3-Stage vs 4-Stage Reverse Osmosis Systems
Written by: Gene Fitzgerald // Last Updated: Aug 14, 2023
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Having multiple filter stages can make a difference when it comes to ensuring clean and pure water.
For one, 3-stage reverse osmosis systems produce cleaner and purer water compared to using just one filter stage.
But what about a 4-stage system? Should you go for one of those instead, just to be on the safe side?
Let’s see.
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Key Takeaways
- A 3-stage reverse osmosis system has a carbon pre-filter, the RO membrane, and a carbon post-filter.
- A 4-stage reverse osmosis system has a sediment pre-filter, a carbon pre-filter, the RO membrane, and a carbon post-filter.
- The best system for you will depend on various factors, such as what is in your feed water and your personal needs.
Difference Between 3-Stage vs 4-Stage Reverse Osmosis Systems
First of all, what is the difference between a 3-stage and a 4-stage reverse osmosis system?
The major difference is that a 4-stage reverse osmosis system will have an additional pre-filtration stage, usually for sediment removal. Aside from that, 3-stage and 4-stage reverse osmosis systems are pretty similar.
3-Stage Reverse Osmosis Systems
A 3-stage RO system has:
- A carbon pre-filter, whether in the form of granular activated carbon or carbon block. This effectively reduces elements in water that contribute to unpleasant tastes and odors, such as chlorine. Its primary purpose is to eliminate chlorine to safeguard the integrity of the RO membrane.
- The RO membrane, the main element of a reverse osmosis filtration system, efficiently removes dissolved solids and other dangerous contaminants such as heavy metals.
- Stage 3, a carbon post-filter, acts as a secondary activated carbon filter that further purifies the water, especially after it has been stored in a tank.
Some 3-stage RO setups have a carbon and sediment composite pre-filter, the RO membrane, and a carbon post-filter, which some consider to be a 4-stage system.
4-Stage Reverse Osmosis Systems
A 4-stage reverse osmosis system usually includes:
- Sediment Pre-Filter: The sediment pre-filter effectively captures dirt, sand, rust, and other tiny particles.
- Carbon Pre-filter
- RO Membrane
- Carbon Polishing Filter
What Do the Different RO Water Filter Stages Do?
A typical RO system consists of three filter stages as a base: Carbon, reverse osmosis, and post-carbon filtration. 4- stage systems have an additional sediment pre-filtration step. Each stage has its own crucial function.
Sediment Pre-Filter
Sediment filtration eliminates dust, dirt, rust, and other particles from water. Typically positioned as the initial filtration stage, it aids in capturing larger particles, enabling subsequent steps to target other contaminants beyond the reach of sediment filtration more effectively.
Consider it as giving the rest of the system a ‘leg up’ to filter the smaller stuff more effectively and prevent later filter stages, the delicate reverse osmosis membrane in particular, from clogging.
Carbon Pre-Filter
During this filtration stage, water passes through carbon media, eliminating tastes, odors, cloudiness, colors, and chemicals like volatile organic compounds (VOCs), chlorine, and sometimes even chloramines.
Aside from improving water aesthetics, the main purpose here is to protect the RO membrane from chlorine which could damage it.
Reverse Osmosis Membrane
During this key stage, water is propelled through a semipermeable membrane with a pore size of 0.0001 microns. A remarkable 99% of impurities, including lead, arsenic, and nitrates, are effectively eliminated from the water.
Carbon Post-Filter
After undergoing reverse osmosis, water is often preserved in a storage tank for later use. Incorporating a coconut shell carbon post-filter eliminates any residual taste that might have leached into the water while being stored.
3-Stage vs 4-Stage Reverse Osmosis Systems – Which Is Better?
The need for a sediment pre-filter depends on the quality of your water. If your water does not contain sediment, then a pre-filter is not necessary. However, if sediment is present and you don’t have a pre-filter, the dirt clogs the carbon filter stage, leading to less effective filtration or clogging of the RO membrane.
Besides, opting for a 4-stage RO system is generally not significantly more expensive than a 3-stage one and is worthwhile if the extra step is needed for your particular water.
Does a Reverse Osmosis System with More Filter Stages Equal Better Filtration?
Manufacturers often increase the number of filter stages in their water filtration systems. Sometimes, they will have seven, eight, nine, or even ten separate filter stages. While it is true that additional filter stages offer more opportunities to remove contaminants as water flows through the system, the effectiveness of filtration is not solely determined by the number of steps.
Other factors contribute to the performance of a filtration system. The design of the filter, its surface area, water flow rate, and the types of filter media used all play a significant role. A ten-stage filter may sound impressive, but it does not guarantee increased contaminant removal if each stage consists of activated carbon alone.
The truth about filter stages lies somewhere in between. More steps can potentially mean incorporating different types of filter media or methods, as well as increasing the volume of the same filter media, both of which can lead to higher rates of contaminant reduction. However, repeating the use of the same sediment or carbon filter consecutively may not improve filtration efficiency.
Further Reading:
How Many Stages Are Needed for Reverse Osmosis?
To ensure the removal of all water impurities, the reverse osmosis stage alone is necessary, but pre-filtration is needed to protect the reverse osmosis membrane. Sediment filtration and chlorine removal through a carbon filter are essential before the water reaches the RO membrane.
Subsequent stages contribute to eliminating residual taste. From a cost and performance standpoint, the recommended setup consists of four stages: sediment filtration, carbon filtration, reverse osmosis, and post-filtration for refining, particularly if you have a system with a storage tank.
Other Considerations
There are a few other things to consider when determining how many filtration stages you require for your setup.
Water Quality
Feed water quality is the main thing to consider.
- For instance, if you need to eliminate bacteria, adding a UV filter might be necessary.
- An additional sediment filtration step could be helpful for those with heavy sediment in their feed water.
To assess, it is best to have your tap water tested to see what it contains, decide what you want to remove and choose how many and which cartridges are suitable for you.
What You Want
Personal preference also plays a big part in deciding the stages of your RO system.
- You may only want to eliminate the residual taste of chlorine from your water, in which case, you may not even need an entire RO system, just a carbon filter.
- Or, you may want water that is guaranteed to be pure, no matter what creeps into your feed water on any given day, in which case, a 3-stage RO is a great idea.
- If you want pure water with minerals readded for taste, you will want a system incorporating a remineralization filter. The possibilities are numerous.
Installation (Space)
Space is definitely a consideration with any kind of water filtration system. Multi-stage filters take up a lot more room than a single-stage unit.
Maintenance
Typically, the number of filtration steps in an RO system correlates with the maintenance required. However, there are exceptions to this rule. Some multi-stage systems incorporate multiple filter materials in a single cartridge, offering easier installation and less frequent maintenance.
Different filter stages typically have varying lifespans. The sediment filter typically lasts approximately 6-12 months, while the reverse osmosis membrane can survive 2 to 5 years. The carbon block filters generally have a lifespan of 6-12 months.
As the number of stages increases, so does the cost of filter replacement.
If you have any questions about 3 vs 4-stage reverse osmosis please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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