Analysis of Common Problems in Reverse Osmosis Systems (Part 1)

1. How often should a reverse osmosis system be cleaned?

Generally, when the standardized flux drops 10～ 15% times, or the system's desalination rate drops 10 ～ 15% or the operating pressure and inter-stage pressure difference increase 10 ～ 15% cleaning RO system is required.

The cleaning frequency is directly related to the degree of pretreatment of the system. When SDI15<3, the cleaning frequency may be annually 4 times; when SDI15 at 5 around, the cleaning frequency may need to be doubled, but the cleaning frequency depends on the actual situation of each project site.

2. What is SDI?

Currently, the most effective way to evaluate colloidal contamination in the RO/NF system influent is to measure the Silt Density Index ( SDI ), also known as the fouling index, of the influent water. This is an important parameter that must be determined before RO design.

at During RO/NF operation, regular measurements must be taken (daily measurements for surface water 2 ～ 3 times), ASTM D4189-82 specifies the standard for this test.

The influent water specification for the membrane system is that the SDI15 value must ≤5 be. SDI Effective pretreatment technologies include multimedia filters, ultrafiltration, microfiltration, etc. Adding a polyelectrolyte before filtration can sometimes enhance the ability of the above physical filtration to reduce the SDI value.

3. Should reverse osmosis or ion exchange be used for general influent water?

Under many influent conditions, both ion exchange resin and reverse osmosis are 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.

Due to the widespread use of reverse osmosis technology, + ion exchange process or multi-stage reverse osmosis or reverse osmosis + The combination of other advanced desalination technologies has become a recognized technically and economically more reasonable water treatment scheme. For further information, please consult a water treatment engineering company representative.

4. 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, cleanability, influent water source, pretreatment, cleaning frequency, and operation and management level. Based on economic analysis, it is usually 5 years or more.

5. 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 the smallest solutes, with a molecular weight less than 0.0001 microns, while nanofiltration can remove solutes with a molecular weight of 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 removal capacity for hardness components, sometimes called "softening membrane ” . Nanofiltration systems operate at low pressure and have lower energy consumption than corresponding reverse osmosis systems.

6. What is the separation capacity of membrane technology?

Reverse osmosis is currently the most 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 7bar （ 100psi ）for brackish water influent to 69bar （ 1,000psi ）for seawater influent.

Nanofiltration can remove particles of 1nm（ 10 angstroms) 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 (such as NaCl or CaCl2 ）is 20 ～ 80% while the removal rate of salts containing divalent anions (such as MgSO4 ）is higher, at 90 ～ 98%。

Ultrafiltration separates macromolecules larger than 100～ 1,000 angstroms ( 0.01 ～ 0.1 microns). All soluble salts and small molecules can pass through the ultrafiltration membrane. Removable substances include colloids, proteins, microorganisms, and macromolecular organic substances. The molecular weight cutoff of most ultrafiltration membranes is 1,000 ～ 100,000 。

The range of particle removal by microfiltration is approximately 0.1～ 1 micrometers. Typically, suspended solids and large colloidal particles can be 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 ～ 3bar。

Seven, Who sells membrane cleaning agents or provides cleaning services?

Water treatment companies can provide specialized membrane cleaning agents and cleaning services. 'Users can purchase cleaning agents for membrane cleaning according to the recommendations of the membrane company or equipment supplier.'

Eight, What is the maximum allowable concentration of silica in the feed water for reverse osmosis membranes?

The maximum allowable concentration of silica depends on temperature, pH value and antiscalants. Usually, when no antiscalant is added, the maximum allowable concentration at the concentrate side is 100ppm Some antiscalants allow the silica concentration in the concentrate to be as high as 240ppm Please consult the antiscalant supplier.

Nine, Chromium What is the impact of chromium on

Some 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+ The higher the oxidation state of the metal ion, the stronger this destructive effect seems to be. Therefore, the concentration of chromium should be reduced in the pretreatment section, or at least Cr6+ should be reduced to Cr3+ 。

Ten, What kind of pretreatment is generally required for RO systems?

The typical pretreatment system consists of the following: coarse filtration (~ 80 μm) to remove large particles, addition of oxidants such as sodium hypochlorite, followed by precision filtration through a multimedia filter or clarifier, then addition of sodium bisulfite to reduce residual chlorine and other oxidants, and 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, softening or the addition of acid and antiscalants is recommended; for water sources with high microbial and organic matter content, activated carbon or anti-fouling membrane elements are also needed.

Eleven, Can reverse osmosis remove microorganisms such as viruses and bacteria?

Reverse osmosis ( RO) is very dense and has a very high removal rate for viruses, bacteriophages, and bacteria, at least 3log or more (removal rate >99.9% ). 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.

Twelve, What is the effect of temperature on permeate flux?

The higher the temperature, the higher the permeate flux, and vice versa. When operating at higher temperatures, the operating pressure should be reduced to keep the permeate flux constant, and vice versa.

Thirteen, What is particulate and colloidal fouling? How is it determined?

Once particulate and colloidal fouling occurs in reverse osmosis or nanofiltration systems, it will seriously affect the permeate flux of the membrane, and sometimes also reduce the desalination rate.

An early symptom of colloidal fouling is an increase in system pressure drop. The sources of particles or colloids in the membrane feed water vary from place to place and often include bacteria, silt, colloidal silica, iron corrosion products, etc. The chemicals used in the pretreatment section, such as polyaluminum chloride and ferric chloride or cationic polyelectrolytes, if not effectively removed in the clarifier or media filter, may also cause fouling.