Why does the reverse osmosis system need to “add alkali”?

Introduction

In the daily operation and maintenance of reverse osmosis water treatment systems, adding alkali to the feed water is a frequent task. However, many people wonder: What is the purpose of adding alkali? How can we avoid membrane fouling and ensure system stability? Today, we will explain the core logic and key operational aspects of adding alkali to the feed water of reverse osmosis.

Why do we add alkali to water?

The core goal of adding alkali is to precisely control the influent pH between 8.0 and 8.5. Within this range, two key benefits are achieved:

Preventing scale: Reverse osmosis membranes are vulnerable to calcium carbonate scaling. When the pH value is lower than 8.0, the calcium and magnesium ions in the water easily combine with carbonate ions to form hard scale on the membrane surface, resulting in a decrease in water production and a sharp drop in desalination rate.Adjusting the pH to 8.0-8.5 effectively inhibits calcium carbonate precipitation, reducing membrane fouling at the source.

Improving efficiency: This pH range improves the membrane's retention of organic matter (such as humic acid and colloids) in the water while reducing the adsorption of certain contaminants to the membrane, indirectly extending the membrane's service life.

Key to practical operation: choose the right reagent and control the parameters

How to choose an alkali dosing agent? Comparison of two mainstream options

Sodium hydroxide (NaOH): The preferred agent. It efficiently adjusts pH, dissolves quickly, and doesn't introduce scaling ions like calcium and magnesium. Its risk of secondary membrane contamination is extremely low, making it suitable for water quality scenarios.

Sodium carbonate (Na₂CO₃): A secondary option. Besides adjusting pH, it reacts with calcium and magnesium ions in the water to form calcium carbonate precipitates (which must be removed by a pre-filter). This is suitable for situations where the inlet water hardness is slightly high (but not exceeding the standard), achieving two goals at once.

Three key parameters to keep an eye on

pH: Importance! Monitor it in real time with an online pH meter. If it drops below 8.0 or rises above 8.5, immediately adjust the dosing pump. A value below 8.0 is prone to scaling, while a value above 8.5 may accelerate membrane hydrolysis and aging, increasing system energy consumption.

Dosage: There is no fixed standard. It must be calculated based on the pH and hardness of the raw water. (For example, the dosage for raw water with a pH of 7.0 and a hardness of 150 mg/L will differ significantly from that for raw water with a pH of 6.5 and a hardness of 250 mg/L.) Manual dosing is strictly prohibited. It is recommended to use a pH meter and an automatic dosing pump to control fluctuations.

Influent Hardness: If the influent hardness is >200 mg/L, alkali addition alone is insufficient. A scale inhibitor must be used in conjunction with the addition of alkali to prevent fine calcium carbonate precipitates from adhering to the membrane surface.

Pit avoidance guide: If you don’t pay attention to these 3 points, the membrane will be scrapped soon

Pretreatment Is Essential

If fine precipitates form in the water after adding alkali (especially when using sodium carbonate), they must be filtered through a 5μm filter. Otherwise, the precipitates will enter the membrane module and quickly clog the membrane pores, causing a sudden increase in differential pressure. Regularly inspect the filter element and replace it if dirty.

Early Detection and Early Treatment Are Crucial for Membrane Fouling

A direct sign of improper alkali addition is membrane scaling. Be vigilant if the following conditions occur:

Water yield drops by more than 10% compared to normal;

System differential pressure (feedwater - reject water) rises by more than 20%;

Significant decrease in salt rejection.

If this occurs, shut down the system immediately and perform a chemical cleanup with a citric acid solution (1%-2%) to prevent scale from hardening and becoming irremovable.

Check Chemical Compatibility in Advance

Alkali chemicals should not conflict with pre-treatment flocculants, biocides, or other agents. For example, mixing certain cationic flocculants with sodium hydroxide may produce flocculent precipitates, which can contaminate the membrane. Before adding alkali, consult the chemical's instructions or conduct a small test to confirm compatibility before commissioning.

Summary

Adding alkali to reverse osmosis feed water isn't a "random addition" exercise; it requires precise control. Choosing the right chemical, controlling pH, and performing pretreatment are crucial for preventing scaling, protecting the membrane, and ensuring long-term stable system operation.