Control of pH and carbonate ions: Crucial influence on reverse osmosis and deionization
Explore the pivotal role of pH and carbonate ions in reverse osmosis and deionization processes. Understand their impact on water purification efficiency, equipment durability, and challenges in achieving ultra-pure water standards.

Water, essential for life and a wide range of industrial applications, undergoes complex purification processes to meet high-quality standards. Among the most fundamental techniques is reverse osmosis, an advanced filtration method that effectively removes a variety of contaminants. However, an often underestimated yet crucial element in this process is the control of water pH and the presence of carbonate ions.

Let's explore how these factors not only influence the efficiency of reverse osmosis but also the durability of the equipment used, as well as the challenges faced during deionization for producing purified water.

Reverse Osmosis

Reverse osmosis is a fundamental technique in water treatment. Operating with semi-permeable membranes, it acts as a selective filter, allowing only water molecules to pass while rejecting impurities, salts, and unwanted particles. This method relies on the application of pressure to overcome natural osmotic pressure, thereby separating pure water from contaminants.

Its power lies in its efficiency: up to 99% of contaminants present in the water can be removed, including minerals, heavy metals, bacteria, viruses, and chemicals, providing highly pure water. This technique is versatile, finding applications in various fields such as drinking water treatment, pharmaceutical production, desalination of seawater, and more!


At the core of reverse osmosis is a semi-permeable membrane acting as a selective filter. Water is forced through this membrane under pressure, allowing only water molecules to pass through microscopic pores. Contaminants, on the other hand, are retained by the membrane and removed, thus producing highly purified water.

This separation method relies on the difference in osmotic pressure: by using a higher external pressure than natural pressure, water is compelled to migrate from the concentrated solution (with impurities) to the less concentrated solution on the other side of the membrane, leaving contaminants behind.

Expected Results:

Following reverse osmosis treatment, a significant reduction in water conductivity is observed. Generally, this reduction can lead to fairly low conductivity values, typically in the range of 3 to 25 microsiemens per centimeter (µS/cm).

However, it's important to note that several factors can influence these figures. The quality of the membrane is essential: high-end membranes can reduce conductivity to the lowest values, while less efficient membranes could allow more ions to pass through, maintaining slightly higher conductivity.

The nature of the initial contaminants also plays a role. If the water contains difficult-to-remove impurities, post-treatment conductivity may be slightly higher.

Impact of Water pH

Water, as an essential component in many purification processes, is subject to various influences. Among these, pH, a measure of water's acidity or alkalinity, plays a crucial role in the operation of reverse osmosis systems. The pH value can significantly influence water treatment quality and equipment durability, especially semi-permeable membranes.

Understanding the impact of pH on reverse osmosis is essential to ensure optimal performance and prolonged equipment life. By examining in more detail the different ways in which pH can affect the reverse osmosis process, it becomes evident that this water characteristic plays a crucial role in the reliability and efficiency of these advanced purification systems.

Impact on Treatment Quality

Let's explore the specific impacts of pH variation on the quality of permeate in reverse osmosis water treatment systems:


pH variation can directly influence the permeate's conductivity. pH level extremes can lead to changes in the ionization of certain substances in water. For example, at higher pH levels, the concentration of hydroxide ions increases, which can result in higher permeate conductivity due to increased ionic content.

Mineral Content

pH affects the solubility of minerals in water. At different pH levels, some minerals become more soluble while others may precipitate. pH variations could influence the mineral content of the permeate. For example, at higher pH levels, the solubility of certain minerals like calcium carbonate increases, potentially resulting in higher mineral content in the treated water.

Water Quality Parameters

pH variations can influence various water quality parameters in the permeate. This includes parameters such as alkalinity, hardness, and the presence of certain ions. pH changes could alter bicarbonate, carbonate, and other ion levels in treated water, affecting its overall quality.

Corrosion Potential

pH imbalances can influence the corrosiveness of the permeate. Low pH (acidic) water can be more corrosive, potentially releasing metals from pipes or storage tanks downstream from the reverse osmosis system. This can impact the overall quality of the permeate and pose safety issues.

Biological Stability

pH variations can also affect the biological stability of the permeate. Extreme pH levels can create an environment that promotes or inhibits microbial growth. Maintaining pH within an optimal range is crucial to prevent microbial contamination of treated water.

Understanding and controlling pH levels within the recommended range for reverse osmosis systems is essential to ensure that the permeate meets quality standards. Monitoring pH alongside other water quality parameters allows for adjustments to maintain consistent and high-quality permeate production from reverse osmosis systems.

Impact on Equipment

pH variation can significantly impact the performance and longevity of a reverse osmosis (RO) system in several ways:

Membrane Degradation

Extreme pH levels (too high or too low) can damage RO membranes. High acidity or alkalinity can degrade membrane material, reducing its effectiveness in separating contaminants and decreasing water permeability. These damages can lead to increased membrane replacement frequency and higher operational costs.

Deposition and Fouling

pH imbalances contribute to the formation of deposits and fouling on the membrane surface. High pH levels can lead to mineral deposits due to salt precipitation, reducing water flow and efficiency. Low pH levels can promote corrosion and organic fouling, leading to decreased system performance and increased energy consumption.

Contaminant Removal Efficiency

pH affects the efficiency of contaminant removal. Some contaminants are better removed within specific pH ranges. Deviations from the optimal pH can reduce the efficiency of the RO system in removing certain ions, minerals, metals, or other contaminants present in water.

Corrosion and Material Compatibility

Extreme pH levels can lead to corrosion of system components, pipes, and fittings downstream of the RO system. Corrosion can result in the release of metals into the permeate, impacting water quality. Maintaining pH within the recommended range helps preserve system material integrity.

Energy Consumption

pH imbalances can increase energy consumption. Fouling and deposition due to pH variations require higher pressure to maintain water flow, leading to increased energy consumption. This can result in higher operational costs and reduced system efficiency.

Managing pH within the optimal range through pretreatment processes, monitoring, and adjustments is essential to ensure the longevity and efficiency of an RO system. Regular maintenance, including membrane cleaning and replacement, is crucial to mitigate the adverse effects of pH variations and maintain system performance.

pH Impact on Downstream Deionizer After Reverse Osmosis

The presence of carbonate ions (CO3²⁻) in water is significantly influenced by pH levels.

In aqueous solutions, carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3), which then dissociates into bicarbonate ions (HCO3⁻) and carbonate ions (CO3²⁻) in a pH-dependent equilibrium.

At lower pH levels (acidic conditions), a higher concentration of

hydrogen ions (H+) in the solution promotes the formation of carbonic acid (H2CO3) and reduces the presence of carbonate ions (CO3²⁻). Conversely, at higher pH levels (alkaline conditions), there is a decrease in H+ ions and an increase in hydroxide ions (OH⁻), favoring the formation of carbonate ions (CO3²⁻) and less carbonic acid.

Therefore, higher pH levels (more alkaline) in water undergoing reverse osmosis favor the prevalence of carbonate ions. Conversely, lower pH levels (more acidic) favor the presence of fewer carbonate ions and more carbonic acid.

Understanding this relationship is crucial for reverse osmosis systems. Higher levels of carbonate ions due to high pH can impact the efficiency and performance of the RO membrane. Larger ions like carbonate can affect membrane selectivity and permeability, potentially influencing the overall efficiency of the RO process.

Moreover, alkaline conditions favoring carbonate ion formation also pose a higher risk of mineral precipitation. Carbonate compounds tend to precipitate and form deposits on the membrane surface, reducing the efficiency of the RO system and requiring more frequent cleaning or maintenance.

In summary, pH variations directly affect the balance between different forms of carbon in water, particularly the formation and prevalence of carbonate ions. This understanding is vital for managing water treatment processes because pH control plays a crucial role in impacting the presence of carbonate ions and subsequently influences the efficiency and maintenance needs of RO systems.

Complication during Ultra-Pure Water Production

In a reverse osmosis (RO) system, conductivity is a crucial parameter to monitor treatment process efficiency. However, as we've seen, managing pH well is important to avoid complications. During ultra-pure water production and the use of a deionizer after the RO system, there is a specific challenge related to the presence of dissolved CO3²⁻ ions.

CO3²⁻ ions refer to carbonate ions, which can pose problems in deionization processes because they do not significantly contribute to the overall conductivity of water. Conductivity meters mainly measure the presence of ions that conduct electricity, and CO3²⁻ ions have a relatively low contribution to conductivity compared to other ions. This means that the RO permeate seems to meet quality standards before deionization, but in reality, there are many more contaminants than perceived.

The issue arises because deionization processes typically rely on ion-exchange resins that remove charged ions (such as cations and anions) from water. Since these resins function based on functional groups allowing them to capture contaminant ions, their treatment capacity is limited, and they need constant regeneration. With the unexpected presence of carbonate ions, the resin saturation level reaches its maximum much faster than expected, and regeneration must be done much more frequently.

Thus, even if the conductivity measurement may seem within an acceptable range after the reverse osmosis step, the presence of carbonate ions remains problematic as it goes unnoticed by the conductivity meters. Premature resin saturation causes a significant increase in regeneration cycles, increasing operational costs and decreasing system efficiency.

Conclusion: Mastering pH and carbonate ions for optimal performance

In conclusion, meticulous control of pH, and consequently, the presence of carbonate ions in reverse osmosis systems, is of paramount importance. These often overlooked factors can significantly impact the quality of treated water and equipment lifespan.

Mastering pH ensures optimal performance of reverse osmosis, ensuring high-quality permeate that complies with required standards. Furthermore, understanding the effects of carbonate ions becomes crucial during the deionization phase, avoiding costly complications and improving the overall efficiency of the system.

Thus, rigorous control of pH and carbonate ions, although initially complex, offers a path to more efficient, cost-effective production of ultra-pure water meeting the strictest quality standards.