The Recovery rate of Reverse osmosis water plant

Reverse osmosis water plant uses reverse osmosis membranes to improve the purity of water by removing impurities and salts contained in the water. When select membrane to design plant, we mainly concerned about 2 factors, recovery rate and desalination rate, which determines the water utilization rate and product water quality.

1. Recovery rate

The standard recovery rate of the membrane is the recovery rate offered by the membrane supplier under the standard test conditions. Generally, the standard recovery rate of brackish water membrane is 15%, and seawater membrane is 10%.

The actual recovery rate of the membrane is the recovery rate when the membrane element is actually used. In order to reduce the pollution rate of the membrane element and ensure the service life of the membrane element, the membrane supplier has made clear regulations on the actual recovery rate of a single membrane element, requiring that the actual recovery rate of a 1-meter-long membrane element should not exceed 18%, but once the membrane is used in the double stage reverse osmosis water plant, the actual recovery rate is allowed to exceed 18%.

The system recovery rate refers to the total recovery rate of the reverse osmosis water plant in actual use. The recovery rate of the system is affected by many factors such as feed water quality, the quantity and arrangement of membrane elements.

 The small capacity reverse osmosis water plant relatively has a low system recovery rate due to the small number of membrane and short water supply process, while the large-scale industrial reverse osmosis water plant actual system recovery rate is generally above 75%, and sometimes even reaches 90%, as it has the large number of membrane and long water supply process

In some cases, a high system recovery rate is also required for small reverse osmosis devices to avoid wasting water resources. At this time, the reverse osmosis water plant can be designed to circulate the waste water. At this time, it can ensure that the surface of the membrane element maintains a certain horizontal flow rate, and can also achieve the system recovery rate required by the user. However, directly adjusting the concentrated water valve to increase the recovery rate of the system is prohibited. It will lead to the contamination of membrane and serious consequences.

Higher recovery rate, less water be consumed, also following issues may occur:

a. The desalination rate of system

b. The precipitation of slightly soluble salt may occur.

c. The product water capacity decreases.

How to determine the system recovery rate

a. According to the length of the membrane elements in series.

b. According to whether there is concentrated water circulation and the size of the circulation flow.

2. Desalination rate

The standard desalination rate of membrane is the desalination rate measured by the membrane supplier under standard conditions. For example, the minimum desalination rate of CPA3 under standard conditions is 99.6% (the average desalination rate is 99.7%).

The actual desalination rate of the membrane is the desalination rate displayed by the membrane element in actual use. The actual desalination rate is sometimes higher than the standard desalination rate, but in most cases it is lower than the standard desalination rate. This is due to the standard test conditions and actual The conditions of use are completely different. Under standard test conditions, the standard test solution is sodium chloride solution, and the standard salt rejection rate of the membrane element is the removal rate of sodium chloride. In actual use conditions, due to the different ion components in the water, the temperature, the average water flux selection value, and the system recovery rate are different from the standard test conditions, and these factors will affect the salt rejection rate of the membrane element.

The desalination rate of the system is shown by the complete set of reverse osmosis water plant. Also due to the different use conditions and standard conditions, the system desalination rate is different from the standard desalination rate. At the same time, because the reverse osmosis device generally has multiple membrane elements in series, and the  actual use conditions of each membrane element are different, so the system desalination rate is also different from the actual desalination rate of the membrane element.

The easiest way to predict the rejection rate of the system is to perform actual calculations through the calculation projection software of the membrane supplier.

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