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Reverse Osmosis vs Ultra Filtration: Which is Better?

Reverse Osmosis Filtration

Reverse Osmosis vs Ultra Filtration are both advanced filtration technologies for purifying drinking water. In this article, we will discuss what each technology is and does and which is better for you.

What is Reverse Osmosis?

Reverse osmosis removes contaminants from unfiltered water, or raw water, when pressure forces it through a semipermeable membrane. Water flows from the more concentrated side (more contaminants) of the RO membrane to the less concentrated side (fewer contaminants) to provide clean drinking water. The fresh water produced is called the permeate. The concentrated water left over is called the waste or brine.

A semipermeable membrane has small pores that block contaminants but allow water molecules to flow through. In osmosis, water becomes more concentrated as it passes through the membrane to obtain equilibrium on both sides. Reverse osmosis, however, blocks contaminants from entering the less concentrated side of the membrane. For example, when pressure is applied to a volume of saltwater during reverse osmosis, the salt is left behind and only clean water flows through.

Reverse Osmosis Filtration

Reverse Osmosis

4 Stages of Reverse Osmosis:

  1. When water first enters an RO system, it goes through pre-filtration. Pre-filtration typically includes a carbon filter and a sediment filter to remove sediment and chlorine that could clog or damage the RO membrane.
  2. Next, water goes through the reverse osmosis membrane where dissolved particles, even too small to be seen with an electron microscope, are removed.
  3. After filtration, water flows to the storage tank, where it is held until needed. A reverse osmosis system continues to filter water until the storage tank is full and then shuts off.
  4. Once you turn on your drinking water faucet, water comes out of the storage tank through another post-filter to polish drinking water before it gets to your faucet.

What is Ultra Filtration?

Ultrafiltration is one membrane filtration process that serves as a barrier to separate harmful bacteria, viruses, and other contaminants from clean water. An ultrafiltration water system forces water through a .01 micron membrane.  Suspended particles that are too large to pass through the membrane stick to the outer membrane surface. Only fresh water and dissolved minerals pass through. See Ultrafiltration and How it Work

Ultra Filtration

Ultra Filtration

Which is Better?

Many ultrafiltration systems use a hollow fiber membrane, which filters water from the inside out. This provides a large surface area for particles to adhere to. Other membranes, like the spiral wound RO membrane, filter from the outside in. The hollow fiber membrane has a high chemical resistance to oxidants and chlorine, but a TFC reverse osmosis membrane cannot tolerate any chlorine.

A Reverse Osmosis System provides the most extensive filtration because the RO membrane has the smallest pore size, but this level of filtration is not always necessary or preferred. A UF system retains beneficial minerals that an RO system removes. However, this means that an ultrafiltration system does not remove salts, fluoride, or TDS dissolved in water. An ultrafiltration system also operates on low water pressure, but a reverse osmosis system needs a booster pump to increase water flow.

Ultrafiltration is the filtration method of choice for people who prefer minerals left in their water but still want microscopic contaminants taken out. A UF system may be selected over an RO system because it wastes less water to the drain. Someone may choose UF in California where water use is regulated. Someone in South Carolina, where the water has few dissolved minerals to begin with, may choose UF since RO wouldn’t be necessary. Sometimes, ultrafiltration is used to recycle effluent water after filtration, so the water can be reused for irrigation.

Final answer: They are both great filtration technologies. The best option for you just depends on your raw water and your goals.

Find out more at: Tap Water Company


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