Electrodialysis (ED), diffusion dialysis (DD), and bipolar membrane electrodialysis (BMED) all belong to the electromembrane separation technology family, but they differ greatly in treatment targets, target products, driving force, and pretreatment requirements. This manual provides a selection comparison from these five dimensions to help quickly identify the technology route during process comparison and selection.
Basic Principles of the Three Technologies
- ED electrodialysis: With a direct current electric field as the driving force, it relies on the selective permeability of anion- and cation-exchange membranes to make ions migrate directionally into the concentrate compartment, achieving desalination or concentration.
- DD diffusion dialysis: With concentration difference as the only driving force, it relies on the acid/alkali permeability and salt retention capacity of the membrane to separate free acid or alkali from salts and impurities.
- BMED bipolar membrane electrodialysis: Under an electric field, water dissociation in the intermediate layer of the bipolar membrane generates hydrogen ions and hydroxide ions, directly converting salts into the corresponding acids and alkalis.
Selection Comparison Across Five Dimensions
| Comparison Dimension | ED Electrodialysis | DD Diffusion Dialysis | BMED Bipolar Membrane Electrodialysis |
|---|---|---|---|
| Driving Force | Direct current electric field | Concentration difference | DC electric field + bipolar membrane water dissociation |
| Treatment target | Saline solutions; feed liquids requiring desalination or concentration | Mixture of free acid, free alkali, and salt | Salt solution (to be converted into acid and alkali) |
| Target products | Diluate (desalinated) and concentrate (concentrated) | Regenerated acid/regenerated alkali and residual liquid | Acid solution and alkali solution |
| Energy consumption characteristics | Dominated by electricity consumption and increases with salt content | No power supply required and very low energy consumption | Primarily electricity consumption, with additional energy needed to overcome water dissociation voltage |
| Typical applications | Desalination, concentration, separation of organics and salts | Waste acid and alkali recovery | Organic acid preparation, acid and alkali regeneration, lithium salt conversion |
How to derive the technical route from the target product
- The goal is to remove or concentrate salt—prioritize ED and focus on the balance between salinity and energy consumption.
- The goal is to recover already present free acid or alkali—prioritize DD and focus on salt rejection and acid permeation.
- The goal is to convert salt into acid and alkali—consider BMED and focus on product concentration requirements and current efficiency.
- The goal is to first recover free acid and then convert the salt in the residual liquid—DD and BMED can be combined into a complete process chain.
Pretreatment and validation boundaries
The three technology types place different emphasis on feedwater requirements, and their boundaries must be defined individually during validation:
- Suspended solids and particulate matter: both require upstream filtration control to avoid clogging of membrane stack channels and membrane surface fouling.
- Hardness and scaling tendency: the concentrate side of ED is prone to supersaturation, so hardness removal and scaling inhibition measures need to be evaluated.
- Organics and COD: they readily adsorb onto the membrane surface and cause flux decline; where necessary, an organics removal or separation stage should be provided.
- Temperature: affects membrane flux and membrane life and must be controlled within the range permitted by the membrane material.
- Oxidizing substances: oxidants such as residual chlorine can damage membrane materials and should be removed upstream of the membrane to a controlled level.
- Operating flux and cleaning cycle: measured data should be obtained through bench-scale testing and pilot-scale testing as the direct basis for engineering scale-up.
It should be emphasized that the feasibility of the electromembrane route depends heavily on the specific water quality. The same technology type may perform well at Plant A but fail at Plant B due to a certain impurity—therefore process selection cannot rely only on experience from similar projects and must return to the water quality itself for validation.