Patients with chronic kidney failure usually have problems with excess fluid, because they have problems passing urine.
To remove excess water from the body during peritoneal dialysis, sugar is added to dialysis fluid. As sugar molecules cannot easily pass through the peritoneum membrane, body water passes through the peritoneum into the dialysis fluid, to balance out the difference in fluid concentration. This process is known as osmosis. By continuously introducing fresh dialysis fluid, excess water that the kidneys cannot remove (and would otherwise collect in the body) can be removed from the blood.
Under hemodialysis, if the pressure outside the hollow fiber is lower than inside, body water is removed from the blood that flows through the hollow fiber. This process is called ultrafiltration and removes excess water from the body.