By specification commercial RO membranes state that they remove 99.9% of bacteria. This in no way indicates that RO removes all bacteria. At any given time the rejection of bacteria may indeed be 100%, but this will never be the case 24/7/365

If the inquiry is to determine if RO water is sterile and can be counted on to be bacteria free, the answer is an absolute no. Here are the three reasons why.

1. Bacteria are living organisms that multiply. Even if only 0.01% of the bacteria in the feed water pass through the RO membrane, that minute amount of bacteria, over time will multiply and can increase exponentially in number. As a result, the water in a storage tank filled with a properly functioning RO system can have profuse amounts of bacteria in it.
2. The rating of 99.9% rejection of bacteria is for a new RO membrane only. Once a membrane has been placed into service it will begin to degrade under normal wear and tear, pressure and friction. With time, the bacterial rejection rate of an RO system will decrease and more bacteria will pass.
3. Bacteria are three dimensional, the pores in an RO membrane are two dimensional. This is also an important factor that forces membrane manufacturers to claim 99.9% and not 100% bacterial removal.

To illustrate: A filter may claim a 0.1 micron cutoff, and it is generally accepted that bacteria are 0.2 microns in size or larger. However, the bacteria may be much smaller width wise than it is lengthwise. Accordingly a 0.2 micron long bacteria could very well pass through a 0.2 micron filter.

Imagine trying to pass an 8 foot long couch sideways through a door opening only 3 ft wide. The couch would not pass. However, flipped lengthwise the couch will pass easily. In essence, an 8 ft “bacteria” will have passed through a filter rated at “three feet”.

These are the reasons it is incorrect to say that an RO membrane removes all bacteria.