Influence of RO Membrane Recovery Rate on System Operation

2022-01-17


The common practice to improve the recovery rate of Reverse Osmosis Membrane is to increase the number of membranes in series, or to return concentrated water.

Increase the number of membrane segments to improve the recovery rate.

For example, the concentrated water discharged after the first stage membrane filtration directly enters the second stage membrane for filtration, or three stages and four stages can be connected in series to obtain a higher recovery rate. After the membranes are connected in series, the water flow rate of the first section is larger than that of the latter section, so generally the number of membranes in the first section is larger than that in the latter section.

The problem is that the more the number of RO membranes in series, the higher the concentration ratio of concentrated water. For a system with a recovery rate of 50%, the concentration ratio is 2 times; when the recovery rate is 75%, the concentration ratio is 4 times; when the recovery rate is 80%, the concentration ratio is 5 times. times; when the recovery rate reaches 90%, it is 10 times relative to the concentration. Due to the existence of concentration polarization in the membrane system, the salt content of the feed liquid on the membrane surface will become higher. Therefore, the contamination of the membrane surface will occur faster than expected due to the concentration of raw water. Generally, the recovery rate is set at about 50-80% in the desalination treatment of brackish water. Factors such as the operating conditions of the system and the characteristic state of the raw water will affect the determination of the recovery rate. Once the recovery rate is too high, it will face the risk of scaling formation and technical pollution.

Concentrated water reflux is to return a part of the concentrated water generated by the reverse osmosis system to the front of the high-pressure pump, mix with the influent water, and then enter the membrane module again for reverse osmosis treatment. This is also an effective means to improve the recovery rate of the reverse osmosis system. It is especially suitable when the water production volume of the system is not large and the water flow cannot flow through the 12m-long membrane module.
However, due to the backflow of concentrated water, the concentration of pollutants at the inlet will increase, and the risk of scaling in the reverse osmosis system will further increase. Therefore, the operation control and management of the reverse osmosis system must be strengthened. If the influent water quality of the reverse osmosis system used by the production enterprise is stable and better than the design value, and the processing capacity of the system still has spare capacity, this method can also be considered to transform the system.
Therefore, it is particularly important to set the recovery rate appropriately according to the actual situation. Based on the water quality analysis data of the raw water, combined with its four-season variation range, considering the design methods of the reverse osmosis system related to the pretreatment, the recovery rate of the produced water, and the operating temperature, the operating conditions are set.