Standard recovery rate, actual recovery rate and system recovery rate

(1) Standard recovery rate of membrane element: the recovery rate adopted by the manufacturer under standard conditions. (the recovery rate of brackish water is 15%, and that of seawater is 10%)

Actual recovery rate of membrane element: the recovery rate of the manufacturer in actual use. (generally, the actual recovery rate of each 1m long membrane element shall not exceed 18%, except for water treatment of the secondary reverse osmosis system)

Membrane element system recovery rate: the above recovery rates are for the recovery rate of a single membrane element, and the second system recovery rate is for the total recovery rate of the whole reverse osmosis device in actual use. (due to many influencing factors, the actual system recovery rate is generally 75% - 90%)

(2) In order to avoid waste of water resources, small reverse osmosis units also require high system recovery rate. Common methods: partial circulation of concentrated water (part of the concentrated water of the reverse osmosis device is discharged, and the rest is circulated into the feed pump to maintain the transverse flow rate on the surface of the membrane element and take into account the required system recovery rate) tips: do not directly adjust the inlet and outlet valves of feed water / concentrated water.

(3) The higher the recovery rate of the system, the more water is saved, but the impact of too high recovery rate: ① the desalination rate of product water decreases. ② Precipitation of slightly soluble salts may occur. ③ The osmotic pressure of concentrated water is too high and the water production of components is reduced.

(4) Generally, the recovery rate of brackish water desalination system is mostly controlled at 75% - 80%.

2 impact of recovery rate on the system

The common way to improve the recovery of membrane components is to increase the number of series sections of membrane or concentrated water reflux.

Increase the number of membrane segments and improve the recovery

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

That's the problem. The more the RO membrane series sections are, the higher the concentration multiple of concentrated water. For the system with a recovery rate of 50%, the concentration multiple is twice; The recovery is 75%, and the concentration is 4 times; When the recovery is 80%, it is concentrated 5 times; When the recovery reaches 90%, it is 10 times higher than that of concentration. Due to the concentration polarization phenomenon in the membrane system, the salt content of the feed liquid on the membrane surface will become higher. Therefore, due to the concentration of raw water, the membrane surface pollution will occur faster than expected. Generally, the recovery rate is set at about 50-80% in brackish water desalination treatment. The operating conditions of the system and the characteristic state of raw water will affect the determination of the recovery rate. Once the recovery rate is too high, it will face the risk of scale formation and technical pollution.

Concentrated water reflux to improve recovery

Concentrated water reflux is to return part of the concentrated water produced by the reverse osmosis system to the front of the high-pressure pump, mix with the inlet water, and then enter the membrane module for reverse osmosis treatment. This is also an effective means to improve the recovery rate of reverse osmosis system. Especially when the water yield of the system is small and the water cannot flow through the 12m long membrane module, it is very suitable.

However, due to the reflux of concentrated water, the pollutant concentration 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 quality of the reverse osmosis system used by the production enterprise is stable and better than the design value, and the treatment capacity of the system still has spare power, 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 raw water, combined with the variation range of four seasons, the design mode of reverse osmosis system related to pretreatment and produced water recovery rate and operating temperature shall be considered, and the operating conditions shall be set.