Dialysis is a life-sustaining treatment for individuals experiencing kidney failure, replacing the natural function of the kidneys by filtering the blood. Hemodialysis requires the preparation of a dialysate solution, a fluid that circulates alongside the patient’s blood within the dialyzer. Since the dialysate is composed almost entirely of water, the quality of the water used is of the utmost importance for treatment effectiveness and patient safety.
Why Water Purity Is Critical for Dialysis Patients
Patients undergoing standard hemodialysis are exposed to a large volume of water during each treatment session. Over a typical four-hour session, a patient’s blood contacts approximately 120 liters of dialysate. This prolonged and repeated exposure makes the patient extremely vulnerable to any contaminants present in the source water.
In a healthy person, contaminants ingested with drinking water are processed and filtered by the gastrointestinal tract and the liver. In dialysis, however, the patient’s blood is separated from the dialysate only by a thin, semi-permeable membrane. Small molecular weight contaminants can diffuse directly across this barrier and enter the bloodstream without passing through the body’s natural defense mechanisms.
Standard municipal drinking water, while safe for the general population, does not meet the purity requirements for dialysis use. The standards set by the Environmental Protection Agency (EPA) for potable water are lower than those mandated for dialysis water by organizations like the Association for the Advancement of Medical Instrumentation (AAMI). Consequently, kidney replacement therapy requires an intensive, multi-stage water purification system to ensure patient well-being.
The Primary Function of Activated Carbon Filtration
The central function of carbon tanks in a dialysis water treatment system is the removal of chemical disinfectants, primarily free chlorine and chloramines. These compounds are intentionally added to municipal water supplies to prevent the growth of harmful bacteria. While effective for public health, these disinfectants are highly toxic when introduced directly into a patient’s bloodstream.
Activated carbon, typically granular, removes these oxidizing agents through catalytic reduction. The carbon surface facilitates the conversion of the disinfectants into harmless chloride ions. This chemical transformation is immediate and occurs rapidly as the water passes through the carbon bed.
Chloramines, a combination of chlorine and ammonia, are challenging because they are not effectively removed by the reverse osmosis unit later in the process. The carbon filter is the designated mechanism for neutralizing these compounds before the water proceeds to sophisticated purification stages. The large surface area of the porous carbon material provides ample sites for this chemical reaction.
To ensure continuous removal and prevent patient exposure, dialysis facilities must use at least two carbon tanks in sequence: a primary and a secondary tank. The first tank performs the majority of the removal, while the second acts as a safeguard. This dual-tank configuration adheres to AAMI standards, ensuring the water meets the mandated maximum level of 0.1 mg/L for total chlorine.
Breakthrough Testing
Technicians routinely test the water exiting the primary tank for residual chlorine and chloramines. The presence of these chemicals indicates “breakthrough,” signaling that the primary tank is exhausted. This prompts an immediate change of the primary carbon bed, as the secondary tank is now performing the removal.
Consequences of Failing to Remove Targeted Contaminants
If the carbon filtration system fails and residual chlorine or chloramines enter the dialysate, the patient faces the severe health risk of hemolysis. Hemolysis is the rapid destruction of red blood cells, which can lead to acute anemia and life-threatening complications. This occurs because chlorine and chloramines are powerful oxidizing agents that directly attack blood components.
These chemicals oxidize the patient’s hemoglobin, the protein responsible for oxygen transport, leading to the formation of methemoglobin. Methemoglobin is incapable of binding and transporting oxygen effectively, causing tissue hypoxia. The oxidizing agents also compromise the red blood cell’s natural defense mechanisms, such as the hexose monophosphate shunt, which protects the cell from oxidative damage.
When the cell’s internal defenses are overwhelmed, the red blood cell membrane is damaged, resulting in premature rupture. A sudden, unexplained drop in a patient’s hemoglobin or hematocrit level during or immediately following a dialysis session often signals a chloramine exposure event. Facilities must perform daily testing of the water for total chlorine and chloramines to confirm the carbon beds are functioning correctly.