypochlorite; then precise filtration through a multimedia filter or clarifier; addition of sodium bisulfite to reduce residual chlorine and other oxidizing agents; finally, a security filter is installed before the high-pressure pump inlet. The role of the security filter, as its name suggests, is as a final safety measure to prevent accidental large particles from damaging the high-pressure pump impeller and membrane elements. Water sources with high particulate matter usually require a higher degree of pretreatment to meet the specified feed water requirements. ;For water sources with high hardness content, it is recommended to use softening or acidification and scale inhibitors. For water sources with high microbial and organic matter content, activated carbon or anti-fouling membrane elements are also needed.

　　Can reverse osmosis remove microorganisms such as viruses and bacteria?

　　Reverse osmosis is very dense and has a very high removal rate for viruses, bacteriophages, and bacteria, at least 3log or more. However, it should also be noted that in many cases, microorganisms may still regrow on the membrane permeate side. This mainly depends on the assembly, monitoring, and maintenance methods. In other words, the ability of a system to remove microorganisms depends on whether the system design, operation, and management are appropriate, not the nature of the membrane element itself.