The arrangement methods of membranes can be categorized as follows:
Stage: Refers to the number of times the feed water permeates through the RO membrane element. A single-stage reverse osmosis can achieve a desalination rate of 98–99%, while multi-stage reverse osmosis can achieve over 99.5% desalination.
Section: Refers to the number of times the concentrate water flows through different pressure vessels.
Depending on requirements, multiple membrane modules can be arranged into one-stage, two-stage, or multi-stage configurations. Within each stage, membrane modules can be arranged in one-section, two-section, or multi-section configurations. A single stage refers to feed water undergoing pressurized reverse osmosis separation once, while a second stage refers to it undergoing pressurized reverse osmosis separation twice, and so on. Within the same stage, a group of identical membrane modules is considered a section.
The basic arrangements and combinations of reverse osmosis membranes can be divided into six main types:
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Single-Stage Single-Section Continuous Type
Water passes through the membrane, and both permeate and concentrate are continuously discharged from the system. This setup has a relatively low water recovery rate. -
Single-Stage Single-Section Cyclic Type
A portion of the concentrate is returned to the feed tank to mix with raw water and pass through the RO membranes again. The permeate quality is slightly reduced compared to the continuous type. -
Single-Stage Multi-Section Continuous Type
The concentrate from the previous section serves as the feed for the next section, and permeate is continuously discharged. This is suitable for large water treatment applications, achieving higher recovery rates and reducing concentrate volume but with a higher solute concentration in the final concentrate. -
Single-Stage Multi-Section Cyclic Type
The permeate from the next section is used as feed water for the previous section. This configuration achieves a higher concentrate level and is ideal for separation processes focusing on concentration. -
Multi-Section Conical Arrangement
Membrane modules are arranged in parallel within sections and in series between sections. This design helps maintain consistent flow conditions within each module and meets recovery rate requirements but requires high-pressure pumps to prevent efficiency loss. -
Multi-Stage Multi-Section Configuration
In cyclic configurations, the permeate from the first stage is used as the feed for the next stage and continues in this manner, with final-stage permeate discharged from the system. Concentrate flows from the later stage back to earlier stages for reprocessing. This arrangement improves recovery and water quality but increases pump energy consumption compared to continuous types.
When selecting and arranging membrane modules, it is essential to follow design guidelines and consider factors such as system purpose, raw water conditions, equipment space limitations, desired output capacity, and target water recovery rate to determine the number of elements per pressure vessel and the appropriate arrangement type.