Final answer:
Dialysis employs a semi-permeable membrane to treat water for dialysis, allowing small solute molecules to pass while restricting larger molecules like proteins, thus cleansing a patient's blood from waste compounds such as urea.
Step-by-step explanation:
The method used to treat water for dialysis is a process that employs a semi-permeable membrane with pores that are designed to allow small solute molecules and solvent molecules to pass through, but not large solute molecules like proteins and starch. This operation takes advantage of the differences in particle size to separate substances when treating dialysis water, ensuring that only desired small molecules and ions can move through the membrane into the dialysate. Within the dialyzer, the blood is cleansed as waste molecules and excess water are forced through the membrane into the dialysate by osmotic pressure, while larger substances such as red and white blood cells are retained within the blood.