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 are classified from Type I to Type IV, where Type I is defined as ‘ultrapure’. Water purity is assessed using various characteristics including:
- TOC (total organic compounds) for Type I water.
Various industries make widespread use of pure, deionised water. They include chemical processing, petrochemicals, pharmaceuticals, power, food and beverage industries along with micro-electronics.
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 build up 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.
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 build up 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.
The low electrical conductivity of deionised water makes it safer to use in fire extinguishing applications for electrical equipment.
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.
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. Also used for washing and rinsing surface prior to paint finish, to improve adhesion and improve 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 Hydroxl 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.
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 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 via 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 positive and negative 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.
The Rapide Strata range of industrial deionisers from Veolia Water Technologies are high-purity water systems offering flow rates from 2.5 to a massive 60 cubic metres per hour. 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.
Another deionisation system from Veolia is the CEDI (continuous electrodeionisation) system which produces a continuous flow of consistent water quality and doesn’t use any chemicals. Flow rates of from 1.8 to around 40 cubic metres per hour are achievable. To ensure that the CEDI system is not overloaded with salts reverse osmosis or deioners are always used at a preceding stage.
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.
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.
About the Author
Peter has a background in analytical chemistry and has gained a wealth of knowledge and experience working for and supplying to a wide range of pharmaceutical, biotech customers and pharmaceutical companies.