3.1 Water Treatment Systems and Standards

Water Quality Standards in Hemodialysis

Hemodialysis patients are exposed to 270 to 500 liters of water weekly, separated from their blood only by a thin dialyzer membrane. Because the gastrointestinal barrier is bypassed, chemical or biological impurities in the water can pass directly into the bloodstream, causing severe illness, toxicity, or death.

To ensure patient safety, the Association for the Advancement of Medical Instrumentation (AAMI), the American National Standards Institute (ANSI), and the Centers for Medicare & Medicaid Services (CMS) enforce strict water quality limits. Dialysis staff must understand the purpose, function, and monitoring requirements of each component in the water treatment system to protect patients from chemical and microbiological hazards.

System Components and Pretreatment

The water treatment process begins with raw feedwater entering the facility. The system consists of a series of purification devices arranged in a specific sequence:

1. Temperature Blending Valve: A blending valve mixes hot and cold feedwater to maintain a stable temperature of 77°F to 80°F. Reverse osmosis (RO) membranes operate optimally at 77°F (25°C). Water that is too cold decreases membrane permeability and water production, while water exceeding 95°F (35°C) damages the membrane and causes hemolysis (the destruction of red blood cells) in patients.
2. Backflow Preventer: Prevents water from the dialysis system from flowing backward into the municipal water supply.
3. Multimedia Filter: Removes suspended particles down to 10 microns using layers of sand, anthracite, and garnet, protecting downstream components. It requires periodic backwashing to flush accumulated debris.
4. Water Softener: Prevents scale accumulation on the RO membrane by removing calcium and magnesium via ion exchange. These hardness ions bind to negative sites on zeolite resin beads inside the softener tank and are exchanged for sodium ions. The system is regenerated daily using salt brine. Staff must check brine tank salt levels daily and verify that hardness remains below 1 grain per gallon (gpg) or 17.2 parts per million (ppm) at the end of each treatment day.
5. Carbon Tanks: Removes chlorine and chloramine (a combination of chlorine and ammonia) from feedwater via adsorption. Chloramine in the blood causes hemolysis and methemoglobinemia (where red cells cannot release oxygen). The system requires a primary (worker) and secondary (polisher) tank connected in series. Water must achieve a minimum empty bed contact time (EBCT) of 10 minutes total (5 minutes per tank) to ensure complete removal.

Testing for Chlorine and Chloramines

Chlorine testing is a critical, non-delegable check performed at the primary tank outlet before the first patient shift and at least every 4 hours. The maximum allowable limit for total chlorine is less than 0.1 mg/L (or parts per million, ppm).

A level of 0.1 mg/L or higher indicates a breakthrough. Staff must immediately test the secondary (polisher) tank outlet. If it is below 0.1 mg/L, treatments can temporarily continue with testing increased to every 2 hours. If the secondary outlet also measures 0.1 mg/L or higher, dialysis must stop immediately to prevent hemolysis. The medical director must be notified.

Reverse Osmosis and Deionization

The reverse osmosis (RO) unit is the core purification device, using high pressure to force feedwater through a semipermeable membrane against the osmotic gradient. This separates feedwater into purified permeate and concentrated reject. A functional RO unit achieves an ionic rejection rate of 95% to 99%. This rate is monitored continuously via conductivity meters and calculated as:

$\text{Rejection Rate (\%)} = \frac{\text{Feedwater Conductivity} - \text{Product Water Conductivity}}{\text{Feedwater Conductivity}} \times 100$

A drop in the rejection rate below 90% indicates membrane degradation, requiring immediate corrective action.

Deionization (DI) tanks serve as backup or polishing systems, using resins to exchange impurities for hydrogen and hydroxide ions. DI does not remove bacteria or endotoxins and can dump toxic chemicals if exhausted. Thus, an inline resistivity monitor is required, and water must divert to drain if resistivity falls below 1 megohm-cm.

Distribution, Microbiological Standards, and Endotoxin Limits

Purified water circulates to dialysis stations via a continuous distribution loop constructed of inert materials like polyvinylidene fluoride (PVDF) or stainless steel. Copper, brass, or lead must never be used due to toxicity. The loop requires smooth internal surfaces and continuous flow (minimum velocity of 1.5 to 3 feet per second) to prevent stagnant "dead legs" where bacteria build a protective biofilm. Biofilms shed bacteria and release endotoxins (bacterial lipopolysaccharides) into the system.

Endotoxins passing into the blood trigger pyrogenic reactions, causing fever, chills, hypotension, or shock. To prevent this, microbiological monitoring is performed monthly. Samples must be collected prior to system disinfection. The AAMI/ANSI chemical and microbiological limits are summarized in the table below:

When an action level is reached, the facility must immediately perform disinfection of the system and repeat testing to prevent the bacteria and endotoxins from exceeding the maximum allowable limits.