In the field of water treatment, reverse osmosis (RO) technology is widely applied in industrial pure water preparation and municipal wastewater treatment due to its high desalination efficiency. As the core component of the system, the operating condition of RO membranes directly determines water treatment efficiency and costs. Over time, membrane elements are susceptible to contamination and blockage, requiring scientific cleaning methods to restore performance. This article will start from the staged operation principles of RO membrane systems, analyze the contamination characteristics of different stages, and ultimately clarify the selection logic for cleaning methods, helping you comprehensively master key reverse osmosis membrane cleaning technologies.
RO membrane systems are divided into first-stage and second-stage. Raw water passes through the first stage to produce permeate water and concentrate water, with the first-stage concentrate water serving as the feed water for the second stage. Eventually, the second-stage concentrate water becomes the total concentrate water of the system.
Due to the higher salt content in the second-stage feed water, its scaling tendency is stronger than that of the first stage. In contrast, the first stage is more prone to fouling caused by suspended solids, colloids, and other substances.
The core principle for determining whether to perform acid cleaning or alkaline cleaning first lies in the pressure difference between the two stages: if the first stage has a higher pressure difference (severe fouling), alkaline cleaning should be conducted first; if the second stage has a higher pressure difference (severe scaling), acid cleaning should be carried out first. The specific choice needs to be based on the actual fouling situation.
I. Staged Operation of RO Membrane Systems: Foundation for Understanding Contamination Differences
RO membrane systems are typically divided into first and second stages, working together to achieve a balance between water quality purification and concentrate discharge. Their specific operating processes and water quality change patterns serve as important prerequisites for determining contamination types.
Raw water first enters the first-stage RO membrane components. Under pressure, water molecules pass through membrane pores to form first-stage permeate (preliminarily purified water that can enter subsequent treatment or serve as supplementary feed water for the second stage), while salts, suspended solids, and other impurities that cannot pass through are retained, forming first-stage concentrate. Since the first-stage concentrate has already concentrated some pollutants from the raw water, its water quality characteristics include higher salt concentration and relatively concentrated impurity content. Therefore, it serves as feed water for the second stage for continued treatment—this design improves raw water utilization and reduces water resource waste.
In the second-stage RO membrane components, the first-stage concentrate undergoes separation again: some water molecules continue to pass through the membrane to form second-stage permeate, which combines with first-stage permeate to become the system's final product water; while the remaining water with highly concentrated salts and impurities becomes the system's total concentrate discharge.
From a contamination risk perspective, the differences between the two stages are significant: the first stage is more prone to fouling by suspended solids and colloids (such as sediment and microbial metabolic products from raw water). These contaminants attach to the membrane surface, gradually blocking membrane pores and increasing water flow resistance. The second stage, receiving first-stage concentrate as feed water, has salt concentrations far higher than raw water, making its scaling tendency (such as precipitation and attachment of salts like calcium carbonate and calcium sulfate) much greater than the first stage, making it more susceptible to scaling contamination. The different contamination types directly determine the selection logic for cleaning methods.
II. Core Judgment for Cleaning Methods: Using Pressure Differential Between Two Stages as Key Indicator
The core of reverse osmosis membrane cleaning is "targeted cleaning"—selecting alkaline cleaning for fouling and acid cleaning for scaling. The key indicator for determining the "condition" is the operating pressure differential between the first and second stages.
Operating pressure differential refers to the pressure difference between the inlet and concentrate outlet of membrane components, directly reflecting the flow resistance inside the membrane. When membranes experience fouling or scaling, water flow encounters resistance, causing pressure differential to increase accordingly. Since the first and second stages have fundamentally different contamination types, their pressure differential changes correspond to different contamination problems. Therefore, cleaning methods must be determined by comparing the pressure differentials of both stages.
1. Priority Alkaline Cleaning Scenarios: High First-Stage Pressure Differential (Severe Fouling)
When monitoring shows significantly elevated first-stage pressure differential (typically exceeding 50% of initial operating pressure differential or reaching system-set alarm thresholds), it indicates severe fouling of first-stage membranes due to suspended solid and colloid accumulation. Alkaline cleaning should be prioritized because: alkaline cleaning agents (such as sodium hydroxide solutions, typically combined with non-ionic surfactants) have excellent dispersion and stripping capabilities, effectively decomposing colloids, organic matter, and microbial slime on membrane surfaces, flushing out suspended solids attached within membrane pores, and restoring membrane water permeability.
For example, after 3 months of operation in an industrial RO system, first-stage pressure differential increased from initial 0.15MPa to 0.32MPa, while second-stage pressure differential only increased from 0.18MPa to 0.22MPa. Combined with raw water quality assessment (high suspended solid content), first-stage fouling was identified as the primary issue. Using 0.1% sodium hydroxide + 0.05% surfactant cleaning solution, circulating at 30°C for 60 minutes, first-stage pressure differential recovered to 0.17MPa, and system water production returned to over 95% of design value.
2. Priority Acid Cleaning Scenarios: Higher Second-Stage Pressure Differential (Severe Scaling)
When second-stage pressure differential is significantly higher than first-stage, or when second-stage pressure differential increases far more than first-stage (such as second-stage pressure differential reaching above 0.4MPa and clearly exceeding current first-stage pressure differential), it indicates severe scaling of second-stage membranes due to salt concentration. Acid cleaning should be prioritized because: acidic cleaning agents (such as citric acid and hydrochloric acid solutions) can chemically react with carbonates, sulfates, and other scales on membrane surfaces, converting insoluble salts into soluble substances (such as calcium carbonate reacting with hydrochloric acid to produce water-soluble calcium chloride), thereby dissolving and stripping scale layers and reducing water flow resistance.
Using a municipal reclaimed water RO system as an example, after 2 months of operation, second-stage pressure differential increased from 0.2MPa to 0.45MPa, while first-stage pressure differential only increased to 0.25MPa. Water quality testing revealed second-stage concentrate LSI (Langelier Saturation Index) of 1.2 (greater than 0, indicating scaling tendency), identifying calcium carbonate scaling. Using 1% citric acid solution (pH adjusted to 3.0-4.0 with ammonia water), circulating at 25°C for 90 minutes, second-stage pressure differential decreased to 0.23MPa, and permeate salt rejection rate recovered from 97% to 99%.
3. Special Cases: Flexible Adjustment for Coexistent Contamination in Both Stages
In actual operation, both stages may experience pressure differential increases (such as during extended system operation or significant raw water quality fluctuations). In such cases, further analysis combining water quality assessment (such as testing concentrate salt composition and suspended solid content) with pressure differential increase magnitude is needed to determine primary and secondary contamination issues: if first-stage pressure differential increases more significantly, alkaline cleaning should still be prioritized, followed by acid cleaning based on second-stage pressure differential conditions after fouling relief; if second-stage pressure differential increase is more prominent, acid cleaning should be performed first to prevent further scale layer hardening, then address minor first-stage fouling.
For example, after 4 months of operation in a power plant RO system, first-stage pressure differential increased from 0.16MPa to 0.3MPa, and second-stage pressure differential increased from 0.19MPa to 0.42MPa. Concentrate water quality testing revealed calcium and magnesium ion concentrations in second-stage concentrate were 2.3 times those in first-stage concentrate, with obvious white scale layers, identifying second-stage scaling as the primary issue. Therefore, 0.5% hydrochloric acid solution acid cleaning (pH=2.0-2.5) was performed first, reducing second-stage pressure differential to 0.24MPa after cleaning; then 0.1% sodium hydroxide solution alkaline cleaning was applied, reducing first-stage pressure differential to 0.18MPa, restoring normal system operation.
III. Operational Considerations: Ensuring Cleaning Effectiveness and Membrane Safety
Based on pressure differential-guided cleaning method selection, the following operational details must be observed to avoid membrane element damage due to improper operation while ensuring cleaning effectiveness:
Cleaning Agent Concentration Control: Whether alkaline or acid cleaning, cleaning agent concentrations must be strictly set according to membrane manufacturer recommendations (such as sodium hydroxide concentrations typically 0.1%-0.5%, hydrochloric acid concentrations 0.5%-1%). Insufficient concentration may fail to completely remove contamination, while excessive concentration may corrode membrane materials (such as cellulose acetate membranes being sensitive to high-concentration alkali).
Cleaning Temperature and Time: Cleaning temperatures are generally controlled at 20-35°C. Low temperatures reduce cleaning agent activity and extend cleaning time; high temperatures may accelerate membrane aging. Cleaning time typically ranges from 30-120 minutes, requiring flexible adjustment based on pressure differential recovery to avoid excessive cleaning causing membrane structure damage.
Post-Cleaning Rinse: After cleaning completion, membrane components must be thoroughly rinsed with pure water or RO permeate until rinse water pH returns to neutral (typically 6-8) and conductivity matches feed water quality, preventing cleaning agent residue from causing continuous membrane corrosion or affecting subsequent permeate quality.
Regular Monitoring and Prediction: Besides pressure differential, regular monitoring of permeate flow and salt rejection rates is necessary—when permeate flow decreases 10%-15% or salt rejection rate drops 2%-3%, even if pressure differential hasn't reached alarm values, contamination conditions should be promptly checked and cleaning scheduled in advance to prevent contamination deterioration (such as scale layer hardening becoming difficult to remove).
Conclusion
The core of reverse osmosis membrane cleaning lies in "assessment first, then cleaning," with understanding RO membrane system staged operation principles and mastering the correspondence between two-stage pressure differentials and contamination types serving as the foundation for scientific judgment. Whether prioritizing alkaline cleaning for first-stage fouling or acid cleaning for second-stage scaling, ultimate decisions must flexibly adjust based on actual water quality, pressure differential change magnitude, and system operating data. Only by following the principle of "targeted cleaning with appropriate operation" can membrane performance be restored while extending membrane service life and ensuring long-term stable RO system operation.
