About Reverse Osmosis Desalination And Recovery Rates

Reverse osmosis technology is currently the highest of all water production technologies mastered by mankind, through reverse osmosis components to improve the purity of water, cleanliness of almost 100%, to remove impurities and salt contained in the water.

In the selection of reverse osmosis equipment, we are mainly concerned with two indicators: the desalination rate and the recovery rate, the former determines the quality of the water produced and the latter determines the utilisation rate of the water.

Standard desalination rate, actual desalination rate and system desalination rate of membrane elements

The standard desalination rate of membrane elements is the desalination rate measured by the membrane element manufacturer under standard conditions, e.g. CPA2 has a minimum desalination rate of 99.2% (average desalination rate of 99.5%) and CPA3 has a minimum desalination rate of 99.6% (average desalination rate of 99.7%) under standard conditions.

The actual desalination rate of a membrane element is the rate of desalination exhibited by the membrane element in actual use. The actual desalination rate is sometimes higher than the standard desalination rate, but more often than not it is lower than the standard desalination rate, due to the fact that the standard test conditions are completely different from the actual conditions of use. Under standard test conditions, where the standard test solution is a sodium chloride solution, the standard desalination rate of the membrane element is expressed as the rate of removal of sodium chloride. In actual use conditions, due to the different ionic composition of the water, the temperature, average water flux selection value, system recovery rate, etc. are different from the standard test conditions, and all these factors will affect the desalination rate of the membrane element.

The system desalination rate is the desalination rate of the complete reverse osmosis unit, also due to the use of conditions different from the standard conditions, the system desalination rate is different from the standard desalination rate, and because reverse osmosis units are generally connected in series with multiple membrane elements, and the actual conditions of use of each membrane element in the unit are different, so the system desalination rate is also different from the actual desalination rate of the membrane elements, for only one membrane element unit, the system desalination rate is equal to the actual desalination rate of the membrane elements.

The easiest way to predict system desalination rates is to carry out actual calculations using the membrane element manufacturer's calculation software.

After understanding the relationship between the standard desalination rate of the membrane element, the actual desalination rate and the system desalination rate, it is important to design the reverse osmosis plant, provide the user with a system performance guarantee, accept the reverse osmosis plant or evaluate the performance of the membrane element based on the actual desalination rate of the system and not on the standard desalination rate of the membrane element.

How to calculate the system desalination rate

System desalination rate = (total feed water salinity - total produced water salinity) / total feed water salinity x 100%

Sometimes, for convenience reasons, the following formula can also be used to approximate the system desalination rate.

System desalination rate = (total feed water conductivity - total produced water conductivity) / total feed water conductivity x 100%

This approximate estimate of the system desalination rate is often lower than the actual system desalination rate, and therefore often causes controversy when accepting reverse osmosis systems.