How Do Reverse Osmosis Filter Systems Work & What Do They Do?

If you’re looking for a water filter that will provide outstanding drinking water for your home or business, Reverse Osmosis (RO) filtration is one of the most popular and best water filtration methods available. In simple terms, reverse osmosis works as water is forced across a semi-permeable membrane, leaving contaminants behind that are flushed down the drain. The clean drinking water collects in a holding tank.

Below you will find diagrams, charts and explanations to answers your questions about RO systems. If you have other questions which are not answered here, please visit our our main water education page or Contact Us.

2How Does Reverse Osmosis Work?

How does Reverse Osmosis remove impurities?

Reverse Osmosis water purification process is a simple and straightforward water filtration process. It is accomplished by water pressure pushing tap water through a semipermeable membrane to remove impurities from water. This is a process in which dissolved inorganic solids (such as salts) are removed from a solution (such as water). 

What happens to the contaminants that don’t pass through the membrane?

When household water pressure pushes water through the RO membrane and additional filters, such as sediment or carbon filters, the impurities are filtered out and subsequently flushed down the drain. What is left, is delicious, clean-tasting drinking water. Note that many RO units incorporate a 4 or stage process for optimal water quality.

You may be interested to know that Reverse Osmosis technology is not new. In fact, RO processes began being utilized by municipalities back in 1977. Since then, Reverse Osmosis has become increasingly popular because it is safe, cost-effective, and easy to maintain.

RO Filter Systems: What are the basic components of an RO System?

Generally, residential "Point Of Use" POU RO systems are installed and stored under the kitchen sink. Regarding the common components of a four to five-stage RO system, note that:

• Most RO systems look similar and have the same basic components.
• Although most RO systems look and work basically the same way --they differ in the QUALITY of their components.

Basic Components of a Reverse Osmosis System:

1. Cold Water Line Valve: Valve that fits onto the cold water supply line. The valve has a tube that attaches to the inlet side of the RO pre filter. This is the water source for the RO system.
2. Pre-Filter(s): Water from the cold water supply line enters the Reverse Osmosis Pre Filter first. There may be more than one pre-filter used in a Reverse Osmosis system, the most common being sediment and carbon filters. These pre-filters are used to PROTECT the RO membranes by removing sand silt, dirt, and other sediment that could clog the system. Additionally, carbon filters may be used to remove chlorine, which can damage the RO membranes.
3. Reverse Osmosis Membrane: The Reverse Osmosis Membrane is the heart of the system. The semipermeable RO membrane is designed to remove a wide variety of both aesthetic and health-related contaminants. After passing through the membrane, the water goes into a pressurized storage tank where treated water is stored.
4. Post filter(s): After the water leaves the RO storage tank, but before going to the RO faucet, the treated water goes through a final “post filter”. The post filter is usually a carbon filter. Any remaining tastes or odors are removed from the product water by post filtration “polishing” filter.
5. Automatic Shut Off Valve (SOV): To conserve water, the RO system has an automatic shut off valve. When the storage tank is full, the automatic shut off valve closes to stop any more water from entering the membrane and blocks flow to the drain. Once water is drawn from the RO faucet, the pressure in the tank drops; the shut off valve then opens to send the drinking water through the membrane while the contaminated wastewater is diverted down the drain.
6. Check Valve: A check valve is located in the outlet end of the RO membrane housing. The check valve prevents the backward flow of treated water from the RO storage tank. A backward flow could rupture the RO membrane.
7. Flow Restrictor: Water flowing through the RO membrane is regulated by a flow restrictor. There are many different styles of flow controls, but their common purpose is to maintain the flow rate required to obtain the highest quality drinking water (based on the gallon capacity of the membrane). The flow restrictor also helps maintain pressure on the inlet side of the membrane. Without the additional pressure from the flow control, very little drinking water would be produced because all the incoming water would take the path of least resistance and simply flow down the drain line. The flow control is most often located in the RO drain line tubing.
8. Storage Tank: The standard RO storage tank holds from 2 - 4 gallons of water. A bladder inside the tank keeps water pressurized in the tank when it is full. The typical under counter Reverse Osmosis tank is 12 inches in diameter and 15 inches tall.
9. Faucet: The RO unit uses its own faucet, which is usually installed on the kitchen sink. Some areas have plumbing regulations requiring an air gap faucet, but non-air gap models are more common
10. Drain line: This line runs from the outlet end of the Reverse Osmosis membrane housing to the drain. The drain line is used to dispose of the wastewater containing the impurities and contaminants that have been filtered out by the reverse osmosis membrane.

Diagram of a Reverse Osmosis System with Basic Components:

4How does a Reverse Osmosis Membrane work?

Reverse Osmosis (RO) is a water treatment process that removes contaminants from water by using pressure to force water molecules through a semipermeable membrane. During this process, the contaminants are filtered out and flushed away, leaving clean, delicious drinking water. Reverse osmosis is capable of removing up to 99 percent of 65 different contaminants (see partial list below), including lead, fluoride, chlorine, dissolved salts, and more.

There are generally four stages in the reverse osmosis process:

Sediment Filter:

This pre-filter stage is designed to strain out sediment, silt, and dirt and is especially important as the sediment filter protects dirt from getting to the delicate RO membranes that can be damaged by sediment.

Carbon Filter:

The carbon filter is designed to remove chlorine and other contaminants that affect the performance and life of the RO membrane as well as improve the taste and odor of your water.

Reverse Osmosis Membrane:

The semipermeable RO membrane in your RO system is designed to allow water through, but filter out almost all additional contaminants.

Polishing Filter:

In a four-stage RO System, a final post filter (carbon filter) will “polish” off the water to remove any remaining taste and odor in the water. This final filter ensures you’ll have outstanding drinking water.

Reverse Osmosis Membrane Operating Conditions and Performance Data

6 How much service and maintenance does an RO system require?

Reverse Osmosis drinking water systems are easy to maintain and upkeep costs are minimal.

1. Replacement filters: RO membranes and filters will need to be replaced every 6 months to 2 years depending on the type of filter, your water quality and your water usage volume.

This guideline is very general. Specific information about how often your membrane or filter should be replaced can be found in the products’ “Specs” or your product owner’s manual. If you’ve purchased a system or filter from us in the past, we will send reminder emails when it comes time to change your filter.

2. Electricity Charges: All of our residential Reverse Osmosis systems require no electricity, running only on water pressure. Some of our larger commercial systems do have to use
electric pumps, but even then the power requirements are minimal.

3. Annual Maintenance: We recommend that you have your RO system sanitized and filters changed once each year.  This can be done by the homeowner or by a local
water treatment professional. A water professional can provide needed maintenance service or consult your RO system’s owner’s manual.  We also carry kits and the filters
to perform the annual sanitation and filter change if you’re a do-it-yourself type.

How Long Will An RO System Last?

If you properly care for your Reverse Osmosis system (annual cleanings plus change out filters/membranes on time), your system should have a very long lifespan; we’ve seen properly-maintained systems last 10-15 years!

We highly recommend that you purchase replacement filters from us, as we only carry top-quality filters that are designed for optimal performance and reliability.

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