Dialysis is a life-sustaining medical treatment that takes over the function of failing kidneys to remove waste products and excess fluid from the blood. This process, known as hemodialysis, requires the continuous creation of a cleansing fluid called dialysate, which draws toxins out of the patient’s blood across a semipermeable membrane. Dialysate is composed of a precise mixture of electrolytes and highly purified water mixed with a concentrate solution. The highly treated, final water product used to create this dialysate is known as “product water.”
Why Water Purity is Essential for Dialysis
The water used in dialysis must be significantly purer than standard drinking water because the patient’s blood is exposed to massive volumes of the fluid. A typical four-hour hemodialysis session exposes the patient to between 120 and 200 liters of water. This contrasts drastically with the 10 to 15 liters a healthy person consumes in a week, which passes through the digestive system and liver.
During treatment, the dialysate water is separated from the patient’s blood only by a thin, semipermeable membrane, allowing small molecular contaminants to diffuse directly into the bloodstream. Since the patient has little or no kidney function, their body cannot effectively excrete these accumulated substances, leading to serious health issues. For example, common tap water additives like chlorine and chloramine, if not removed, can cause hemolytic anemia by damaging red blood cells.
Other contaminants, such as aluminum, can accumulate in the patient’s bones and brain, potentially leading to bone disease and a severe neurological condition known as dialysis encephalopathy. Bacteria and their toxic fragments, called endotoxins, can also pass across the membrane, causing acute symptoms like pyrogenic reactions (fever and chills) during treatment. Even trace amounts of heavy metals (copper or zinc) or elements like fluoride can cause chronic toxicity. This necessitates the extensive purification process to protect the patient.
The Steps to Produce Product Water
The production of product water is a multi-stage process that begins with pre-treatment of the incoming tap water supply. This initial phase involves sediment filters to remove large particles and temperature blending to maintain a constant temperature, optimizing later stages. Water softeners exchange hardness ions (calcium and magnesium) with sodium ions, which prevents scaling and protects the delicate reverse osmosis membranes downstream.
Following pre-treatment, the water moves through activated carbon filters, a stage fundamental for removing chemical disinfectants added by municipal water treatment plants. These carbon tanks effectively adsorb chlorine, chloramine, and various organic compounds that could otherwise damage purification components or harm the patient. Often, two carbon tanks are installed in series to ensure complete removal.
The core of the system is the Reverse Osmosis (RO) unit, which uses hydrostatic pressure to force water through a semi-permeable membrane. This process is highly effective, removing over 90% of the dissolved ions, bacteria, and endotoxins present in the water. In some advanced systems, a two-stage RO process is used, where the water passes through a second membrane to achieve even greater purity and provide an additional safety margin.
The water may undergo post-RO purification steps before being distributed to the dialysis machines. Deionization (DI) tanks may be used as a final polishing step or an emergency backup to remove any remaining ionic contaminants. Because DI systems can sometimes promote bacterial growth, the final product water is often treated with ultraviolet (UV) light, which destroys microorganisms by damaging their DNA.
Quality Standards and Monitoring
The quality of product water is governed by rigorous regulatory standards, most notably those established by the Association for the Advancement of Medical Instrumentation (AAMI). These standards are dramatically stricter than those for potable water, reflecting the direct and high-volume exposure route during dialysis. The AAMI standard for routine dialysis water sets the maximum allowable bacterial count at 100 Colony-Forming Units per milliliter (CFU/mL) and the maximum endotoxin level at 0.25 Endotoxin Units per milliliter (EU/mL).
Monitoring water quality is an ongoing requirement, starting with daily checks for conductivity, which measures the water’s ability to conduct electricity and indicates the level of ionic contaminants. Regular testing for chemical contaminants, such as heavy metals and aluminum, is performed weekly or monthly to ensure compliance. Microbiological testing, including both bacterial counts and endotoxin levels, is conducted at least monthly.
These microbial samples are sent to a certified laboratory and incubated for an extended period, often seven days, at a temperature that mimics the water environment to accurately detect viable, slow-growing waterborne bacteria. If the product water fails to meet any specified chemical or microbiological standards, the system must be immediately taken offline. Mandatory corrective action must be implemented before the water can be used again for patient treatment.