What are the Different Stages of Reverse Osmosis Filtration?

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.

The Reverse Osmosis Filter

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

Depending on the quality of the source water, different types of filters are used to remove contaminants and generally improve the quality of your water.  In this section, we’ll be looking at the various types of filters that are used during the different stages of water filtration.

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.

Alkaline Filters

Alkaline filters increase the PH of water to make it less acidic. They usually consist of electrolysis plates that ionize the water. When the water is ionized, the charged plates attract acidic minerals and chemicals in the water. The water leaving the ionizer will generally have a PH of 8 – 9.

Water Softeners

Hard water contains high concentrations of minerals like calcium and magnesium. Most water softeners remove hard minerals using sodium obtained from regular salt (NACL). The hard minerals are flushed away down the drain.

Salt-free water softeners use resin beads that trap the hard minerals.

Remineralization

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.

Anti-bacterial Filters

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.

Pre-Filters

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.