Is UV Required With Reverse Osmosis (RO)?

Finding the right water purification system for your home may seem like a daunting task but understanding the mechanism of each filtration system can go a long way. When it comes to UV and RO purification, here’s what you need to know.

UV purification with Reverse Osmosis is required if the water supply has high microbial content. RO systems remove up to 90% of total dissolved solids (like sediments, chemicals, and heavy metals) but do not remove microbes. Bacteria, viruses, and algae (including microbes from the RO holding tank) can be killed only by passing the water through a UV disinfection system.  

Before discussing which type of water filtration is better for your home, it is important to know the mechanism of these filtration systems, and what exactly they can and cannot remove.

Types of UV Light

Ultraviolet (UV) light refers to non-ionizing radiation that has a wavelength between 100 and 400 nm (nanometers). While the primary natural source of UV light is the sun, artificial UV light can be used in the purification of water.

There are three bands in the UV wavelength range and it’s important to understand them before exploring the UV filtration mechanism.


UVA light falls between the wavelengths of 315-400 nm. Natural UVA causes sunspots, wrinkles, and other signs of aging.


UVB light has a wavelength between 280-315 nm. Natural UVB may cause sunburn, redness, and skin cancer on prolonged exposure.


UVC light has the shortest wavelength, ranging between 100-280 nm. Natural UVC from the sun is mostly absorbed by ozone, therefore it does not cause much harm to the body.

  • Artificial UVC light is used in water treatment systems because it has highly potent germicidal properties.
  • This penetrating light attacks bacteria and viruses at the genetic level and kills them.
  • Waterborne bacteria like E. coli and types of pseudomonas, salmonella, clostridium, and staphylococcus are killed.
  • In addition, UVC is also effective against saccharomyces yeasts, penicillium and mucor spores, and viruses like influenza, hepatitis, poliovirus, and rotavirus.
  • Chlorella algae are also destroyed by UVC.

UV Filtration Mechanism

The UV mechanism is fairly straightforward – a UVC lamp (with wavelength between 240 and 280 nm) is brought to contact with water for an exposure time ranging between 10-20 seconds.

Depending on the condition of the water, especially the concentration of suspended solid particles, UVC light effectively kills most microbes.  

In this method, additional chemicals and disinfecting agents are not added to the water. Therefore, this disinfection is done without causing major physical and chemical changes in the water.

It does not alter the taste or smell of the water either.

This makes it safe for human consumption.

In addition, UV purification is faster than RO. It is less expensive than RO filtration as only the UV lamp needs to be replaced. UV purification is more eco-friendly as there is no wastage of water.

However, UV filtration does not leave any residual disinfectant to avoid future contamination.

 There is no mechanism to remove any remnants of the dead microorganisms from the water either. UV filtration also does not remove any dissolved or suspended salts. It does not remove other impurities like sand, sediments, heavy metals, or particulate matter.

The turbidity (cloudiness) of water remains unchanged after UV filtration.

RO Filtration Mechanism

RO filtration uses a semi-permeable membrane that filters suspended solids and dissolved contaminants in the water. By placing this membrane between multiple vats of water – one with a higher concentration of contaminants and one with a lower concentration of contaminants.

The filter membrane is made of the polymer matrix that removes suspended and dissolved solids. In this process, pressure is applied to push water between the membrane, leaving contaminants behind. Here, only purified water is pushed to the other side.

This process is called Reverse Osmosis because it goes against the osmotic movement of the solvent (in this case, water) from a region of lower solute concentration to a region of higher solute concentration.

RO filtration removes dissolved minerals from hard water, improving its taste and color. RO water cleans muddy and dirty water by removing sediments with their pore size of 0.0001 microns. Nitrates, pesticides, organic compounds, and sodium can be effectively removed through RO filters.

RO filtration is more expensive than UV as the filters must be changed periodically. RO systems also need more electricity when compared to UV filters. RO water purification also leads to wastage of water as more water is used to flush the contaminants out.

RO filtration cannot remove bacteria and viruses from the water because of their pore size. In fact, there is no physical membrane that can remove microbes effectively.

Is UV required after RO?

It is important to understand the quality of your water supply before opting for either purification mechanism.

The Total Dissolved Solids (TDS) metric refers to the volume of dissolved contaminants in the water, including:

  • Nitrates
  • Calcium
  • Sodium
  • Potassium
  • Chlorides
  • Sulfates
  • Carbonates.

The World Health Organization dictates that less than 300 mg/L of TDS is excellent-tasting water.

A TDS level of 500 parts per million is acceptable for drinking.

If your water supply is predominantly hard with a high TDS value, an RO filter can remove minerals and prevent scaling.

If you want to desalinate water, an RO filter is your best bet. If your water supply is softened (with low TDS levels) but you are worried about microorganisms, a UV filter alone will do.

Water from reservoirs, rivers, ponds, and wells may be high in organic contaminants – they may contain a high number of microorganisms in addition to sediments and chemical runoffs. In this case, an RO filter removes contaminants and heavy metals.

Here, using an additional UV filter after RO purification can help disinfect the water and eliminate microbes.

Water stored in the RO holding tank can get contaminated with microbes if maintained incorrectly. Bacterial and algal colonies may form in the tank if not cleaned properly. In this case, running the water through a UV disinfection system purifies the water and makes it safe for consumption.

In short, while an RO filtration system can remove nitrates, organic compounds, and excess minerals, only a UV filtration system can kill bacteria, viruses, and algae in your water.

What is RO UV?

The RO UV filtration mechanism combines the best of both worlds, so to speak. Not only does it remove dissolved minerals and suspended particles, but it also removes biological contaminants like bacteria and viruses with a UV disinfection system. This means that water purified through an RO UV system is disinfected and has lowered TDS levels. 

Which is better – RO UV or RO UF?

RO UF (reverse osmosis/ultrafiltration) purifiers involve combining the RO mechanism with the ultra-filtration process. UF purifiers use a microfilter (of size 0.01 micron) to remove suspended solid particulate matter.

Much like a simple coffee filter, this purification system removes solid particles from the water, making it clearer. No water goes to waste here, and UF is much faster than RO.

However, UF filters cannot remove bacteria and viruses from water. UF filters cannot remove dissolved impurities either.

An RO UV system, on the other hand, helps remove maximum impurities from water, since the RO components removes dissolved and suspended impurities, and the UV purification system eliminates microbes.

Therefore, an RO UV system does a better job of removing maximum contaminants from the water, including bacteria, viruses, algae, heavy metals, nitrates, carbonates, sulfates, minerals, and other particulate matter.

Final Verdict

Passing water through a UV purification system after RO filtration is the best way to remove the maximum number of contaminants. This system can effectively remove microbes, dissolved contaminants, and suspended solids, ensuring that your water is safe for consumption.

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