ANSI/AAMI ST108: The Guide to Water Quality for Processing of Medical Devices

About the ANSI/AAMI ST108 Guide

Surgical site infections are a leading cause of hospital re-admissions, many times due to water quality. Most sterilizers, washers and disinfectors used in the cleaning of surgical instruments are considered medical devices and require treated water within standards set by AAMI and IFU’s of the various device manufacturers. This Guide provides information on:


Contaminants in Water and Technologies for Removal

Water is the universal solvent with the ability to dissolve almost everything it encounters.  The properties of water are constantly changing. Wherever it flows, it takes along chemicals, minerals, and nutrients whether it be from the air, the ground and even our bodies.

Although municipalities remove these contaminants to meet US Drinking Water Standards, water becomes re-contaminated as it travels to the final point of use. Additional treatment may be needed based on the specifications for use.

Water for Processing Medical Devices

Water that has been treated to be suitable for drinking is not necessarily suitable for Sterile Processing. This is especially true during the final rinse cycle where high quality / low endotoxin water is needed.

Treated water is for both instrument processing, and final flushing at the sink. The manufacturers of the processing equipment (sterilizers, washers, etc.) each have their own set of specifications and instructions for use (IFU) specifying the quality and quantity of water required for their specific piece of equipment. See Reference Table 1 for a list of equipment and the general water quality specifications.

Download AAMI ST108 Readiness Checklist

Table 1 – Water Quality Requirements for Sterile Processing Equipment

water quality requirements for ST108 sterile processing equipment

Properly Sizing Water Treatment Equipment

A water treatment company will use the manufacturers specifications to properly size the correct system based on quality and usage. To guarantee no down time, the water treatment system should be sized to supply all the sterile processing devices for one hour. This can be determined by multiplying the water usage for each cycle by the number of cycles per hour by the quantity of devices in the department. Sizing this way guarantees there will be enough water whether the department is running 10 or 24 hours a day and will minimize the size of the pure water storage tank.


sizing equipment for ST108 implementation

AAMI TIR34 has become a Standard, ANSI/AAMI ST108

What does this mean for you? TIR is a technical information report to be used as a guideline. After several years, this guideline has become a standard that will be audited and monitored by regulatory agencies.

This leads to many questions:

  • How will the new standards affect your sterile processing department?
  • Do you have the knowledge and team in place to handle the new standards?
  • Have you budgeted for upgrades or replacement of your current system?
  • Do you have updated procedures in place?
  • Are you prepared for monthly sampling and testing of your water treatment system?
  • Do you have a maintenance schedule and documentation?

Let’s take this one step at a time.


No doubt, there will be many helpful articles, webinars, and consultants offering information on the new standards.  However, it is strongly encouraged that all facilities purchase and study the new written standard from AAMI.


Name of Standard: AAMI ST108:2023 – Water for Processing of Medical Devices


Compare the new standards against your current system. It’s not just about the equipment you have, but also the maintenance and monitoring. Depending on your time and capabilities, it may be beneficial to bring in an expert who knows the new standard. A few suggestions of who to contact would be:

  • A water treatment vendor who specializes in SPD water
  • An industry consultant who works with compliance for hospitals


The new standard includes the requirement of having a Water Management Program. This team is responsible for ensuring all requirements are put in place with continued monitoring and maintenance. A successful team would include an executive who has the authority to allocate resources, facilities engineering personnel, infection prevention and control personnel, medical device processing personnel, clinical engineering personnel, surgical procedures personnel, and a water treatment specialist.


There are several areas in the new standard that suggests you pay special attention to:

Categories of medical devices and the risk levels associated with each:

  • Critical
  • Semi-critical
  • Non-critical

The difference between Utility Water, Critical Water and Steam

  • New Limits

The need for a Water Management Program

  • The role and responsibilities of the team
  • Determining who should be on the team

How water quality affects the SPD process

  • According to the stage in the process
  • According to the type of contaminant

This document does not provide details on the standard but can be used as guidance to help you understand where to begin.

Categories and Recommended Levels of Water Quality for Medical Device Processing

There will be three categories of water quality that may require treatment:

1. Utility Water

Utility Water is the hospitals water source coming straight from the tap. Since this water is used only for flushing, washing, and rinsing the acceptable contaminant levels are less stringent than for Critical Water, but still may require treatment.

2. Critical Water

Critical Water is used for the final rinse on the washer/disinfector and steam generation. For the health and safety of the patient, it’s important to remove microorganisms such as bacteria and endotoxin.

3. Steam

Steam is vaporized water that is produced by a boiler or heat exchanger. The condensate testing should meet the requirements as shown in table 2.

Table 2 – Water Quality for Medical Device Processing

This list is not comprehensive and shows only an overview of the main contaminants.

For the full list refer to AAMI ST108.

Water Quality for ANSI AAMI ST108

Contaminants and Treatment Technologies

Contaminant Control Technology ANSI AAMI ST108

Contaminant Control Technology ANSI AAMI ST108

Typical Layout of Water Treatment System for Critical Water

Typical ANSI AAMI ST108 Layout

Typical ANSI AAMI ST108 Layout for Critical Water

Typical Layout of Sterile Processing Medical Devices

Sterile Processing ANSI AAMI ST108 Layout

Sterile Processing ANSI AAMI ST108 Layout

The Importance of a Proper Loop Design

High purity water needs to be distributed in high quality plastic or stainless-steel piping to prevent contamination. Recirculation and prevention of dead legs is important for regular disinfection of the distribution system. To prevent biofilm in the piping and connections, distribution loops should recirculate at a continuous rate of 3-5 feet per second and the pipe feeding the device should be less than six times the pipe diameter of the loop. See chart below for recommended recirculation rates in distribution loop:

*Based on Schedule 80 PVC. Other piping systems may be slightly different.

Maintenance and Monitoring of a Critical Water Treatment System

A critical water treatment system requires monitoring, testing, disinfection, and on-going maintenance including consumable component replacements. Every day the systems are operating, parameters should be logged assuring that the equipment is performing properly.

Frequency for Water Quality Monitoring

Most likely you are already monitoring the critical water. It will now be necessary to monitor the Utility Water as well since it is included in the new standards. Water sampling should always be taken before disinfection. If results are above level, disinfect and rinse to the level of no detection, then retest. The best disinfectant for reverse osmosis, storage tank and distribution loop is 1% Peracetic Acid or Ozonation.


Guide Summary

Water quality varies from season to season, place to place, and even day to day.

Because it plays a significant role in the sterilization process, it is important to team up with a knowledgeable water treatment company who specializes in the health care industry and follows the guidelines set by AAMI/ANSI ST108:2023. In addition, and possibly most important, the team of sterile processing professionals must remain informed and vigilant in following all procedures to ensure the safety of their patients.