Reverse osmosis (RO) membrane technology has been widely applied in fields such as seawater desalination, wastewater treatment, and industrial wastewater reuse. However, membrane fouling remains a critical factor limiting the long-term stable operation of reverse osmosis systems.
During operation, reverse osmosis membranes are subjected to "attacks" from various contaminants, which primarily include: - Inorganic deposits (such as calcium and magnesium scale): These can clog membrane pores, reducing water production efficiency. - Organic contaminants (such as microorganisms and grease): These can block the membrane surface, lowering filtration efficiency. - Colloidal particles: These can clog membrane pores, diminishing membrane performance. These contaminants adhere to the membrane surface, leading to gradual performance degradation. Therefore, regular cleaning is essential.
During the cleaning process, people often face a dilemma: should the reverse osmosis membrane be acid-washed or alkali-washed first? This question directly impacts the cleaning effectiveness and service life of the membrane. If the sequence is incorrect, not only will the cleaning effect be poor, but it may also accelerate membrane wear, and in extreme cases, even necessitate the replacement of the membrane module.
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01
Common Film Washing Formulations
Option 1
A mixed solution of 0.2% sodium hypochlorite (NaClO) and 0.1% sodium hydroxide (NaOH) was employed, which 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, suitable for removing iron contamination and blockages caused by carbonate crystallization.
Option 3
A mixed solution of 0.3% hydrogen peroxide (H₂O₂) and 0.3% sodium hydroxide (NaOH) is suitable for cleaning membrane module fouling caused by glutamic acid fermentation broth.
Option 4
A 0.5% aqueous solution of nitric acid (HNO3), which is suitable for removing lead phosphate blockages on membrane modules caused during the electrophoretic paint treatment process (this cleaning step should be performed after other routine chemical cleaning procedures).
Option Five
1% formaldehyde solution, suitable for bacterial contamination cleaning of ultrafiltration membrane modules.
Option Seven
9% sodium dodecyl benzene sulfonate, 9% surfactant, 0.4% sodium hydroxide (NaOH), 0.15% anhydrous sodium carbonate, 11% sodium phosphate, 10% sodium silicate. The pH value must be strictly controlled during the cleaning process. Some membrane modules are sensitive to high-pH cleaning solutions, so please choose carefully. This solution is primarily used to remove membrane fouling caused by oily wastewater.
Option Eight
20% sulfuric acid (H2SO4), this solution is primarily used to remove contamination caused by silicon scale crystallization.
Option 9
3% phosphoric acid (H3PO4), 0.5% disodium ethylenediaminetetraacetate, this solution is primarily used to remove membrane fouling caused by protein and oil contaminants.
02
Precautions for Chemical Cleaning
Water for Cleaning Agent Preparation
Softened water or product water, free from heavy metals, residual chlorine, or other oxidizers
Cleaning agent dosage
Each 8-inch element: 40-80 liters (depending on dirt level)
Each 4-inch element: 10-20 liters (depending on dirt level)
Cleaning operation pressure
Lower pressure (0.1-0.3 MPa), typically not exceeding 0.4 MPa
Cleaning flow rate
Each 8-inch pressure vessel: 6-9 m³/hr
Each 4-inch pressure vessel: 1.8-2.3 m³/hr
Cleaning solution temperature
As high as possible, but the maximum temperature is 45°C. It is usually set between 30-35°C. If the temperature of the cleaning solution exceeds 45°C, cooling equipment should be installed
Cleaning Type
Each pressure vessel alternately undergoes recirculation and soaking
cycle period
It is recommended to do it every 0.5-1 hour (repeat 2-3 times)
soaking time
2-24 hours (depending on the degree of dirt)
cleaning method
It is best to perform segmented cleaning
cleaning time
Minimum 1-2 hours, depending on the level of contamination and the cleaning method used
The principle and purpose of acid washing and alkali washing: 1. Acid washing: Purpose: Acid washing is mainly used to remove inorganic salt deposits on the surface of membranes, such as calcium carbonate, iron oxide, etc. Principle: Acidic solutions (such as citric acid, nitric acid, etc.) dissolve salt deposits or convert them into soluble substances, thereby causing the deposited inorganic salt deposits to detach from the membrane surface. Common medications: citric acid, nitric acid, hydrochloric acid. 2. Alkali washing: Purpose: Alkali washing is mainly used to remove organic and biological pollution. Principle: Alkaline solutions (such as sodium hydroxide, potassium hydroxide, etc.) can saponifie oil, decompose organic matter, and remove organic and biological pollution by dissolving extracellular polymers of microorganisms. Common chemicals: Sodium hydroxide, trisodium phosphate, sodium hypochlorite (low concentration, specifically for microbial contamination). The optimization of acid washing and alkali washing sequence for reverse osmosis membrane cleaning usually includes two steps: acid washing and alkali washing. Acid washing is mainly used to remove inorganic salt deposits, such as calcium carbonate, calcium sulfate, iron oxides, etc; Alkali washing: mainly used to remove organic matter and biological pollution, such as oil, microbial mucus, etc. Scenario 1: First acid, then alkaline acid washing priority: Remove inorganic scale: Inorganic salt scale is usually the main factor causing membrane surface coverage and blockage. If acid washing is carried out first, these hard scale layers can be effectively removed, making the subsequent alkaline washing process more effective.
Alkali washing follow-up: Thoroughly remove organic matter: After acid washing, the inorganic salt scale on the membrane surface has been removed, and the residual organic matter and biofilm can be more effectively exposed and react with alkaline solution to achieve better cleaning effect. Scenario 2: First alkaline, then acid-base washing. First alkaline solution can effectively decompose organic matter and microbial dirt, reducing obstacles for subsequent acid washing. This step can make the membrane surface cleaner, allowing the acidic solution to better contact the inorganic deposits and ensure better cleaning effect.
Acid washing followed by: The purpose of acid washing is to remove inorganic deposits. Due to the removal of organic pollution by alkaline washing, acidic cleaning agents can directly act on inorganic salts, achieving better descaling effects.
05 Operation process of acid washing and alkali washing
1. System shutdown and emptying: Shut down the reverse osmosis system, empty the incoming water from the membrane components, and ensure that all water and residue are drained. Before starting chemical cleaning, rinse the membrane system with clean water to remove loose contaminants on the membrane surface and reduce the consumption of chemical agents.
2. Prepare an alkaline cleaning solution (such as sodium hydroxide solution) with a concentration of 0.5-1%, and control the temperature of the cleaning solution at 30-40 ℃. Cycle the alkaline cleaning solution through the membrane module for 30-60 minutes, depending on the degree of contamination. During the cleaning process, it is necessary to maintain a uniform flow rate and pressure to avoid membrane damage. After alkaline washing, thoroughly rinse the membrane module with clean water to ensure that all alkaline solution is discharged, and rinse until the pH value of the effluent is close to neutral (about 7).
3. Prepare an acidic cleaning solution (such as citric acid or oxalic acid solution) with a concentration of 1-2% during the pickling process, and control the temperature at 25-35 ℃. Cycle the acidic cleaning solution through the membrane module for 30-60 minutes, adjusting according to the degree of inorganic deposits. During the acid washing process, regularly observe the cleaning effect of the membrane system to ensure that inorganic pollutants (such as calcium carbonate and iron oxides) are effectively removed. After acid washing, rinse the membrane system with clean water again to ensure that acidic residues are completely removed, and rinse until the pH value of the effluent returns to neutral.
4. After completing the inspection and cleaning, check the operation of the system to ensure that the membrane components are clean and restore normal water production rate. Conduct routine testing on the system to confirm that the filtration performance of the membrane has returned to normal and the effluent quality meets the standard.
