Under working conditions of RO membranes contaminated by composite pollutants consisting of inorganic scaling, organic matter and algae, the unified industrial standard cleaning sequence is acid washing first (to remove inorganic hard scale), followed by alkaline washing (to eliminate colloids, organics and algae). Reverse sequence (alkaline washing first then acid washing) is strictly prohibited. This rule is rooted in the physicochemical properties of contaminants, chemical reaction mechanisms and membrane protection requirements. Detailed principles and hazards of reverse cleaning are elaborated below.
I. The Necessity of Acid Washing Before Alkaline Washing
1.1 Core Function of Acid Washing: Remove Hard Blockages on Membrane Surfaces
During RO membrane operation, inorganic scales (calcium carbonate, calcium sulfate, silicates, etc.) preferentially cover and encrust membrane surfaces and flow channels, while encapsulating colloids, organic substances and microbial slime trapped beneath the scale layer.
The acidic cleaning solution dissolves rigid inorganic scale, unclogs membrane pores and flow passages, and breaks the dense scale covering layer, creating favorable conditions for the penetration and stripping of soft contaminants by subsequent alkaline cleaning solution.
1.2 Core Function of Alkaline Washing: Complete Removal of Deep-Seated Soft Contamination
Colloidal slime, organic matter and algae are categorized as soft contaminants, mostly attached to membrane surfaces and gaps within scale layers. After acid washing eliminates scale and clears surface channels of the membrane, oxidative alkaline cleaning solution can fully contact the membrane substrate and deep pores. It thoroughly oxidizes and decomposes organics, strips residual colloidal particles and kills algal spores to achieve complete cleaning.
1.3 Matching the Natural Layered Structure of Composite Contamination in Actual Operation
Composite pollutants in practical systems form a layered structure: organic slime on the upper layer and inorganic hard scale attached closely to the membrane surface at the bottom. Only by breaking the underlying hard scale via prior acid washing can the surface soft contaminants be stripped from the root without residual pollutants.
II. Severe Drawbacks of Reverse Cleaning (Alkaline Washing First, Then Acid Washing)
2.1 Compaction and Solidification of Soft Contaminants Causing Irreversible Deep Blockage
If alkaline washing is performed first, the dense and rigid inorganic scale layer on the membrane remains intact, preventing alkaline solution from penetrating through the scale to reach the membrane substrate. Meanwhile, the circulating pressure during alkaline washing compresses soft colloids, organics and algal slime under high pressure, squeezing them into membrane pores and scale crevices. The originally easily removable soft contaminants become solidified blockages. Subsequent acid washing fails to dissolve the solidified organic impurities, resulting in unrecoverable water production capacity and permanent blockage of membrane pores.
2.2 New Precipitates Generated by Acid-Base Neutralization Trigger Secondary Scaling Contamination
Prior alkaline washing leaves abundant alkaline cleaning solution, hydroxide ions and dispersed colloidal particles trapped in membrane flow channels and micropores. Subsequent acid washing triggers direct acid-base neutralization reactions. Meanwhile, calcium and magnesium ions in water combine with residual alkaline substances to form fine new precipitates of calcium carbonate and magnesium hydroxide.
These newly formed precipitates feature extremely tiny particle sizes that embed deep inside membrane micropores. They are far harder to remove than original scale, drastically increasing membrane differential pressure and reducing salt rejection rate.
2.3 Sharply Reduced Cleaning Efficiency with Severe Residual Contamination
The intact inorganic scale layer blocks alkaline liquid penetration, so alkaline washing only removes a small amount of surface contaminants while deep organic and algal pollution remains completely trapped. Subsequent acid washing can only partially dissolve scale, yet solidified organic blockages and new precipitates cannot be eliminated. The overall cleaning efficiency falls below 50%. System operating parameters cannot be restored, and rapid re-contamination and membrane clogging occur within a short period.
2.4 Elevated Risk of Accelerated Membrane Element Degradation and Damage
Reverse cleaning requires repeated alternating acid and alkaline circulation to achieve marginal improvement. Excessive alternating exposure of the polyamide separation layer to acid and alkaline fluids causes persistent scouring and corrosion, accelerating membrane material aging, leading to permanent decline in salt rejection and drastically shortening the service life of RO membrane elements.
III. Summary
1. Standard sequence (acid first, then alkaline): Remove hard blockages followed by soft contaminants via layer-by-layer stripping, leaving no residual pollutants or secondary scaling. It maximizes the recovery of membrane performance and protects membrane elements.
2. Reverse sequence (alkaline first, then acid): Causes compaction and solidification of soft pollutants, secondary membrane clogging from new precipitates, ineffective cleaning and membrane damage. This non-compliant cleaning operation is one of the primary causes of premature scrapping of industrial RO membranes.
Translation Notes
1. Professional water treatment & membrane industry terminologies adopt internationally recognized standard expressions:
RO membrane 反渗透膜；differential pressure 压差；salt rejection rate 脱盐率；polyamide separation layer 聚酰胺分离层
2. Mineral scaling and chemical compound names follow chemical English norms (calcium carbonate, magnesium hydroxide, silicate, etc.)
3. Sentence structure conforms to English technical document habits, logical layers consistent with the original Chinese text
4. Industrial operation specifications, hazard descriptions and core conclusions retain the original technical rigor for overseas engineering & after-sales documents.
