ISO 26722: Water Treatment Equipment for Haemodialysis Applications

1. Overview of ISO 26722

ISO 26722:2014 specifies requirements for water treatment equipment used in haemodialysis applications and related therapies. Developed by ISO/TC 150 (Implants for surgery) Subcommittee SC 2, this standard addresses a critical patient safety concern: dialysis patients are exposed to 300 to 600 liters of water per week, making water purity a direct life-or-death matter. Unlike healthy individuals whose digestive systems provide a barrier against contaminants, dialysis patients have direct blood contact with dialysis fluid across the semi-permeable membrane of the dialyzer.

The standard covers all devices, piping, and fittings from the point where potable water enters the treatment system to the point of use of dialysis water. This includes water purification devices such as reverse osmosis systems, deionization units, carbon adsorption beds, sediment and cartridge filters, water softeners, online water quality monitors (conductivity, resistivity, temperature), ultraviolet irradiators, and distribution piping systems. It applies equally to multi-patient dialysis facilities in hospitals and clinics and single-patient settings such as home dialysis or acute hospital care.

2. Water Quality Requirements

2.1 Chemical Contaminant Limits

The dialysis water must meet the contaminant limits defined in ISO 13959. Key maximum allowable concentrations include:

2.2 Microbiological Limits

The standard requires that dialysis water meet specified action levels for total viable microbial count and endotoxin concentration. Microbial testing must use validated methods such as membrane filtration, spread plates, or pour plates with TGEA (tryptone glucose extract agar) or R2A culture media. Incubation conditions are specifically defined as 17 to 23 degrees Celsius for 168 hours (7 days). The calibrated loop technique is explicitly prohibited as it lacks the sensitivity required for dialysis water monitoring.

3. Water Treatment Equipment Requirements

3.1 System Design Principles

ISO 26722 provides detailed technical specifications for each component in the water treatment cascade. Understanding these requirements is essential for biomedical engineers and facility designers.

Carbon Beds (Clause 4.2.8): When feed water chloramine concentration exceeds 1 mg/l, two carbon beds must be installed in series. Each bed must provide at least 5 minutes Empty Bed Contact Time (EBCT) at the maximum product water flow rate, achieving a total minimum EBCT of 10 minutes. Granular activated carbon with an iodine number greater than 900 is considered optimal for chlorine and chloramine removal. The use of regenerated carbon is strictly prohibited. Sample ports must be installed between the two beds to detect early breakthrough of chlorine.

Reverse Osmosis (Clause 4.2.10): RO systems must be validated at installation to demonstrate capability of meeting water quality requirements with the user typical feed water. Online conductivity monitors with temperature compensation to 25 degrees Celsius are mandatory. The system must include a mechanism to divert product water to drain or shut down when conductivity exceeds preset limits, with corresponding visual and audible alarms meeting IEC 60601-1-8 requirements.

Deionization (Clause 4.2.11): DI systems must produce water with specific resistivity of 1 M-cm or greater (conductivity of 1 microS/cm or less). Feed water for deionization must be pretreated with activated carbon to prevent nitrosamine formation. If a deionization system is the last treatment process, it must be followed by an endotoxin-retentive filter.

Ultraviolet Irradiation (Clause 4.2.13.3): UV devices must emit light at 254 nm wavelength and provide a minimum radiant energy dose of 30 mW-sec/cm2. Calibrated ultraviolet intensity meters are required to prevent sublethal dosing that could lead to development of resistant bacterial strains.

3.2 Materials Compatibility

All surfaces that contact dialysis water must be fabricated from non-reactive materials, specifically plastics or appropriate stainless steel. Copper, brass, galvanized material, and aluminium are explicitly prohibited at any point beyond the water treatment device used to remove contaminating metal ions. This requirement directly addresses the historically documented cases of catastrophic hemolysis caused by copper leaching from improperly specified plumbing components.

4. Storage, Distribution, and Disinfection

The standard places strong emphasis on biofilm prevention, which represents the most persistent challenge in dialysis water systems.

Continuous Recirculation: Distribution loops must maintain continuous flow with dedicated return lines to prevent water stagnation. Areas of stagnant flow (dead zones) are explicitly forbidden in the piping design. Direct feed systems must include backflow prevention.

Storage Tanks: Must have conical or bowl-shaped bases that drain completely from the lowest point. Bladder tanks and pressurized surge tanks are prohibited. Vents must be fitted with hydrophobic 0.45 micrometer air filters. Sight tubes should be avoided due to algae and fungi growth risk.

Disinfection Methods: Three validated approaches are recognized: hot water disinfection at temperatures exceeding 80 degrees Celsius, ozone disinfection at 0.2 to 0.5 mg/l for 10 minutes contact time, and chemical disinfectants. All methods must include fail-safe mechanisms preventing patient exposure during disinfection cycles.

Alarm Performance Requirements: Audible alarms must achieve at least 65 dBA sound pressure level at 3 meters distance and cannot be silenced for more than 180 seconds. Alarms must be positioned to ensure prompt personnel response in patient care areas, aligning with IEC 60601-1-8 classification for medical electrical equipment alarm systems.

5. Engineering Design Insights

ISO 26722 provides critical guidance for biomedical engineers and clinical technology professionals responsible for dialysis water system design, specification, and operation.

Redundancy Architecture: The double-carbon-bed requirement for chloramine removal exemplifies defense-in-depth design thinking. A single bed failure could expose patients to life-threatening hemolytic crisis. Series redundancy provides a critical safety buffer during the interval between routine monitoring tests, which may be several days apart in some facilities.

Sampling Port Placement Strategy: Sample ports must be strategically located between series carbon beds (to detect early breakthrough of chlorine or chloramine) and at the most distal points of the water distribution loops. This placement enables proactive maintenance scheduling based on trend analysis rather than reactive failure response after water quality has already degraded.

Bypass Valve Hazard Awareness: The standard explicitly warns that bypass valves allowing individual treatment devices to be isolated must be designed to prevent inadvertent operation during normal clinical use. Multiple documented patient injury incidents have been traced to unauthorized or accidental bypass of critical water treatment steps.

Disinfection Validation Protocol: Following chemical disinfection procedures, mandatory residual disinfectant testing must be performed before patient treatments can resume. While the standard does not specify numerical residual limits, equipment manufacturers must provide validated test methods and acceptable concentration ranges for their specific disinfection protocols.

6. Frequently Asked Questions