4 Stage Vs 5 Stage Reverse Osmosis [With Benefits]

Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to remove ions, molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective”, this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely.

In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.

Here is an in-depth comparison table for 4 stage vs 5 stage reverse osmosis systems:

Parameter4 Stage RO5 Stage RO
Number of Filtration Stages45
Filtration Stages1) Sediment Pre-Filter 2) Carbon Block Filter 3) Carbon Block Filter 4) RO Membrane1) Sediment Pre-Filter 2) Carbon Block Filter 3) Carbon Block Filter 4) RO Membrane 5) Post Carbon Filter
Contaminant RemovalRemoves up to 99% TDS, chemicals, metals, microorganismsRemoves up to 99% TDS, chemicals, metals, microorganisms plus additional filtration from post filter
Water PurityHigh purity waterUltra pure water
Taste ImprovementGood – removes chlorine and contaminantsExcellent – post filter polishes taste
CostMore affordable system15-30% higher initial cost
Operating CostsLower cost of filter replacementsAdded post filter replacement cost
Efficiency99% rejection ratesUp to 2-3% higher rejection rates
Typical ApplicationsDrinking water, food service, hydroponics, aquariums, pre-treatmentHigh-tech manufacturing, medical, laboratories, pharmaceutical
MaintenanceAnnual filter and membrane replacementsAdditional post filter maintenance
Filter LifespanSediment & carbon – 6-12 months RO membrane – 2-3 yearsSediment & carbon – 6-12 months RO membrane – 2-3 years Post carbon – Annually
Space RequirementsCompact systemLarger footprint

How Does Reverse Osmosis Work?

Reverse osmosis applies external pressure to reverse the normal osmotic process resulting in desalination (removal of salt) or impurity reduction in water by pushing it through a semi-permeable membrane that doesn’t allow salts or impurities to pass through.

The principle of RO is fairly simple – pressure is applied on the high concentration side of the semi permeable membrane to force the water across from high concentrate to low concentrate. The amount of pressure required depends on the salt concentration of the feed water. The higher the concentration, the more pressure it requires.

As the water crosses the membrane, the rejected contaminants become more concentrated. To keep these contaminants from building up on the membrane and reducing its effectiveness, cross flow is maintained to sweep it away from the membrane.

The membrane rejects salts, silts, clays, dissolved organic matter, bacteria and pyrogens. The degree of separation depends on the membrane type and raw water quality.

What are the Basic Components of an RO System?

A reverse osmosis system contains four main components:

  • Pre-treatment – The incoming feed water is pre-treated through sediment and carbon filters to remove suspended particles that can clog membranes.
  • Pressurized tank – The pre-treated water is pumped into a pressurized tank to provide the pressure needed to push water through the semi-permeable RO membrane.
  • Semi-permeable RO membrane – The pressurized pre-treated feed water is then pushed through this fragile membrane that removes a high percentage of contaminants and minerals from the water.
  • Storage tank – The treated water passes from the RO membrane into a storage tank. This is the final purified water that is now ready for use per the system’s application.

What are the Key Differences Between 4 Stage and 5 Stage RO Systems?

The main differences between 4 stage and 5 stage RO systems are:

Number of Filters

  • 4 stage RO systems – Contain 4 main filtration components i.e. sediment pre-filter, 2 carbon block pre-filters, RO membrane.
  • 5 stage RO systems – Contain 5 main filtration components i.e. sediment pre-filter, 2 carbon block pre-filters, RO membrane, and a post carbon filter.

The additional 5th stage in a 5 stage RO is a post carbon filter or polishing filter after the RO membrane.

Contaminant Removal

  • 4 stage RO – Removes up to 99% of total dissolved solids (TDS), chemicals, heavy metals, microorganisms etc.
  • 5 stage RO – Removes up to 99% TDS, chemicals, metals, microorganisms, plus the post carbon filter provides extra filtration for improved contaminant reduction and taste enhancement.


  • 4 stage RO – Produces cleaner, better purified water vs tap water though may have slight mineral taste depending on source water.
  • 5 stage RO – The additional post carbon filtration provides enhanced taste by removing residual impurities and chemicals that standard 4 stage ROs may leave behind.


  • 4 stage RO – Requires regular filter changes i.e. sediment, carbon blocks, RO membrane.
  • 5 stage RO – Requires filter changes for sediment, carbon blocks, RO membrane and additional post carbon filter.

So in summary, 5 stage RO systems add an extra carbon filter to provide superior contaminant reduction and improved taste compared to standard 4 stage systems. The trade-off is the extra filter increases cost and maintenance requirements.

What are the Key Benefits of 4 Stage and 5 Stage RO?

Benefits of 4 Stage RO

  • Removes up to 99% of water contaminants and impurities.
  • Eliminates microorganisms like bacteria and protozoa.
  • Significantly reduces Total Dissolved Solids (TDS).
  • Removes heavy metals like lead, mercury, arsenic, chromium.
  • Removes pesticides, herbicides, industrial solvents, VOCs.
  • Softens hard water by removing calcium and magnesium.
  • Improves taste and appearance of drinking water.
  • Affordable and cost effective water filtration.
  • Widely used for residential drinking water and commercial applications.

Added Benefits of 5 Stage RO

  • Removes trace chemicals and contaminants missed by 4 stage RO.
  • Provides a higher purity water quality.
  • Enhances taste by removing residual odors and chemicals.
  • Ideal when source water has high chemical contamination.
  • Used when highest quality water is required e.g. laboratories, aquariums, dialysis.

In summary, 4 stage RO provides excellent broad spectrum water filtration for most residential and commercial needs. 5 stage RO offers that additional level of purification and crisp taste for applications requiring ultra pure water.

What are the Main Applications of 4 Stage and 5 Stage RO?

4 Stage RO Applications

  • Point of use drinking water filtration – Removing contaminants from municipal tap water and well water to improve taste, odor and health safety.
  • Water softening – Eliminates hard water minerals like calcium and magnesium.
  • Commercial food and beverage – Soda fountains, coffee makers, juice dispensers, beer brewing.
  • Car washes – Purifies and softens wash water.
  • Hydroponics – Purifies nutrient solutions.
  • Aquariums and fish farms – Removes chlorine and purifies fish tank water.
  • Dialysis clinics – Pre-treatment to remove impurities before further polishing filtration for dialysis machines.

5 Stage RO Applications

  • Laboratory testing and research facilities – Requires ultra pure water for accurate test results.
  • Medical facilities – Used for sterilization equipment, surgical instruments, kidney dialysis equipment.
  • Pharmaceutical manufacturing – High purity water needed for making pharmaceuticals and drugs.
  • Electronics manufacturing – Used extensively for rinsing and processing semiconductors and computer chips.
  • Commercial food and beverage – Bottled water, enhanced steam purity for coffee machines.
  • Photo labs – Used for developing film to prevent spotting and defects on prints.

So in summary, 4 stage RO systems are widely used for broad residential and commercial applications where high purity water is desired. 5 stage RO provides that additional level of filtration for applications needing ultra pure water.

What Factors Should be Considered When Choosing Between 4 Stage and 5 Stage RO?

There are several factors to consider when deciding between a 4 stage and 5 stage reverse osmosis system:

Water Quality

  • 4 stage RO – Ideal for removing common contaminants from municipal and well water supplies. Sufficient for most residential and commercial applications.
  • 5 stage RO – Recommended for higher contaminated water sources or water with chemical pollution like pesticides, industrial solvents, heavy metals. The extra stage provides added protection.

Desired Purity Level

  • 4 stage RO – Produces cleaner, tastier water than tap water. Reduces TDS up to 99%. Good for typical drinking water applications.
  • 5 stage RO – Creates ultra pure water exceeding 4 stage RO. Essential when highest water purity is critical. Necessary for scientific, medical, or food and beverage applications.

Taste Requirements

  • 4 stage RO – Improves taste by removing chlorine and contaminants. May have slight mineral taste depending on water source.
  • 5 stage RO – Provides enhanced taste via the post carbon filter to polish and freshen RO water.


  • 4 stage RO – Typical rejection rates up to 99%. Good for general household use.
  • 5 stage RO – Increased contaminant reduction with the extra filtration stage. Produces highest quality water possible via RO.


  • 4 stage RO – More affordable system. Lower upfront cost plus ongoing filter replacements.
  • 5 stage RO – Initial system cost is 15-30% higher than 4 stage. Added expense of extra filter changes. Worth the investment when water quality is critical.


  • 4 stage RO – Filter changes every 6-12 months for pre-filters and 2-3 years for membranes.
  • 5 stage RO – Additional post carbon filter must be changed every 6-12 months adding to maintenance costs.

Space Limitations

  • 4 stage RO – Compact 4 stage systems allow flexible installation under sinks or in tight spots.
  • 5 stage RO – Larger footprint may require more installation space for the extra filtration canister.

Taking these factors into account will help determine whether a 4 stage or 5 stage RO system is the right choice for a particular application and budget.

How are the Stages Configured in 4 Stage and 5 Stage RO Systems?

RO systems are modular, allowing different filtration components to be assembled based on the number of stages desired. Here is how the stages are typically configured:

4 Stage RO Composition

  • 1st stage – Sediment pre-filter made of spun polypropylene that filters particulates down to 5 microns.
  • 2nd stage – Granular activated carbon (GAC) block filter that adsorbs chemicals, chlorine, pesticides, herbicides, VOCs and improves taste and odor.
  • 3rd stage – Carbon block filter providing a second phase of carbon filtration.
  • 4th stage – Semi-permeable thin film composite (TFC) membrane that removes dissolved salts, metals, minerals and other contaminants.

5 Stage RO Composition

  • 1st through 4th stages – Same as 4 stage system.
  • 5th stage – Inline granular activated carbon post-filter. Final polishing filter further improves taste, appearance, and purity.

The modular design allows installing each filtration component in sequence to achieve the desired level of water purification. The raw feed water is processed through each progressive stage becoming incrementally more purified.

What are the Typical Rejection Rates of 4 Stage and 5 Stage RO Systems?

Reverse osmosis systems are rated on their rejection efficiency – the percentage of dissolved contaminants removed from the raw water supply. Here are the typical rejection rates:

4 Stage RO Rejection Rates

  • Total dissolved solids (TDS) – 95-99% rejection
  • Fluoride – 90-95% rejection
  • Nitrates – 85-95% rejection
  • Sodium – 93-98% rejection
  • Arsenic – 90-96% rejection
  • Copper – 95-98% rejection
  • Lead – 96-98% rejection
  • Cysts – 99% rejection
  • VOCs, pesticides, herbicides – 99% rejection

5 Stage RO Rejection Rates

  • TDS – 97-99% rejection
  • Fluoride – 93-96% rejection
  • Nitrates – 90-96% rejection
  • Sodium – 95-99% rejection
  • Arsenic – 93-98% rejection
  • Copper – 96-99% rejection
  • Lead – 98-99% rejection
  • Cysts – 99% rejection
  • VOCs, pesticides, herbicides – 99% rejection

The rejection rates above can vary based on factors like raw water quality, system age, temperature and pressure. But overall, 5 stage RO can achieve up to 2-3% higher contaminant removal vs 4 stage systems.

What are the Maintenance Requirements of 4 Stage and 5 Stage RO?

Proper maintenance is essential to optimize performance and lifespan of any RO system. The maintenance needs of 4 vs 5 stage systems are:

4 Stage RO Maintenance

  • Sediment pre-filter – Replace every 6-12 months.
  • Carbon block filters – Replace every 6-12 months. Replace both at same time.
  • RO membrane – Replace every 2-3 years depending on water quality and consumption.
  • Sanitization – Sanitize entire system annually using chlorine bleach to prevent biological growth.

5 Stage RO Additional Maintenance

  • Post carbon filter – Replace every 6-12 months along with pre-filter cartridges.
  • Increased sediment buildup – The extra stage can increase particulate buildup requiring more frequent filter cleanings.
  • Membrane replacement – May need to replace membrane every 2 years instead of every 3 years.
  • Higher running costs – More and costlier frequent filter replacements. Annual costs around 20-30% higher than 4 stage RO.

Following the manufacturers recommended maintenance schedule is crucial to get optimal lifespans of filters and membranes for both 4 stage and 5 stage systems.

What are Indications that Filter or Membrane Replacement is Needed in RO Systems?

Here are key signs that filters or membranes need replacing:

Sediment and carbon filters:

  • Decreased water production and slower fill times.
  • Cloudiness or odors returning in filtered water.
  • Higher than normal TDS levels in filtered water.
  • Filters more than 6-12 months old.

RO membrane:

  • Unusually high TDS levels in RO water.
  • RO water production and efficiency drops.
  • Foul tastes return in RO water.
  • Filters not removing odor or chlorine tastes.
  • RO membrane over 2-3 years old.

5 stage RO post carbon filter:

  • Taste or odor issues reappear.
  • Water has cloudy appearance again.
  • Chemical or chlorine taste returns after filtration.
  • Annual replacement as per manufacturer.

Catching deteriorating performance early allows filters and membranes to be replaced before damage occurs. This restores a system back to peak performance levels.

Frequently Asked Questions

What is the typical lifespan of RO system filters and membranes?

Sediment and carbon filters typically last 6-12 months. RO membranes last 2-3 years on average. Post carbon filters need annual replacement. Proper maintenance maximizes component lifespans.

Can a 4 stage RO system be upgraded to 5 stages?

Yes, a 4 stage RO system can easily be upgraded to a 5 stage by adding a post carbon filter canister. This provides the benefits of 5 stage filtration.

Does RO remove beneficial minerals from water?

A: Yes, RO removes up to 99% of salts and minerals which improves purity but may result in flatter tasting water lacking beneficial minerals. Remineralizing filters can add back some beneficial minerals.

What factors affect the lifespan of RO membrane?

Water quality, pre-filtration effectiveness, pressure, temperature, and consumption rates impact membrane life. Proper maintenance can achieve 2-5 years from quality membranes.

Can RO effectively remove pharmaceuticals from drinking water?

Yes, the tight pore size of advanced RO membranes provide 99% reduction of trace pharmaceuticals, hormones, antibiotics and endocrine disruptors found in some water supplies.


In summary, choosing between 4 stage and 5 stage reverse osmosis water filtration depends on factors like feed water quality, desired purity level, taste preferences, efficiency needs and budget.

4 stage RO effectively removes up to 99% of contaminants for most general residential and commercial uses. 5 stage RO offers enhanced filtration, often necessary for high purity industrial, laboratory, or medical applications where water quality is critical.

While 5 stage RO provides the highest quality water, it comes at a cost of higher system purchase price, maintenance overheads, and more frequent filter replacements. For many homeowners and businesses, a 4 stage RO system provides ample water purification at greater economy.

Understanding these key differences allows selecting the right RO system based on purification needs and cost considerations. With routine maintenance, both 4 stage and 5 stage RO systems can provide decades of optimized water filtration.

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