Deionised water, or DI water, is sometimes called demineralised water, or DM water. It is water which has had (most of) the ions removed. Ions are atoms or molecules which have either more electrons than protons, making a negative ion (an anion) or fewer electrons than protons, making a positive ion (called a cation).
Normal tap water is full of ions derived from the natural environment, the pipes and other sources. Typically, these might include Sodium (Na), Calcium (Ca), Iron (Fe) and Copper (Cu). The deionisation process removes these mineral ions and is an important aspect of water purification for various applications.
Deionised water is not the same as distilled water, although they are both forms of purified water. While the deionisation process produces high purity water by removing mineral ions, it will not significantly remove any uncharged organic molecules. Whereas the distillation process boils water and then condenses the vapor which leaves both solid, mineral contaminants and organic contaminants behind.
What is Deionised Water Used For?
Deionised water is used in various applications which require water of high purity. Here are some examples:
Chemistry and Laboratory Applications
Water of known purity is often required in chemistry and laboratory applications. Accurate test results can only be obtained when the purity of the water used in experiments is reliably known. Adequately pure water is also needed for cleaning lab equipment and sterilisation.
Water purity levels are classified from Type I to Type IV, where Type I is defined as ‘ultrapure’. Water purity is assessed using various characteristics, including:
- Resistivity
- Conductivity
- TOC (total organic compounds) for Type I water.
For information on this topic, read our blog: Understanding Different Types of Pure Water for Laboratory Applications
Industrial Processes
Various industries make widespread use of pure, deionised water. They include chemical processing, petrochemicals, pharmaceuticals, power, food and beverage industries along with micro-electronics.
Cooling Applications
Regular tap water contains varying levels of dissolved contaminants, including minerals. These contribute to the electrical conductivity of the water, which makes it unsuitable for many cooling applications. These dissolved minerals will also contribute to the buildup of scale, corrosion and erosion. Deionised water is therefore preferred in these applications as it has a far lower level of electrical conductivity and will not result in unwanted scale build-up.
The design of cooling systems require special consideration in order to minimise or avoid any downtime. Such systems require tight control and continuous monitoring of the deionised cooling water conductivity to ensure that required temperatures are maintained.
Learn more about optimising a cooling system in our blog: Cooling Tower Water Treatment - Why is it Important?
Boiler Feed
Boiler feedwater is the water that is pumped into the boiler steam drum, where it is turned into vapor. If this water contains significant dissolved minerals, then there will be a buildup of scale in the steam drum.
The quality of the boiler feedwater is clearly important as it affects boiler performance, scale build up and corrosion. Deionised water is typically used as this helps to minimise these issues and prolong boiler life.
Read more about boiler water treatment in our blog: The Importance of Water Quality for Boiler Efficiency
Extinguishing Fires
The low electrical conductivity of deionised water makes it safer to use in fire extinguishing applications for electrical equipment.
Aquariums
Regular tap water is not ideal for most aquariums as the impurities promote the growth of algae. Deionised water is often preferred for its purity which helps to improve the overall health of aquatic life.
Automotive
Deionised water is commonly used in engine cooling systems as the low level of mineral content means there is minimal scale build-up, thus prolonging the life of the system. It is also often used to top up lead-acid batteries. It is also used for washing and rinsing surfaces prior to the paint finish, to improve adhesion and surface finish.
How to Make Deionised Water?
The process of deionising water exchanges all of the charged ions, both positive (cations) and negative (anions), for Hydrogen and Hydroxyl ions which combine to form pure H2O. Various techniques and technologies are used, depending upon the quality of the water source and the required water purity levels.
Deionisation with Electrodeionisation
Electrodeionisation (EDI) involves the use of electricity. Ion exchange membranes and deionising resins are used in the process.
Water is passed between a positive anode and a negative cathode. Special ion-selective membranes are used to separate positively charged ions, which are attracted to the negatively charged cathode, and negatively charged ions, which are attracted toward the positive anode. Capacitive deionisation (CDI) is a form of EDI.
Some EDI systems are referred to as continuous electrodeionisation systems (CEDI) as the electrical current is responsible for continuously regenerating the required resin mass.
Deionisation with Reverse Osmosis
Deionisation is often used after water has been passed through a reverse osmosis (RO) system. Reverse osmosis can remove up to around 99.9% of contaminants. This leaves only the smallest particles to be filtered via the deionisation membranes and resin. This process results in high-purity water.
Making DI Water at Home
Deionisation systems for home use are readily available. Some are designed for specific applications, such as cleaning (e.g. automobiles) or for aquariums. The simplest of these systems will typically use two chambers filled with the positively and negatively charged resins, which are responsible for the deionisation process. The output water contains far lower levels of dissolved minerals, which can otherwise result in noticeable spots when cleaning an automobile.
Where to get Deionised Water?
There are many outlets which sell deionised water for various applications. It is often used to fill smoothing irons, as it leaves minimal scaling, and is popular for various automotive applications. So you can often find deionised water on the shelves at DIY stores and automotive equipment sellers. It is also readily available from various well-known online retailers.
Veolia Deionisers
The Rapide Strata range from Veolia Water Technologies delivers high-purity deionised water with flow rates ranging from 2.5 to 60 cubic metres per hour, making it suitable for a wide variety of industrial applications. Regeneration downtime is minimised to just 30 to 45 minutes. Regeneration is the process by which the anion and cation resins are regenerated. The conductivity of the purified water is continuously monitored and includes an auto service shut-off and alarm facility. Typical applications include general industrial, pharmaceutical industries, food and beverage industries and more.
Is it Safe to Drink Deionised Water?
People often ask - is it safe to drink deionised water? While deionised water is not poisonous, there are reasons why you should only ever consume small amounts of DI water and you should never use DI water as your main form of drinking water.
Regular tap water contains many minerals, which are actually good for our health. The deionisation process removes most of these, such as Calcium and Magnesium, which are desirable minerals in our diets. And if you use DI water for cooking it will naturally result in loss of minerals in the cooked food, which is not good. It should also be noted that deionising water, which starts out as non-potable, will not make it safe to drink.
Interested in learning more about deionisation technologies and how they benefit water treatment processes? Click here.
