Diffusion dialysis recovers acid using a concentration difference of solution on both sides of the ion exchange membrane.

In one example of diffusion dialysis for recovering acid, only anion exchange membrane is put between waste acid compartment and acid recovery compartment. With supplying waste acid containing metal ion through the bottom of waste acid compartment and water through the top of acid recovery compartment, free acid from the waste acid permeates through the anion exchange membrane into the water side by diffusion, driven by the concentration difference between each solution while metal ion does not permeate due to the rejection from the anion exchange membrane (Figure 10).