Why RO Membranes Require Acid Cleaning First Followed by Alkaline Cleaning, and Can the Sequence Be Reversed?
July 9, 2026
For chemical cleaning of reverse osmosis (RO) membranes, the sequence of acid cleaning first then alkaline cleaning must be strictly followed. This order cannot be reversed under any circumstances; otherwise, contaminants will solidify and membrane fouling will be aggravated. The reasoning can be explained by contaminant categories and chemical reaction mechanisms.
I. Contaminant Types Dictate Cleaning Sequence
RO membrane fouling is typically composite fouling, consisting of intertwined inorganic mineral scale, organic matter and biological slime that form a rigid layer similar to concrete. Inorganic scale acts as the structural framework, while organic substances serve as adhesive fillers.
Inorganic mineral scale: Calcium carbonate, calcium sulfate, iron and aluminum oxides, etc., which can only be dissolved by acid cleaning.
Organic matter and microorganisms: Bacterial exudates, humic acid, algae and other substances that can only be eliminated via alkaline cleaning.
II. Core Logic of Acid Cleaning Prior to Alkaline Cleaning
The correct sequence works by first breaking down the structural framework with acid, then thoroughly removing residual contaminants with alkali.
Functions of acid cleaning
Acid preferentially dissolves inorganic mineral deposits such as calcium carbonate and metal oxides in scale. Besides partial contaminant removal, it disintegrates the integrated composite fouling layer, turning the dense contaminant coating porous and loose to create penetration channels for subsequent alkaline cleaning.
Functions of alkaline cleaning
Subsequent alkaline solution penetrates deep into the fouling layer through the opened channels. It saponifies, hydrolyzes and strips exposed organic matter and microbial residues, flushing them entirely out of the membrane system.
III. Consequences of Reversing the Sequence (Alkali First, Then Acid)
If alkaline cleaning is carried out first, the alkaline solution directly reacts with metal ions (calcium, magnesium, iron, etc.) within the fouling layer, generating massive water-insoluble metal hydroxide precipitates such as flocculent colloids of calcium hydroxide and magnesium hydroxide.
Contaminant solidification
The newly formed hydroxide colloids act like paste, firmly trapping organic matter and slime inside membrane pores. This renders the fouling layer denser and harder, severely worsening membrane blockage.
Ineffective subsequent acid cleaning
Even if acid cleaning is performed afterward, acid solution can barely permeate the dense mixed layer of hydroxides and organics, completely losing its cleaning efficacy.
Brief Summary
Acid cleaning breaks down the fouling framework and opens penetration paths; alkaline cleaning penetrates the layer for thorough removal. Reversing the order turns the mineral framework into paste and clogs membrane pores permanently. This mandatory sequence is determined by chemical principles and must be fully complied with.
Special Circumstances and Contamination Identification Methods
While acid-first-alkali-second is the general rule, minor adjustments shall be made on-site based on actual fouling types.
How to Identify Fouling Categories
Open the membrane housing for inspection:
Rough, dry, white or yellowish deposits = dominant inorganic scaling, requiring extended acid cleaning;
Slippery, viscous foulants with fishy odor = severe microbial fouling, requiring intensified alkaline cleaning.
In addition, slippery, odorous security filter cartridges also indicate heavy biological contamination.
Three-Step Cleaning Process for Severe Microbial Fouling
If microorganisms are confirmed as the primary pollutant, adopt the sequence: Alkaline Cleaning → Acid Cleaning → Alkaline Cleaning
High-pH chemicals first hydrolyze and strip most organic contaminants;
Low-pH acid dissolves residual inorganic scale and flushes away debris;
A final alkaline wash completely removes residual biofilm on membrane surfaces.
The intermediate acid cleaning step is critical to prevent hydroxide solidification that would occur with alkaline-only cleaning.
