What is Reverse Osmosis & How Does it Work?

Jan. 21, 2022

Breaking down reverse osmosis technology and how it works

About the author:

Brian Campbell founder for WaterFilterGuru.com. Campbell can be reached at [email protected].

What is Reverse Osmosis and How Does it Work?

One of the most effective water treatment methods is reverse osmosis. RO systems greatly improve the quality of water, making it healthy and safe to drink. They can be used in residential, commercial and industrial applications and offer a broad range of benefits. 

This article will look at how the reverse osmosis process works and the advantages and disadvantages of this type of water purification. 

What is Reverse Osmosis?

Reverse osmosis is a process that forces water through a semi-permeable membrane at a high pressure. The pores in an RO membrane are tiny - usually around 0.0005 microns or less - which makes it impossible for the majority of contaminants to pass through. 

Water particles are small enough to advance through the membrane, but larger impurities are rejected and linger in the RO chamber. During the reverse osmosis process, these impurities are flushed down a drain with wastewater at a constant rate. 

How Does RO Work?

There are three stages to a standard reverse osmosis system: a carbon prefilter, an RO membrane and a carbon post-filter. 

1. Carbon Pre-filter

After entering the RO system, water travels through an activated carbon pre-filter. Using adsorption, this filter grabs onto sediment, VOCs, chlorine, lead taste and odor. These contaminants become trapped in the filter media and are unable to pass onto the next stage of purification. 

2. RO Membrane

While carbon filters offer effective filtration, they can’t remove dissolved salts or minerals from water. This is where an RO membrane can help. A good semi-permeable membrane can remove up to 99% of all inorganic material. The majority of impurities are simply too small to squeeze through the membrane’s pores. 

3. Carbon Post-filter

There are a few contaminants that are small enough to make it through the RO membrane. These are removed during the final polishing stage, which typically uses a carbon post-filter. Again, chemicals and sediment will be targeted in this filtration stage. 

4. Optional Remineralization Stage

It’s becoming more and more common for some of the best reverse osmosis systems to include a final filter stage: a remineralization filter. Reverse osmosis removes dissolved minerals and salts such as calcium, magnesium and potassium. Reintroducing these minerals can improve the alkalinity and taste of water, and make it less susceptible to future contamination. 

What Contaminants Does Reverse Osmosis Remove?

Reverse osmosis removes up to 99.9% of all TDS (total dissolved solids) from water. This includes:

  • Heavy metals
  • Chlorine and chloramines
  • Dissolved minerals and salts
  • Fluoride
  • Bacteria and viruses

There are certain contaminants that reverse osmosis may not remove entirely, including hydrogen sulfide, pesticides and herbicides, and certain organic compounds. 

What Are the Advantages of Reverse Osmosis?

Enhances Taste

Reverse osmosis removes contaminants that could affect the taste of your water, including chlorine, iron and other chemically derived contaminants. This helps to improve water’s overall taste. If reverse osmosis water has been remineralized, it’ll taste even better. 

Improves Water Quality

Because reverse osmosis is one of the most effective purification methods out there, it’s favored by those looking to make their water safe, free from contaminants, and healthier to drink. 

Reduces Plastic Waste

Reverse osmosis produces water that’s as clean and filtered as bottled water (sometimes even more so). Those who drink a lot of single-use plastic bottled water can eliminate their plastic waste by switching to RO water. 

Low-Cost to Run

Aside from maintenance, reverse osmosis systems cost virtually nothing to run. They’re usually connected to a main water line, and don’t necessarily require electricity to operate. Using a reverse osmosis filter instead of buying drinking water can help save hundreds of dollars per year. 

Are There Disadvantages With RO?

Expensive Up-Front Purchase

Reverse osmosis is one of the most expensive water treatment options. A residential RO system can cost upwards of $500, so it may not be an option for smaller budgets. Industrial RO systems can cost thousands. 

Costly to Maintain

To function properly throughout its lifespan, a reverse osmosis system requires regular maintenance. The system’s pre- and post-carbon filters will require changing every 6-12 months, and the RO membrane will need to be replaced once every two years. 

Wastes Water

Reverse osmosis is the only water treatment option that wastes water. The impurities that can’t pass through the RO membrane have to be removed, and the only effective way to do this is to empty them down a drain along with a small amount of water. Though reverse osmosis filters are becoming more efficient, water waste will always be a part of the RO process. 

Pretreatment for Reverse Osmosis Systems

The filters and membrane that make up an RO system can quickly become damaged by certain contaminants, greatly reducing the lifespan of the unit. The best method of damage limitation is to use a pre-treatment to prevent fouling, scaling, chemical damage and mechanical errors. 

Though pretreatment is not usually used for residential reverse osmosis, industrial applications can benefit greatly from this process. It is often seen as essential to pretreat water before the RO process, and it certainly makes sense from an economical perspective. 

There are typically 4 stages of a reverse osmosis pretreatment system:

1. Screening of Solids

First, solids are removed from water, preventing membrane fouling caused by biological or particle growth. This also reduces the potential for damage to the pump. 

2. Cartridge Filtration

Next, particles of between 3 and 5 microns in size are removed using a filter cartridge. This cartridge will generally be made from string-wound polypropylene or similar. 

3. Dosing

Bacteria and other pathogens are removed using an oxidizing biocide like chlorine. This chlorine is then deactivated with bisulfite dosing. Inhibitors are also used to prevent bacteria from accumulating on the surface of the membrane. 

4. pH Adjustment

Water hardness, alkalinity and pH can all result in scaling in an RO unit. This stage of pre-treatment adjusts water’s pH and reduces calcium carbonate scaling. Scale inhibitors will also be used to prevent all types of scale formation. 

About the Author

Brian Campbell