Principle of pH Control in Reverse Osmosis Operation - 2

Section II: RO Membrane Module Operation Stage: Precise Salt Rejection Balance, Scaling and Contamination

Objective: Maximize desalination rate and water flux, minimize scaling (inorganic scaling, silica scaling) and contamination (organic matter, biological contamination) on the membrane surface, and extend the membrane lifespan. pH control during this stage is precise and crucial.

2. Control Logic and Principles:

(1) Influence of Membrane Surface Charge on Interaction with Pollutants:

The principle is: Most polyamide composite membranes carry a negative charge on their surface. Reducing the pH (increasing H+) will: Partially neutralize the negative charge on the membrane surface. May neutralize or reduce the surface negative charge of negatively charged organic pollutants and colloidal particles in water.

Effect: Reducing the electrostatic repulsion between the membrane and contaminants, theoretically enhancing the adsorption tendency of organics and colloids on the membrane surface. However, in practice, a moderate low pH combined with good pretreatment often effectively controls such pollution. High pH (>10) typically increases the negative charge of both the membrane and contaminants, strengthening the repulsion, which is beneficial for reducing this type of pollution, but other risks should be cautious.

Control inorganic scaling (calcium carbonate, silica, metal hydroxides):

Calcium Carbonate: The principle is the same as pretreatment acidification. Even if pretreatment has been acidified, with the increase of the concentration factor in the RO process, the LSI/S&DSI on the concentrate side may still rise. Typically, the LSI/S&DSI on the concentrate side is maintained within a safe range (such as <1.8 or as determined by the performance of the anti-scalant) by continuously adding acid (H2SO4/HCl) or using a dedicated anti-scalant.

Silica: The solubility of SiO2 increases significantly with rising pH levels, particularly after pH exceeds 7. For high-silica raw water:

Strategy 1 (Low Recovery Rate/Scale Inhibitor): Operate at lower pH levels (such as 7-8), rely on potent silicon scale inhibitors, and limit the system recovery rate to prevent over-saturation of SiO2 on the concentrated water side.

Strategy 2 (High pH Operation): Key Original Points: This is a critical strategy for efficiently treating high silica water. Increase the incoming water pH to 10-11 (usually using NaOH):

This pH condition significantly increases the solubility of silicate ions (SiO3-)^2-The ratio and total solubility (can increase by several times) enable the system to operate at higher recovery rates without silicon scaling, significantly enhancing water production efficiency. Meanwhile, high pH strongly inhibits biological growth (biological contamination).

However, strict monitoring is essential:

Scale Risk of Calcium and Magnesium: pH > 10 when CaCO3₃Scaling tendency has sharply increased, and magnesium hydroxide [Mg(OH)₂[ ] andCa₃(PO₄)₂The risk of scaling has significantly increased. It is essential to use a high-pH, strong-scale inhibitor, and it may be necessary to moderately reduce recovery rates or enhance softening.

· Membrane Hydrolysis Risk: Polyamide membranes may hydrolyze under continuous high pH (especially >11), leading to a permanent decrease in desalination rate. Adhere strictly to the pH and temperature limits set by the membrane manufacturer.

3. Enhanced Specific Ion Removal Rate

CO₂When the pH is low and water enters, CO₂High percentage, almost completely permeating through the membrane into the product water, resulting in high conductivity of the product water and low pH. When the incoming water has a high pH, CO₂Converted to HCO3^-/CO3^2-Effective removal achieved, resulting in an increased pH of the produced water.

Ammonia: Primarily in the form of NH4⁺ (Low pH) Easily removable, NH₃High pH values are difficult to remove. Controlling pH affects its morphology and removal rate.

Boron: As mentioned above, boron is mainly present as the non-charged borate [B(OH)₃Form exists in neutral pH water with a low RO removal rate (~50-70%). Raising the pH to 9-10.5 converts boron into the negatively charged borate ion [B(OH)4-], significantly increasing the membrane's removal rate to over 90%, which is crucial for meeting stringent drinking water boron content standards.

Weak electrolytes (silicic acid, CO)₂, Ammonia): RO Membrane for Dissolved Gases (CO)₂, NH₃(Note: There is no Chinese content provided to translate. Please provide the actual Chinese text for translation.)₃BO₃, H₄SiO₄Removal rate is very low.