How to Reduce Chemical Damage to RO Membranes in RO Systems

In the previous article, we introduced the physical damage that may occur to RO membranes during system operation, as well as methods to avoid such damage. Proper operation can help extend the service life of the membranes.

"Chemical damage" refers to the damage caused to RO membrane elements through chemical actions. Some types of chemical damage will directly destroy the desalination layer of the membrane. Once it occurs, it is irreversible and permanent damage, and the only solution is to replace the RO membrane elements. Other types of chemical damage can be mitigated through chemical cleaning to restore membrane performance to a certain extent.

1.1

The desalination layer of RO membrane elements can be damaged by strong oxidizing substances such as residual chlorine and other halogens. When excessive sodium hypochlorite is added to the raw water, but insufficient reducing agent is dosed before the RO system, residual chlorine enters the RO system and causes oxidative damage to the membrane elements.

1.2

If the activated carbon filter is used for too long, its adsorption capacity decreases, or if it operates beyond the designed flow rate, residual chlorine may pass through and enter the RO system, causing oxidation damage to the membrane elements.

1.3

When UF (ultrafiltration) and RO share the same chemical cleaning system and pipelines, if sodium hypochlorite is used for UF cleaning and the pipelines are not thoroughly flushed or replaced, residual cleaning solution may enter the RO system and cause membrane oxidation.

1.4

Incorrect dosing of strong oxidizing chemicals or the use of unqualified chemicals (such as low-quality antiscalants or non-compliant non-oxidizing biocides) may result in these chemicals entering the RO system through dosing devices and pipelines, causing oxidation or contamination of the membrane elements.

1.5

Using strong oxidizing chemicals during chemical cleaning (for example, mistakenly using sodium hypochlorite to clean RO membranes) can directly result in the scrapping of the entire set of RO membrane elements, causing significant economic losses.

1.6

Using water containing residual chlorine (such as tap water) for low-pressure flushing can also cause continuous oxidation damage. For example, in one project, RO membranes experienced a drop in salt rejection to 90% within only two weeks due to flushing with tap water.

When RO feed water contains a large amount of colloids or organic matter, the SDI value will significantly exceed the standard, resulting in a rapid increase in the pressure difference of cartridge filters and frequent replacement. When colloids or organic matter leak into the RO system, they will be retained by the front-stage membrane elements, causing an increase in differential pressure, a noticeable rise in feed pressure, and a decrease in permeate flow.

Because organic matter can provide nutrients for microbial growth, organic fouling can also lead to subsequent bacterial and microbial contamination.

Under suitable temperature conditions (20–35°C) and sufficient nutrient supply, bacteria and microorganisms can reproduce and grow very rapidly, showing exponential growth. Microbial contamination usually occurs in spring and summer, and is alleviated in winter.

In some projects, after oxidation incidents, operators are afraid to use sodium hypochlorite and instead overdose reducing agents at the RO inlet to control ORP values (while testing residual chlorine levels). Although residual chlorine may meet standards, excessive reducing agents can create anaerobic conditions, which instead promote the growth of anaerobic bacteria.

For customers in the food industry, microbial contamination is very common. Once contamination occurs, the total bacterial count and indicators such as Pseudomonas aeruginosa may exceed standards, making normal production impossible and seriously affecting water quality and plant efficiency.

In addition, RO systems in the food industry are frequently started and stopped. If low-pressure flushing is not performed after long shutdown periods, the concentrated organic matter and inorganic salts on the concentrate side will become nutrients for microorganisms, leading to rapid microbial growth.

To reduce the impact of microbial contamination on membranes, anti-fouling RO membranes can also be used, such as YIME anti-fouling membrane series.

If excessive PAM (polyacrylamide) is dosed in the pretreatment system and not fully precipitated, it may enter the membrane system. If an ultrafiltration system is present, it will first foul the UF system, then pass through to foul the cartridge filter, and eventually enter the RO membrane system.

This type of fouling is very difficult to remove through conventional chemical cleaning or flushing methods. Even if performance is partially restored, it cannot return to the original state of the membrane elements.

RO membrane surfaces are negatively charged and tend to adsorb cations. Therefore, the use of cationic PAM is not recommended. When using PAM, overdosing must be avoided, and jar tests should be conducted to determine the optimal dosage.

Inorganic scaling is one of the most common phenomena in membrane systems. It usually occurs at the tail-end membrane elements in the second or third stage of the RO system. This is because the feed water in these stages has already been concentrated by the upstream membrane elements. For example, when the overall recovery rate is 75%, the salt concentration can increase by approximately four times. When the concentration of a certain ion exceeds its solubility product, scaling will occur.

After scaling occurs, various methods such as visual inspection, raw water quality analysis, acid and alkali dissolution tests, and elemental analysis can be used to determine the nature of the scale.

Depending on the feed water quality, possible types of inorganic scale include calcium carbonate, calcium sulfate, barium sulfate, calcium fluoride, silica scale, etc. Sometimes, more than one type of scale may exist simultaneously.

Among them, carbonate scales can be effectively cleaned using hydrochloric acid or citric acid. However, for scales such as calcium sulfate, calcium fluoride, and silica, which are very difficult to remove, most cleaning agents show limited effectiveness.