How to Clean Reverse Osmosis Membranes: Should Acid Washing or Alkaline Washing Come First?

Reverse osmosis (RO) membrane technology is widely used in seawater desalination, wastewater treatment, and industrial wastewater reuse. However, membrane fouling remains a key factor limiting the long-term stable operation of reverse osmosis systems.

During operation, reverse osmosis membranes are subjected to various contaminants, primarily including: inorganic deposits (such as calcium and magnesium scale), which clog membrane pores and reduce water recovery; organic contaminants (such as microorganisms and oils), which coat the membrane surface and decrease filtration efficiency; and colloidal particles, which block membrane pores and degrade membrane performance. These contaminants adhere to the membrane surface, causing a gradual decline in membrane performance; therefore, regular cleaning is essential.

During the cleaning process, a common dilemma arises: should reverse osmosis membranes be acid-washed or alkali-washed first? This decision directly impacts the effectiveness of membrane cleaning and the service life of the membranes. If the sequence is incorrect, not only will cleaning be ineffective, but it may also accelerate membrane wear; in extreme cases, membrane modules may even need to be replaced.

Common Membrane Cleaning Formulations

Option 1
A mixed solution of 0.2% sodium hypochlorite (NaClO) and 0.1% sodium hydroxide (NaOH). This option is suitable for removing membrane fouling caused by organic matter and active microorganisms.

Option 2
A 1%–2% citric acid solution or a 0.4% hydrochloric acid (HCl) solution. This option is suitable for removing iron-based contamination and blockages caused by carbonate crystallization.

Option 3
A mixture of 0.3% hydrogen peroxide (H₂O₂) and 0.3% sodium hydroxide (NaOH). This is suitable for cleaning membrane module contamination caused by glutamic acid fermentation broth.

Option 4
0.5% aqueous nitric acid (HNO₃) solution. This option is suitable for removing blockages caused by lead phosphate during the electrophoretic coating process (this cleaning step should be performed after other routine chemical cleanings).

Option 5
1% formaldehyde solution. Suitable for cleaning bacterial contamination on ultrafiltration membrane modules.

Solution 6
A mixture of 9% sodium dodecylbenzenesulfonate, 9% surfactant, 0.4% sodium hydroxide (NaOH), 0.15% anhydrous sodium carbonate, 11% sodium phosphate, and 10% sodium silicate. The pH must be strictly controlled during the cleaning process. Some membrane modules are sensitive to high-pH cleaning solutions; please select this option with caution. This solution is primarily used to remove membrane fouling caused by oily wastewater.

Solution 7
20% sulfuric acid (H₂SO₄). This solution is primarily used to remove fouling caused by silica scale crystallization.

Solution 8
3% phosphoric acid (H₃PO₄), 0.5% disodium ethylenediaminetetraacetate. This solution is primarily used to remove membrane fouling caused by protein and grease contaminants.

Precautions for Chemical Cleaning
Water for cleaning solution preparation: softened water or product water, free of heavy metals, residual chlorine, or other oxidizing agents

Cleaning solution volume: 40–80 liters per 8-inch element (depending on the degree of fouling)

Per 4-inch element: 10–20 liters (depending on the degree of fouling) Cleaning operating pressure

Low pressure (0.1–0.3 MPa), typically not exceeding 0.4 MPa Cleaning flow rate

Per 8-inch pressure vessel: 6–9 m³/hr Per 4-inch pressure vessel: 1.8–2.3 m³/hr

The cleaning solution temperature should be as high as possible, but not exceeding 45°C; typically, 30–35°C is selected. If the cleaning solution temperature exceeds 45°C, cooling equipment must be installed.

Cleaning Method: Each pressure vessel should undergo alternating cycles of recirculation and soaking.

Cycle Interval: Recommended every 0.5–1 hour (repeat 2–3 times)

Soaking time: 2–24 hours (depending on the degree of fouling)

Cleaning method: Segmented cleaning is preferred

Cleaning duration: At least 1–2 hours, depending on the degree of contamination and the cleaning method used



Principles and objectives of acid and alkali cleaning

1. Acid cleaning:

Objective: Acid cleaning is primarily used to remove inorganic salt deposits from the membrane surface, such as calcium carbonate and iron oxide.

Principle: Acidic solutions (such as citric acid, nitric acid, etc.) dissolve salt deposits or convert them into soluble substances, thereby dislodging deposited inorganic salt deposits from the membrane surface.

Common agents: Citric acid, nitric acid, hydrochloric acid.

2. Alkaline Cleaning:

Purpose: Alkaline cleaning is primarily used to remove organic and biological contaminants.

Principle: Alkaline solutions (such as sodium hydroxide, potassium hydroxide, etc.) saponify fats and oils, decompose organic matter, and dissolve the extracellular polymers of microorganisms, thereby removing organic and biological contaminants.

Common agents: Sodium hydroxide, trisodium phosphate, sodium hypochlorite (low concentration, specifically for microbial contamination).

Optimizing the Sequence of Acid and Alkaline Washing

The cleaning of reverse osmosis membranes typically involves two steps: acid washing and alkaline washing.

Acid washing is primarily used to remove inorganic salt deposits, such as calcium carbonate, calcium sulfate, and iron oxides;

Alkaline washing: Primarily used to remove organic matter and biological contaminants, such as oils and microbial slime.

Scenario 1: Acid First, Then Alkaline

Prioritizing Acid Washing: Removing Inorganic Scale: Inorganic salt scale is typically the primary cause of membrane surface fouling and blockage. Performing acid washing first effectively removes these hard scale layers, thereby making the subsequent alkaline washing process more effective.

Follow-up alkaline cleaning: Thorough removal of organic matter: After acid washing, the inorganic salt deposits on the membrane surface have been removed. At this point, the remaining organic matter and biofilms can be more effectively exposed and react with the alkaline solution, achieving a better cleaning result.



Scenario 2: Alkaline first, then acid

Alkaline Cleaning First: Alkaline solutions effectively break down organic matter and microbial fouling, reducing obstacles for the subsequent acid wash. This step ensures the membrane surface is cleaner, allowing the acidic solution to better contact inorganic deposits and guarantee superior cleaning results.

Acid Cleaning Next: The purpose of acid cleaning is to remove inorganic deposits. Since the alkaline wash has already cleared organic contaminants, the acidic cleaning agent can directly act on inorganic salts, achieving better descaling results.



Operational Procedures for Acid and Alkaline Cleaning

1. System Shutdown and Drainage

Shut down the reverse osmosis system and drain the feed water from the membrane modules, ensuring all water and residues are completely removed. Before starting chemical cleaning, rinse the membrane system with clean water to remove loose contaminants from the membrane surface and reduce chemical consumption.

2. Alkaline Cleaning Procedure

Prepare an alkaline cleaning solution with a concentration of 0.5–1% (e.g., sodium hydroxide solution) and maintain the solution temperature at 30–40°C. Circulate the alkaline cleaning solution through the membrane modules for 30–60 minutes; the exact duration depends on the degree of contamination. Maintain a consistent flow rate and pressure during the cleaning process to prevent membrane damage. After the alkaline wash, thoroughly rinse the membrane modules with clean water to ensure all alkaline solution is flushed out, continuing until the effluent pH approaches neutral (approximately 7).

3. Acid Wash Procedure

Prepare an acidic cleaning solution with a concentration of 1–2% (e.g., citric acid or oxalic acid solution), maintaining a temperature of 25–35°C. Circulate the acidic cleaning solution through the membrane modules for 30–60 minutes, adjusting the duration based on the extent of inorganic deposits. During acid washing, periodically monitor the cleaning effectiveness of the membrane system to ensure that inorganic contaminants (such as calcium carbonate and iron oxides) are effectively removed. After acid washing, rinse the membrane system again with clean water to ensure that all acidic residues are thoroughly removed, continuing until the effluent pH returns to neutral.

4. Final Inspection

After cleaning is complete, inspect the system’s operation to ensure the membrane elements are clean and have resumed normal water production rates. Conduct routine tests on the system to confirm that membrane filtration performance has returned to normal and that the effluent quality meets standards.