Water is one of the most important raw materials used in industries. However, not every process requires ordinary tap water. Many industrial operations need water that contains almost no dissolved minerals. This is where DM water or demineralized water becomes essential. By removing dissolved salts and mineral ions, DM water helps prevent scaling, corrosion, contamination, and equipment failure.
Today, demineralized water is used in power plants, pharmaceutical manufacturing, laboratories, chemical processing, electronics, food industries, and the growing green hydrogen sector. The quality of this water directly affects equipment performance, product quality, and operating costs.
In this article, we’ll explain how demineralized water is produced, why it behaves differently from ordinary water, where it is used and whether it is safe to drink.
What is Demineralized Water?
Demineralized water, commonly called DM water, is water from which nearly all dissolved mineral salts have been removed.
The purification process eliminates positively charged ions (cations) such as:
- Calcium (Ca²⁺)
- Magnesium (Mg²⁺)
- Sodium (Na⁺)
- Potassium (K⁺)
- Iron (Fe³⁺)
It also removes negatively charged ions (anions), including:
- Chloride
- Sulfate
- Nitrate
- Silica
- Carbonates
The result is extremely low-conductivity water with almost zero Total Dissolved Solids (TDS).
Unlike ordinary drinking water, DM water contains almost no minerals.
Why Remove Minerals?
Minerals may be beneficial for drinking water, but they create serious problems in industrial systems.
Even a thin layer of mineral deposits inside equipment can:
- Reduce heat transfer efficiency
- Increase energy consumption
- Block pipelines
- Damage pumps
- Cause boiler tube failures
- Reduce turbine efficiency
- Affect chemical reactions
- Contaminate manufactured products
Using DM water eliminates these issues and improves equipment reliability.
Conductivity of Different Types of Water
| Water Type | Typical Conductivity (µS/cm) |
| Ultra-pure water | 0.055 |
| High-quality DM water | 0.1–1 |
| Standard DM water | 1–10 |
| Reverse Osmosis water | 10–100 |
| Municipal tap water | 300–700 |
| Drinking water (recommended upper limit) | <1000 |
Lower conductivity indicates higher purity.
Demineralized Water vs Distilled Water
Many people assume these are the same, but they are produced using different technologies.
| Feature | Demineralized Water | Distilled Water |
| Production method | Ion removal | Evaporation and condensation |
| Removes dissolved salts | Yes | Yes |
| Removes bacteria | Not always | Yes |
| Removes viruses | Not always | Yes |
| Removes heavy metals | Mostly | Yes |
| Energy consumption | Low | High |
| Operating cost | Lower | Higher |
| Industrial use | Very common | Limited |
Which One Is Better?
Neither is universally better.
Distilled water is ideal when sterilization is important.
(Note: Sterilization is the process of completely removing or killing all microorganisms, including bacteria, viruses, fungi, and their spores. It ensures that water, equipment, or materials are completely free from harmful germs, making them safe for sensitive applications like pharmaceutical manufacturing and medical use.)
Demineralized water is preferred for large industrial systems because it is more economical to produce while achieving very low mineral content.
How is Demineralized Water Produced?
Modern treatment plants rarely rely on a single technology. Instead, they combine multiple purification stages to achieve the desired water quality while minimizing operating costs.
The most common technologies include:
- Ion Exchange
- Reverse Osmosis (RO)
- Electrodeionization (EDI)
Let’s examine each process.
Ion Exchange Process
Ion exchange is the most widely used method for producing demineralized water.
The process uses synthetic resin beads that exchange unwanted dissolved ions for hydrogen (H⁺) and hydroxyl (OH⁻) ions.
These ions combine to form pure water.
Reaction:
H⁺ + OH⁻ → H₂O
How Ion Exchange Works
The treatment usually consists of two resin vessels.
Cation Exchange Unit
Removes positively charged ions such as:
- Calcium
- Magnesium
- Sodium
- Iron
These ions are replaced with hydrogen ions.
Anion Exchange Unit
Removes negatively charged ions including:
- Chloride
- Sulfate
- Nitrate
- Silica
These ions are replaced with hydroxyl ions.
The hydrogen and hydroxyl ions then combine to produce pure water.
Advantages of Ion Exchange
- Produces very high-quality water
- Low operating cost
- High removal efficiency
- Suitable for boiler feed water
- Well-established industrial technology
- Easy to automate
Limitations
The resin eventually becomes exhausted.
It must be regenerated using:
- Hydrochloric acid or sulfuric acid (cation resin)
- Sodium hydroxide (anion resin)
Proper handling of these chemicals is essential for safe plant operation.
Reverse Osmosis (RO)
Reverse Osmosis is a membrane-based purification process.
High-pressure pumps force water through a semi-permeable membrane.
The membrane allows water molecules to pass while rejecting dissolved salts and impurities.
RO removes:
- Dissolved salts
- Heavy metals
- Organic contaminants
- Microorganisms
- Pesticides
- Radionuclides
Why RO Alone Is Not Enough
Although RO removes most impurities, it usually produces water with a conductivity of 10–100 µS/cm.
Many industrial applications require much higher purity.
For this reason, RO is commonly used as a pre-treatment before ion exchange or EDI.
This significantly extends the life of downstream purification equipment.
Electrodeionization (EDI)
Electrodeionization is one of the most advanced water purification technologies.
It combines:
- Ion exchange resins
- Ion-selective membranes
- Direct current electricity
Unlike conventional ion exchange, EDI continuously regenerates the resin without using chemicals.
Benefits of EDI
- Continuous operation
- No chemical regeneration
- Lower operating costs
- Very high water purity
- Reduced environmental impact
- Suitable for pharmaceutical and semiconductor industries
Proper pretreatment is essential because membrane fouling can significantly reduce system performance.
Why Industries Prefer Multi-Stage Treatment
Instead of relying on a single purification method, most modern plants use multiple stages.
A typical arrangement is shown in the following image.
This approach improves water quality, reduces maintenance, and increases the life of expensive equipment.
Why DM Water Matters
Producing demineralized water is not simply about removing minerals. It is about protecting industrial assets, improving product quality, reducing downtime, and extending equipment life.
Choosing the right treatment technology depends on:
- Feed water quality
- Required purity
- Plant capacity
- Operating costs
- Industry standards
- Maintenance requirements
Physicochemical Properties of Demineralized Water
Demineralized water behaves very differently from ordinary water. Once dissolved minerals are removed, its chemical stability changes significantly.
Without natural salts and alkalinity, DM water becomes highly reactive. It readily absorbs gases from the atmosphere and dissolves materials from surfaces it comes into contact with. These properties make it ideal for industrial applications but also create challenges in handling, storage, and distribution.
Conductivity and Resistivity
The purity of demineralized water is measured primarily by its electrical conductivity or resistivity.
Conductivity indicates how easily electricity passes through water. Since dissolved ions conduct electricity, lower conductivity means fewer impurities.
Resistivity is simply the inverse of conductivity. Higher resistivity indicates purer water.
Typical Water Quality
| Water Quality | Conductivity (µS/cm) | Resistivity (MΩ·cm) |
| Theoretical pure water | 0.055 | 18.2 |
| Ultra-pure water | 0.055–0.1 | 10–18.2 |
| High-quality DM water | 0.1–1.0 | 1–10 |
| Standard DM water | 1–10 | 0.1–1 |
| RO permeate | 10–100 | <0.1 |
| Tap water | 300–700 | Very low |
Most industrial plants continuously monitor conductivity because it provides an instant indication of water quality.
Why Conductivity Matters?
High conductivity can cause several operational problems:
- Boiler scaling
- Turbine deposits
- Product contamination
- Reduced chemical reaction efficiency
- Higher maintenance costs
- Equipment failure
For industries such as pharmaceuticals and semiconductor manufacturing, even a slight increase in conductivity may indicate contamination that requires immediate corrective action.
pH of Demineralized Water
Many people believe pure water always has a pH of 7.
This is only true at 25°C.
The neutral pH of water changes with temperature because the self-ionization of water increases as temperature rises.
Neutral pH at Different Temperatures
| Temperature (°C) | Neutral pH |
| 0 | 7.47 |
| 10 | 7.27 |
| 25 | 7.00 |
| 50 | 6.63 |
| 100 | 6.14 |
This does not mean water becomes acidic at higher temperatures. It remains chemically neutral because the concentrations of hydrogen and hydroxyl ions remain equal.
Modern industries therefore use instruments with Automatic Temperature Compensation (ATC) to obtain accurate pH measurements.
Why Stored DM Water Becomes Acidic
Freshly produced DM water has almost no dissolved minerals.
However, once exposed to air, it immediately begins absorbing carbon dioxide (CO₂).
Carbon dioxide reacts with water to form weak carbonic acid.
As a result:
- pH decreases
- Conductivity increases
- Water purity gradually declines
Within a short time, the pH may drop from approximately 7.0 to around 5.5–6.0.
For this reason, laboratories and industries often store DM water in sealed tanks fitted with air filters or nitrogen blanketing systems.
The “Hungry Water” Phenomenon
One of the most important characteristics of demineralized water is its tendency to dissolve materials from its surroundings.
Because it contains almost no dissolved minerals, it naturally seeks chemical equilibrium.
This aggressive behavior has earned it the nickname “Hungry Water.”
Instead of depositing minerals like hard water, DM water removes them from surfaces.
What Does Hungry Water Dissolve?
It can gradually dissolve:
- Iron
- Copper
- Zinc
- Lead
- Brass
- Concrete minerals
- Carbonate deposits
- Protective oxide layers
This is one reason why material selection is critical in DM water systems.
Corrosion in DM Water Systems
Ordinary hard water often forms a thin protective calcium carbonate layer inside pipes.
This layer reduces corrosion.
Demineralized water removes this protective coating and exposes bare metal.
Once exposed, metals corrode much faster.
Common problems include:
- Uniform corrosion
- Pitting corrosion
- Flow-assisted corrosion
- Erosion-corrosion
- Dezincification of brass fittings
Without proper system design, these failures can become expensive and dangerous.
Materials Suitable for DM Water
The choice of construction material greatly affects system life.
Recommended Materials
- Stainless Steel 316L
- PVDF (Polyvinylidene Fluoride)
- HDPE (High-Density Polyethylene)
- Polypropylene
- PTFE (Polytetrafluoroethylene)
- High-purity PVC (Polyvinyl Chloride)
Materials to Avoid
- Mild steel
- Carbon steel
- Galvanized steel
- Brass with high zinc content
- Copper in high-purity systems
Although stainless steel has a higher initial cost, it significantly reduces maintenance and contamination over the system’s lifetime.
Corrosion Prediction Indices
Engineers use several water chemistry indices to predict whether water will form scale or cause corrosion.
These indices are valuable tools for selecting treatment methods and maintaining system reliability.
Langelier Saturation Index (LSI)
The LSI predicts whether water will dissolve or deposit calcium carbonate.
| LSI Value | Interpretation |
| Positive | Scale-forming water |
| Zero | Stable water |
| Negative | Corrosive water |
Since DM water contains almost no hardness or alkalinity, its LSI is strongly negative.
This confirms its highly corrosive nature.
Ryznar Stability Index (RSI)
The RSI provides another way to estimate corrosion risk.
| RSI Value | Water Condition |
| Below 6 | Scaling tendency |
| 6–7 | Stable water |
| Above 8.5 | Highly corrosive |
Most untreated DM water falls into the highly corrosive category.
Calcium Carbonate Precipitation Potential (CCPP)
CCPP estimates how much calcium carbonate the water will dissolve or precipitate.
A strongly negative value indicates aggressive water that can attack pipelines and concrete surfaces.
Industrial Applications of Demineralized Water
DM water is used wherever mineral contamination can affect equipment performance or product quality.
Some of the major applications include:
- Thermal power plants
- Chemical manufacturing
- Pharmaceutical production
- Food processing
- Textile industries
- Electronics manufacturing
- Laboratories
- Automotive industries
- Battery manufacturing
- Green hydrogen production
Each industry has different purity requirements depending on the process.