Types of Water Contaminants Removed By Reverse Osmosis

bill_r
By bill_r / November 16, 2016
Contaminated Water RO Filter

Is tap water really safe for drinking? Well, that is a controversial question. The truth is that municipalities do their best to ensure that they provide clean water. Unfortunately, the water they deliver is not clean enough.  With a Reverse Osmosis (RO) system in place you can be certain that the water you drink is free of most types of contaminants.

The process of reverse osmosis involves water being filtered through a semi-permeable membrane (see our page for additional information about how a RO filter works). A typical RO system has four filters (the sediment filter, carbon filter, the reverse osmosis filter, and the post carbon filter). Each filter is responsible for removing certain types of contaminants.

Types of water contaminants

Generally, RO systems get rid of contaminants such as viruses, bacteria, protozoa and chemicals. The list below shows the types of chemicals removed and the percentage of effectiveness of the system in their removal.

Contaminant % of removal Contaminant % of removal
Aluminum 97-98% Nickel 97-99%
Ammonium 85-95% Nitrate 93-96%
Arsenic 94-96% Phosphate 99+%
Bacteria 99+% Polyphosphate 98%-99%
Bicarbonate 95-96% Potassium 92%
Boron 50-70% Pyrogen 99+%
Bromide 93-96% Radioactivity 95-98%
Cadmium 96-98% Radium 97%
Calcium 96-98% Selenium 97%
Chloride 94-95% Silica 85-90%
Chromate 90-98% Silicate 95-97%
Chromium 96-98% Silver 95-97%
Copper 97-99% Sodium 92-98%
Cyanide 90-95% Sulphate 96-98%
Ferro cyanide 98-99% Sulphite 96-98%
Iron 98-99% Zinc 98-99%
Lead 96-98% Insecticides 97%
Magnesium 96-98% Detergents 97%
Manganese 96-98% Herbicides 97%
Mercury 96-98% Virus 99+%
Total Dissolved Solids TDS 95-99% Hardness 93-97%

The percentages indicated above are generally accepted in the industry. However, an RO system could perform better or worse than the figures noted above because other factors could affect the filtration process. Such factors include the PH, temperature, pressure and chemicals in the water.  The above chart is handy for comparing the Consumer Confidence Report that details what's in your water to potential pollutants.

The filtration process

Reverse Osmosis is primarily based on the physical process of filtration. Filtration occurs when matter (gas, liquid or solid), passes through the pores of a medium or an absorbent material. Where water contaminants are concerned, filtration is dependent on the size of contaminant, and where applicable, its charge. To increase the effectiveness of the RO filters, pretreatment of drinking water may be necessary. The pretreatment might involve adjusting the pH, adding coagulants in the water or altering the chlorine levels.

There are three types of filtration described by the CDC and understanding them will give you a clearer idea of how a good water filtration system works.

  • Water first goes through the microfiltration to remove large particles that would otherwise destroy delicate filters such as the RO membrane.
  • The 2nd stage is ultrafiltration to trap larger chemicals, bacteria and protozoa. In many systems, there may be 2 of these filters.
  • The last filtration stage is nanofiltration or Reverse Osmosis (of these 2 choices, reverse osmosis is better) which gets rid of most chemicals to produce safe drinking water.

1. Microfiltration

Microfiltration filters have a pore size of about 0.1 microns. Some microfiltration filters have a pore size as small as 0.05 microns while others have a pore as big as 5 microns. Due to the pore size, microfiltration is effective in removing some contaminates as show below:

  • Yes - Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Maybe - Filter is moderately effective at removing bacteria such as E. Coli and Salmonella
  • No - Filter is not effective at removing viruses such as Rotavirus or Hepatitus A.
  • No - Filter is not effective at removing chemicals

2. Ultrafiltration

Ultrafiltration filters have a pore size of about 0.003 microns with a range from 0.003 on the lower scale to 0.05 on the upper scale. The Molecular Weight Cut Off (MWCO) on these filters is between 13,000 and 200,000 Daltons. In the RO system, the ultrafiltration filter is the carbon filter.

  • Yes - Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes - Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Maybe - Filter is moderately effective at removing viruses such as Rotavirus and Hepatitus A.
  • No - Filter is not effective at removing chemicals

3. Nanofiltration

Nanofiltration filters have even smaller pores with pore sizes between 0.001 microns and 0.01 micron. The MWCO of these filters is between 200 and 2,000 Daltons. Like the ultrafiltration filters, the effectiveness of nanofilters is affected by the size of particles, the weight, and the charge. Nanofilters aren't typically used in a RO filter because the even smaller pores of the Reverse Osmosis membrane are used.

  • Yes - Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes - Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Yes - Filter is effective at removing viruses such as Rotavirus and Hepatitus A.
  • Maybe - Filter is moderately effective at removing chemicals such as metal ions of copper, chromium, or lead.

4. Reverse Osmosis Membrane

Reverse Osmosis membrane filters have the smallest pores at about 0.0001 micron or 0.1nm. These filters are best at filtering out ions and molecules that the other filters can't.  In the RO system, the RO membrane filter is the last stage and filters out the smallest particles.

  • Yes - Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes - Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Yes - Filter is effective at removing viruses such as Rotavirus and Hepatitus A.
  • Yes - Filter is effective at removing chemicals such as chromium, copper, sodium, and lead.

Conclusion​

RO systems are cheap and easy to install. Also, these systems do not require lots of maintenance but they do require filter changes are regular intervals - typically every 6 months to a year depending upon the amount of contaminants in the water.

Take a look at our Reverse Osmosis Filter Reviews page to see our top picks for RO Filter systems.​

About the author

bill_r

Hi and welcome to my website. I'm Bill Robertson and I worked my way through school as a handyman. The skills I learned have been useful as I've made changes to my own home. As a father of a 3 children, having a healthy water supply is important. Because the Central Texas well water was so hard, a water softener and reverse osmosis filter were critical. The water softening process was interesting and resulted in such a difference in water quality that I wanted to know why. This website is the result. Central Texas is famous for its hard water and I likes sharing what I learn experience with other readers.

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