Matthew Barta

Matthew Barta

R.O Why 4 Stages of Filtration?

3 minutes reading

The 4 Stages of R.O Filtration

What does each stage do?

The heart of the P-ZIP filtration system is the Reverse Osmosis (RO) membrane (third stage). The RO membrane is actually a very thin film coated on the surface of a multi-layered filter sheet which is rolled into a spiral to form the RO filter cartridge. The RO membrane allows water molecules to pass through, but not large molecules and ions.  This thin membrane is very sensitive to both physical damage and damage by certain chemicals (i.e. chlorine). If the inlet water contains sand or other particles in it they could scratch the membrane surface. If there is chlorine present it will deteriorate the membrane. So the two prefilters are to protect this RO membrane.

If the inlet water is properly pre-filtered the RO membrane can last many years. If not pre-filtered the membrane may be damaged or deteriorate in just a few months. So changing the prefilters regularly is essential to the preservation of the membrane and maintaining good water quality.


Stage 1: Polypropylene (PP) pre-filter:

This is a very fine mesh material that physically strains any particles from the water. This will stop sediment particles that are as fine as 5 microns (1 micron = 1/1000 of a mm) such as grit, dust, rust, mud and algae..  Besides protecting the RO membrane from damage, it protects the membrane from plugging up.


Stage 2: Activated carbon (charcoal) pre-filter:

Activated carbon is a very fine porous material that acts like a micro sponge. Water can flow through this sponge-like structure and certain types of molecules will be adsorbed onto the surfaces of said sponge.
Because chlorine is added to all municipal water, and this chlorine will very quickly deteriorate the RO membrane, it is essential that it be removed. The activated carbon filter will accomplish this.
Eventually the surfaces of the carbon will be completely covered by and no longer able to absorb the chlorine. So it is extremely important to change the carbon filter regularly.

Stage 3: RO Membrane:

This is the stage that removes the bulk of chemicals and other contaminants from the water, down to a molecular level. It works through a combination of both physical separation and an electrostatic barrier on the surface of the membrane. This membrane is actually only a very thin coating of material on the surface of a multi-layered filter sheet. Water flows over the surface of the membrane and only the smaller water molecules can pass through the membrane. Besides physically blocking contaminant molecules from passing through, the surface of the membrane has a slight polar electrostatic charge that pushes ions and polar molecules away from the surface, preventing the membrane from clogging.

With pore size less than 0.0005 micron the membrane effectively screens out micro-particles. It repels heavy metal complexes including aluminium, lead, mercury; micro organisms; colloidal matter and most cysts and virtually eliminates mineral ions including fluorides and sodium.

Stage 4: Mineral/pH Post-Filter:

After filtering through the RO membrane the water will be very pure, containing almost no minerals, and will be slightly acidic (see here to learn why RO water is slightly acidic). By reintroducing calcium and magnesium into the water it improves the taste and adds healthy minerals. Besides the improved taste, if you are going to be using this water for making coffee the pH and alkalinity (which you can learn more about here) are crucial in extracting the maximum amount of flavors from the bean. This addition of minerals and pH adjustment is accomplished by a balanced combination of minerals and activated carbon in the post filter.


P-Zip R.O Membrane has been tested for performance to comply with NSF Standards. The National Scientific Foundation is a trusted third-party not-for-profit testing organisation for product manufacturers that deliver, and treat, drinking water. A NSF mark tells customers as well as the regulatory community that your products are certified safer.

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