Reverse Osmosis Scaling Right After Startup? Here's What You Need to Know About Prevention and Treatment!

Reverse Osmosis Scaling Right After Startup? Here's What You Need to Know About Prevention and Treatment!

Reverse osmosis scaling not only reduces membrane flux and increases energy consumption but can also cause irreversible damage to the membrane, thereby affecting the stability of the entire system and the quality of the treated water. Therefore, understanding the causes of reverse osmosis scaling, prevention mechanisms, and treatment measures is crucial for ensuring efficient system operation, extending membrane lifespan, and reducing operational costs.

I. Causes of Reverse Osmosis Scaling

Scaling during routine reverse osmosis operation is not uncommon in certain regions, primarily due to the following reasons:

1. Failure to conduct comprehensive water quality analysis during design phase, resulting in feedwater containing scaling particles during operation;

2. Changes in water source during operation without corresponding adjustments to scale inhibitors, leading to scaling; 3. Incompatibility between scale inhibitors and the system, causing membrane fouling or scaling;

4. Excessively high recovery rate control during operation, resulting in secondary scaling;

While these represent common causes of scaling, troubleshooting often reveals specific factors. Below is a case study of scaling incidents caused by improper installation of primary and secondary membrane elements.

➡Failure Description:

1. After membrane module installation, operational time was minimal (cumulative duration <20 hours) with intermittent operation;

2. During operation, a significant overall decline in Stage 2 desalination rate was observed (Stage 1 average product water conductivity: 10 μS/cm; Stage 2 average product water conductivity: 268 μS/cm);

3. Inspection after removing the end cap revealed obvious scaling on the last two Stage 2 membrane elements. The end cap appeared white, and treatment with hydrochloric acid produced abundant bubbles (indicating carbonate scaling);

4. Upon removing Stage 2 membrane elements, it was discovered that the overall installation orientation of Stage 2 membrane elements was incorrect. The consequence of this incorrect installation was that a portion of the feedwater bypassed filtration through the membrane elements and was directly discharged from the system as part of the concentrate stream. Thus, the concentrate flow rate at this time was membrane concentrate plus bypassed water. The actual concentrate volume produced by the membrane elements was less than the concentrate volume detected by the system. With unchanged product water flow, the actual recovery rate for the membrane elements was higher than the system recovery rate.

This implies that under scaling conditions, incorrect installation accelerates membrane scaling compared to proper installation, with scaling deposits preferentially forming at the end of the second stage. During membrane installation, the concentrate seal ring must be positioned on the feed side. Sufficient attention must be paid to this during new system installation; otherwise, abnormal phenomena (such as abnormal increases in product water conductivity or membrane damage) will be observed during operation.

II. Prevention and Treatment of Reverse Osmosis Fouling

Calcium carbonate fouling is a common operational issue in reverse osmosis systems. Below, we use calcium carbonate fouling as an example to share methods for identifying, preventing, and addressing it.

1. Index for Assessing Calcium Carbonate Scaling Propensity

For brackish water with TDS ≤ 10,000 mg/L, the Langelier Saturation Index (LSI) indicates the likelihood of CaCO₃ scaling. LSI = pHC - pHS

Where: pHC is the concentrate pH, pHS is the pH at CaCO₃ saturation. When LSI ≥ 0, CaCO₃ scaling occurs. Most untreated natural waters exhibit a positive LSI value. For high-salinity brackish water or seawater sources with TDS > 10,000 mg/L, the Steffen and David Saturation Index (S&DSI) indicates CaCO₃ scaling potential. S&DSI = pHC - pHS

Where: pHC is the pH of the concentrate, pHS is the pH at CaCO₃ saturation. When S&DSI ≥ 0, CaCO₃ scaling occurs. For most untreated natural high-salinity sources, S&DSI is typically positive. To prevent CaCO₃ scaling, acid must be added to make it negative. If scale inhibitors are used to prevent CaCO₃ precipitation, it can remain positive.

2. Prevention of Calcium Carbonate Scaling

(1) Raw Water Quality Assessment: Input raw water quality test results into reverse osmosis design software to automatically calculate LSI or S&DSI indices. Develop scaling prevention strategies based on these values.

(2) Pretreatment Softening Processes: Primary softening methods include lime softening, lime-soda softening, and resin softening. Select the appropriate process based on water volume and economic feasibility;

(3) Acid and Scale Inhibitor Dosage in RO Feedwater: Reserve equipment and piping for acid and scale inhibitor injection during design.

(4) Regular Conductivity Testing of Individual Membrane Housings in RO Stages: Conduct regular conductivity tests on individual membrane housings in both stages. If abnormal conductivity increases are detected in the second stage, promptly investigate the cause.

3. Determining Calcium Carbonate Scaling

(1) Water Quality Assessment: If RO feedwater exhibits high alkalinity and hardness, and the LSI or S&DSI index calculated from concentrate pH exceeds zero, scaling risk is indicated;

(2) Operational Data Review: Typical system symptoms include elevated second-stage pressure drop, reduced permeate flow, and significant increase in second-stage permeate conductivity;

(3) Noticeable Acid Cleaning Effect: When cleaning with hydrochloric acid or citric acid, the cleaning solution pH rises rapidly. After acid cleaning, product water flow and salt rejection rate recover.

(4) End Cap Removal: The surface of the last membrane element or end plate at the terminal end shows white or pale yellow inorganic scale. The inorganic scale can be dissolved with acid.

4. Treatment of Calcium Carbonate Scaling

Prompt chemical cleaning is essential after calcium carbonate scaling occurs. Acid washing is conventionally recommended, preferably using hydrochloric acid. For severe scaling requiring online cleaning, observe the following precautions:

a) Maintain acid washing solution pH ≥ 3;

b) Use low-flow circulation for dissolution; avoid high-flow circulation to prevent inorganic scale from scratching the desalination layer;

c) Replenish acid promptly to sustain cleaning solution pH.