The main aim for most RO systems is to separate pure water away from the dissolved contaminants present in the feed water. Simplistically, the rejection can be calculated as:
Where CP is salt concentration (TDS) in the Permeate, and CF is the salt concentration (TDS) in the feed.
In reality, it is much more complicated than this, as the Feed TDS gradually increases through the membrane. Similarly the Permeate TDS gradually increases across the membrane. Also, different salts will be rejected differently depending on the size and charge of the salt.
Rejection will also be heavily dependent of system temperature and system flux. As temperature increases, the amount of salt passing through will increase. Conversely, as the temperature decreases, the salt passage decreases. The effect of flux is a little more difficult to understand. For a given temperature and a given membrane, the salts will always pass through the membrane at a constant rate. If we increase the water flux (i.e. by increasing feed pressure), then more water is passing through for the same amount of salt which in effect dilutes the salt. Thus at higher flux, rejection appears to have improved.
Salt passage is the opposite of rejection, and given by the formula:
Since rejection is heavily dependent on temperature and flux, it is important to normalise operating data before comparing membrane performance from one day to the next. Software for normalising data is available from the membrane manufacturers.