What is the difference of ro membrane among standard testing recovery, actual recovery, and system recovery ?

1. Standard Testing Recovery

The standard recovery of membrane components is the recovery rate adopted by membrane component manufacturers under standard testing conditions. The standard recovery rate for brackish water membrane components is 15%, while for seawater membrane components it is 10%.

2. Actual Recovery

The actual recovery rate of membrane components refers to the recovery rate of membrane components during actual use. In order to reduce the pollution rate of membrane elements and ensure their service life, membrane element manufacturers have made clear regulations on the actual recovery rate of single membrane elements, requiring that the actual recovery rate of each 1-meter-long membrane element should not exceed 18%. However, when the membrane element is used for water treatment in the second stage reverse osmosis system, the actual recovery rate is not limited by this and is allowed to exceed 18%.

3. System Recovery

The system recovery rate refers to the total recovery rate of the reverse osmosis device during actual use. The system recovery rate is influenced by various factors such as the quality of the feedwater, the number and arrangement of membrane elements, etc. Small reverse osmosis devices generally have low system recovery rates due to the small number of membrane elements and short feedwater flow, while industrial large-scale reverse osmosis devices generally have a system recovery rate of over 75%, sometimes even up to 90%, due to the large number of membrane elements and long feedwater flow.

In some cases, a high system recovery rate is also required for small reverse osmosis devices to avoid wasting water resources. In this case, different measures need to be taken when designing reverse osmosis devices. The most common method is to use concentrated water partial circulation, that is, only a part of the concentrated water from the reverse osmosis device is discharged, and the remaining part is circulated into the inlet of the feed pump. At this time, it can ensure that the surface of the membrane element maintains a certain lateral flow rate and achieve the system recovery rate required by the user. However, it is not advisable to directly adjust the inlet and outlet valves of the feed/concentrated water to improve the system recovery rate. If this operation is carried out, it will accelerate the pollution rate of the membrane element, leading to serious consequences.

The higher the system recovery rate, the less water is consumed, but if the recovery rate is too high, the following problems may occur:

① The desalination rate of product water decreases.

② Possible precipitation of slightly soluble salts.

③ The osmotic pressure of concentrated water is too high, resulting in a decrease in the water production of the components.

Generally, the recovery rate of desalination systems for brackish water is controlled at 75%, which means that the concentrated water is four times more concentrated. When the salt content of the raw water is low, sometimes 80% can also be used. If the content of a certain slightly soluble salt in the raw water is high, sometimes a lower system recovery rate is also used to prevent scaling.