By Rehan Iqbal
Reverse osmosis (RO) is generally considered as the best for home, business, and commercial water filtration. If you’ve been shopping around for a home water filtration system, you would have come across numerous manufacturers claiming that their RO water filter is better than all the others. You will be bombarded with information about all the stages of reverse osmosis.
With anything from 3 to more than 10 RO filter stages, it can be tough making sense of all the sales jargon. How many stages of RO filtration do you actually need? What are all the stages of a water filtration system?
If it seems like you have more questions than answers at the moment, relax. I’ll be explaining all that you need to know about reverse osmosis filter stages.
How a Reverse Osmosis Filter Works
In order to come to grips with RO stages, you need to have a basic understanding of how these filters work. Realistically, many stages of a reverse osmosis system have nothing to do with filtration. Some stages replace minerals, alter the PH of the water, or improve the taste. Technically, this is not filtration, it’s more about water quality.
You may also come across post-filters. Understandably, you could be quite confused about how an RO system works. We should start off by explain what an RO filter is and how it works. While this is but one (possibly two) of the stages used for a filtration system, it is the basis for any reverse osmosis filter for water.
At the heart of every RO system is the reverse osmosis membrane. This is a filter cartridge containing several layers of semipermeable material, known as reverse osmosis membranes. Originally, the membranes were made using cellulose acetate (CA). Though it has become more common to use thin film composite (TFC).
The purpose of the membrane is to remove small particles from the water like dissolved salts, sediment, certain chemicals, minerals, volatile organic compounds (VOCs), and other toxins the water. An RO membrane can remove 75% to over 90% of contaminants from the water, depending on the type of membrane and what impurities are in the source water.
An RO filter works by forcing the water through the membrane using water pressure. Contaminants are flushed away through the drain and the filtered water is stored in a tank, ready for use. Several stages of filtration can be added before it reaches the RO filter, after passing through the RO filter, and after leaving the storage tank.
The first stage of any RO system is called a prefilter or sediment filter. This removes debris like silt, dirt, and rust from the water. The purpose of the prefilter is to prevent subsequent filters from becoming blocked.
Various types of activated carbon filters make most of the additional stages. These filters remove chemicals like chlorine, along with improving the taste, color, and odor of the water.
Other filtration stages can include silver or UV filters to kill bacteria, water softeners and conditioners to remove hard minerals, and remineralization to add healthy minerals that have been removed during the filtration process.
Types of Filters Used for RO Stages
Activate Carbon Filters
After the RO membrane, carbon filters are the most common stage of water purification. There can be several stages of pre and post activated carbon filters.
Activated carbon filters, also known as activated charcoal filters, are either solid or granular. During the manufacturing process, small pores are introduced into the carbon to increase the surface area that comes int contact wit the water.
As water passes through the filter, impurities are absorbed by the carbon. It works very much like sponge, drawing in the contaminants and trapping them inside the filter media. Activated carbon completely removes up to 80 chemicals found in water. They are also very effective at removing about 30 known chemicals, and moderately effective at removing another 22.
Activated carbon filters are recommended for treating water with the following common chemicals:
Granulated activated (GAC) filters are made using grains of activated carbon, approximately 1 millimeter in size. They are usually made from coconut shell, wood, or coal. Coconut shell is the most environmentally-friendly material used to make activated carbon filters as it is the most renewable. GAC filters are incredibly effective, removing more chemicals that any other type of filter. However, as water flows between the grains, they can cut pathways into the filter media, making them less effective over time.
Carbon Block Filters are made using incredibly small particles of activated carbon, measuring 1 micron or less, resembling a fine powder. The carbon powder is bonded, using a binding agent, to form solid blocks. Because the blocks are solid, the problem of channeling (common to GAC filters) is eliminated. The only issue with these filters is their density which can restrict water flow. Carbon block filters are more commonly used in post-filtration, as most other contaminants would already have been removed. This reduces the chance of blockages that could restrict water flow.
During the filtration process, many healthy minerals are removed, and this can leave the water tasing bland. To reintroduce these minerals, making the water healthier and tastier, mineral ball and stone filters are used. This process is known as remineralization.
Generally used as the last stage of filtration, once the water leaves the storage tank, UV or silver filters are used to kill bacteria and other pathogens in the water.
Generally referred to as sediment water filters, a prefilter prevents larger debris from passing through the more advanced filtration stages. These filters are usually made from polypropylene and have a pore size of approximately 5 micron. This means that the other filters, which can filter particles as small as 1 micron (sometimes less), are less likely to become blocked by sediment in the water.
Types of Multistage RO Water Filters
The amount of stages used to filter the water can affect the purity, taste, and good health of the water. These can range from 3 to 7 stages, with options to increase filtration efficacy by adding even more stages.
It is not always easy to determine how many stages of water filtration you need. Having your water tested in a laboratory may help a lot. By knowing what contaminants are in the water, and in what quantities they exist, provides a good insight into how many stages of filtration will be best.
A basic 3-stage system is okay for a good municipal water supply. Though some water utilities are not as thorough in their testing and quality control of the water they supply. Excessive use of chemicals like chlorine and fluoride at water treatment facilities may require additional carbon filtration.
For the best removal of chemicals from the water, it is usually best to utilize at least two stages of active carbon filtration. Using both GAC and carbon bock filters will ensure that your water is perfectly pure, clear, and tastes great.
In hard water areas, a water softener is essential as this improves the quality of your water and prevents limescale that can damage your plumbing and appliances. Remineralization will improve the taste and good health of your water by adding essential minerals after the water has been filtered.
If you use well water or are uncertain about the utility water in your area, anti-bacterial filters will provide peace of mind, knowing that you and your family will not fall ill from drinking tap water.
Apart from the stages mentioned, some manufacturers will provide additional stages, like extra activated carbon filtration and alkaline water ionizers.