The difference between reverse osmosis water and deionized water is the level of purity.

Reverse osmosis (RO) removes solids to purify and improve the quality of drinking water, but the water is still saturated with salts and oxygen.

Deionized water (DI) removes these salts and oxygens, but the deionized process does not remove the dissolved solids found in the water. This is why it is common to combine reverse osmosis and deionized systems together to remove at least 95 percent of the impurities in your drinking water.

What is Reverse Osmosis and RO Water

Reverse Osmosis (RO) is one of the most efficient and modern water purification technologies. Originally designed to filter salt out of ocean water, RO can filter most known water impurities—up to 95 percent of all dissolved solids— found in municipal tap water. 

The impurities RO removes include:

• Organic Chemicals
• Chlorine
• Lead
• Silicates
• Pesticides
• Herbicides
• Arsenic
• Asbestos
• Radionuclides
• Cysts

How Does an RO System Work?
The RO process involves several stages of filtration. It usually starts with a pre-cleaning process. A pre-filter removes sediment رواسب, such as dirt and rust.

Some systems may also use a carbon filter to reduce volatile organic compounds (VOCs) such as chlorine and other contaminants that give water a bad taste or odor.

Water is placed under high pressure to force it through a reverse osmosis membrane.

This membrane is a thick polyamide film with tiny pores large enough to allow water to pass through, but is small enough to prevent passing organic particles.

The filtered water is held in a storage tank until needed.

What is Deionization and DI Water?
Deionized water (DI) is exactly what the name implies, water that has had the ions removed from it. An ion is a particle that has a positive or negative charge. How does water get ionized?

Minerals from the surrounding soil, pipes and plumbing water passed through get dissolved on the way to your tap. When these minerals dissolve, they attach themselves to water particles; this attachment causes the ionization, meaning the particles have a negative or positive charge.

Normal unfiltered tap water is full of these ionized mineral particles (which is why DI is also sometimes called demineralized). These include:

Salt (sodium)
Magnesium
Calcium
Chloride
Iron
Carbonates
Nitrates

How Does a Deionization Water System Work?
An ion exchange system consisting of small plastic resin beads attracts and retains the electrically charged mineral particles. Water passes through the beads, the mineral particles bind to the resin beads, and the cleaned deionized water passes to a storage tank where it can be called upon as needed for drinking or bathing.

Deionized water is sometimes confused with distilled water, which is not the same.

Distilled water

Distilled water is obtained through a process of boiling and steamingالغليان والتبخير.

Distilled water can still be ionized, and while it is a purer form of water, deionized water is more pure.

Distilled water is a kind of purified water that has been taken directly through the distillation process

This process is one of the methods that can be used to purify water.

Distillation works byطريق يعمل عن boiling water and collecting the steam that results, which will return to water once cooled.
This purification process has proven to be highly effective at getting rid of such contaminants as:

• Many different viruses and bacteria
• Certain protozoa like giardia
• Various chemicals such as sulfate and lead
• Because of how pure distilled water is, it’s commonly used in laboratories and medical facilities. While purified water is more commonly drunk than distilled water, it’s still possible to drink distilled water since it’s entirely free of impurities.

Purified water 

Purified water is a kind of water that has been effectively filtered with the goal of removing various impurities from the water. Purified water can be made from both tap water and groundwater, which means that it’s readily available for practically all homeowners.

The many impurities and contaminants that are removed from water during the purification process include:

• Numerous chemical pollutants
• Bacteria
• Fungi
• Algae
• Certain metals like lead and copper
• Different parasites

Purified Water The water produced by reverse osmosis units ending in ion exchange units is the water that is called purified water or filtered water, and it is sometimes followed by oxidation units by ultraviolet rays (Ultraviolet Oxidation). Purified water is one of the purest types of water and is used in pharmaceutical and other manufacturing processes.

ماء المنقى أو الماء المصفى Purified Water
الماء الناتج عن وحدات التناضح العكسي والمنتهية بوحدات التبادل الأيوني هو الماء الذي يطلق عليه الماء المنقى أو الماء المصفى، ويتبع أحياناً بوحدات أكسدة بواسطة الأشعة فوق البنفسجية (Ultraviolet Oxidation). الماء المنقى هو أحد أنقى أنواع الماء ويستخدم في عمليات التصنيع الدوائي وغيرها.

There are several different methods that can be used to purify water, which include:

• Filtration
• Sedimentation
• Coagulation
• Disinfection

While these four processes differ significantly in how they purify water, they are able to provide similar results. In order for water to be classified as purified water, it must have lower than 10 PPM, which means that around 99 percent of all contaminants must be removed from the water.

What is the difference between distilled water and purified water (i-chemeng.com)