RO membrane is similar to attempting to force water through a plastic bag. The pore size is approx. 0.001 microns which is usually large enough for only water to pass through, but too small for larger molicules including bacteria, cysts and minerals.
The membrane is not considered a filter as it does not function to collect and hold filtered contaminants, but rejected contaminants must be flushed away on a continual basis. This is the purpose for water continually flowing to drain while the unit is producing RO water.
When flushing is inadequate, then the ppm of rejected dissolved solids attempting to pass through the membrane will become too great which depending on the composition of the contaminants, can block the membrane, or overwhelm the membrane, thereby increasing TDS within the product water.
I anticipate due to the extremely small pore size, water continually passing through over time will cause each pore to become worn and enlarged, thereby permiting smaller unwanted contaminants to also pass through.
There is no standard TDS ppm amount to determine when to replace the membrane. You will need to consider the composition of the raw water to determine the contaminants you desire to be eliminated and also consider your tolerance for some percentage of those contaminants to remain within your water.
While a TDS meter will not indicate which specific contaminants represents the TDS in the RO water, most users when deciding when to replace the membrane, generally consider the percentage of TDS reduction differential between the raw and product water will represent that percentage of all contaminants within the raw water.
If a new membrane results in 96% TDS rejection, depending on the contaminants to be eliminated, some users might decide to replace the membrane when rejection drops to 90% whereas another user with different contaminants might decide to replace at 85% rejection.
Not all contaminants are necessarily removed by the RO membrane. Because the best performing and most common membrane, TFT has low tolerance to oxidizing chemicals such as chlorine, one or more carbon filters will be utilized to remove chlorine and other chemicals prior to the membrane. With no exposure to a chemical sanitizing agent, biofouling on the membrane can occur over time, thereby reducing production volume, increasing the volume of rejection flow to drain and increasing the potential for bacteria to pass through the membrane into the RO water.
Sanitizing the entire system and sanitary practices while occaisionally replacing the membrae will reduce the potential for bacteria problems.
