Is the recovery rate of the reverse osmosis membrane system the higher the better?

Reverse osmosis technology is the highest of all water production technologies currently mastered by humans, and its cleanliness is almost 100%. The reverse osmosis element is used to improve the purity of water and remove impurities and salts contained in the water.
When using reverse osmosis membranes to treat raw water, the recovery rate of the membrane system, as an important performance index, usually receives great attention. A high recovery rate means a strong water output capacity. So is the recovery rate of the reverse osmosis membrane system the higher the better?

 1. What is the recovery rate of the reverse osmosis membrane system?

The recovery rate refers to the percentage of the influent water in the membrane system that is converted into product water or permeates, which is determined by the pretreatment influent water quality and water requirements. The recovery rate of the system is affected by many factors such as the quality of the water supply, the number, and arrangement of membrane elements. The small-scale reverse osmosis device has a low number of membrane elements and short water supply process, so the system recovery rate is generally low. Due to a large number of membrane elements and long water supply process in the device, the actual system recovery rate is generally above 75%, and sometimes even reaches 90%.

2. Recovery rate algorithm of the reverse osmosis membrane system

The recovery rate of a conventional single-branched reverse osmosis membrane element varies according to the quality of the influent water, and the recovery rate mostly ranges between 12%-50%. The recovery rate of conventional reverse osmosis membrane systems also varies from 30% to 90% depending on the scale of the system and the quality of the raw water.

The calculation formula of the recovery rate is as follows:

R%=Fp/Ff×100%
In the formula: Ff ---- reverse osmosis membrane (system) inlet water flow rate (m³/h)
Fp ---- Water flow rate of reverse osmosis membrane (system) (m³/h)
R ---- Recovery rate of reverse osmosis membrane (system)

3. Is the recovery rate of the reverse osmosis membrane system as high as possible?

In some cases, a high system recovery rate is also required for small reverse osmosis devices to avoid wasting water resources. At this time, when designing reverse osmosis devices, different countermeasures need to be adopted. The most common method is to use concentrated The water is partially circulated, that is, only part of the concentrated water of the reverse osmosis device is discharged, and the remaining part is circulated into the inlet of the feedwater pump. At this time, it can ensure that the surface of the membrane element maintains a certain horizontal flow rate, but also can achieve the system recovery rate required by the user. It is not possible to directly adjust the feed water/concentrated water inlet and outlet valves to increase the recovery rate of the system. If this is done, the pollution rate of the membrane elements will increase, leading to serious consequences.

Although the higher the recovery rate of the system, the less concentrated water discharge, but an excessively high recovery rate will cause the following problems
①The desalination rate of produced water drops, resulting in poor water quality
② Increasing the recovery rate of the system will increase the concentration difference on both sides of the reverse osmosis membrane, resulting in an increase in the salt content of the product water
③The increase in the recovery rate will also lead to a decrease in the net driving pressure of the system, which will eventually lead to a decrease in the water production of the system. To maintain the same water production, the operating pressure of the system must be increased, leading to an increase in system operating costs.

Generally, the recovery rate of brackish water desalination system is mostly controlled at 75%, that is, the concentrated water is concentrated by 4 times. When the salt content of the raw water is low, sometimes 80% can be used. A lower system recovery rate is also used to prevent fouling.

Therefore, the recovery rate should be set according to the conditions of the reverse osmosis system, not the higher the better.

HID™ Membrane Co., Ltd. specializes in the R&D, design, production, and sales of water treatment products include RO Membrane, Reverse Osmosis (RO) System, RO Membrane Housing, RO Pre-Filters, RO Flat Sheet, etc. Its products are widely used in the water supply needs of industries such as electronics, medicine, chemicals, drinking water production, beverages, food brewing, boiler feed water, breeding, and planting.