Pre-filtration or Primary Filtration?
While one refers to the first filtration performed at the time of water intake and the other consists of a water preparation step for a finer filtration, how do we differentiate them?

Primary filtration

It consists of the first filtration step when the water is drawn in its natural state. It is generally when the water comes from a surface reservoir such as a lake, river or tank that this step is most necessary.

Primary filtration is used to remove the larger contaminants found in the water. Depending on the type of primary filtration used, and the cleanliness of the water, these contaminants may be suspended solids, twigs, algae, or any other particles that are visible to the naked eye.

As mentioned above, the type of primary filtration will have an impact on the extracted contaminants since there are several varieties. These include screening, decantation, flocculation and even some types of filtration.



Pre-filtration is a step to prepare the water for its main treatment. For example, when reverse osmosis is used for water treatment, the pores of osmotic membranes are 0.0001 microns in size. With such small pores, the membranes can quickly become clogged if there is too much contaminant present. The problems caused by these contaminants can be due to their size or to their physico-chemical properties such as colloidal silica.

In short, depending on the type of filtration used, prefiltration varies greatly, but generally speaking, we find technologies such as ion exchange, microfiltration or media filters as prefilters.


Are these filtration steps necessary?

Actually, not always. It all depends on the quality of water you need and the quality of water available to you. For example, if you are collecting city water and your water treatment system consists of ultrafiltration, you will most likely not need a pre-filtration or primary filtration step.


In short, enhance your understanding and knowledge by looking at these articles on related topics:

in FAQ
What is Membrane Flux?
Simply defined, membrane flux refers to the amount of liquid that can be treated over a defined time.