I’ve written a lot about the upcoming AAMI ST108: “Water for the processing of medical devices.” This standard will be a giant step forward in the practical and critical aspect of ensuring that water quality is what it needs to be for successful, repeatable sterile processing outcomes. In this article, let’s look at the first steps in the supply chain of water for processing.

Contaminant content

Utility and critical water are what are prescribed to be used for cleaning of reusable medical devices in the sterile processing department (SPD). The contaminant content for these water types is well defined, and should be just fine when made from water as it comes into the healthcare facility from the municipal supply:

  • If the municipal supply conforms to the EPA safe drinking water standards.
  • If the water mains are not 150+ years old and made of wood (I’m talking to you, New York City, but not only NYC).
  • If there has not been a recent water main break.
  • If there is not record rain and snowfall or even spring snowmelt.
  • If there is no agricultural runoff into the water table.
  • And so on, and so on, and so on.

No one can assure that all of these conditions will never exist. So, as the Boy Scouts say, be prepared.

But how?


Let’s triage this. The most obvious problem with a water supply is the presence of particulates in the water. These can come from mountain stream runoff into the water table; ambient conditions, such as fine sand in the water in New Mexico; or water main breaks. When the water flow is restored after a water main break, the crud at the bottom of the water main gets stirred up and mixed into the water.

How do we handle this situation? Most particulates are on the order of 10 microns or larger. A particulate filter should be a requirement for a water supply going into a healthcare facility. I have paid a few month’s living expenses because a number of my clients have called me in to find no filters on the incoming water supply. I can only assume that they are representative of most facilities. I am thankful for the work, but if things were done right in the first place, money, cancelled cases, and time could have been saved.

Particulate filters are called sand or bag filters. These are large, can be sized to meet the flow requirements of the SPD or the entire building, and are very cheap to maintain (the filter element is sand). Filter pore sizing can be as low as 5 microns, but you have to ask for that. These are cheap insurance and money well spent.

Total dissolved solids

The next issue that is typical is high total dissolved solids (TDS). These are chemical compounds that break up into ions in water.

According to the EPA safe drinking water standards, the upper limit for conductivity in utility water is 500 µS/cm. Conductivity is a proxy for total ionic dissolved solids. But in California (Bay Area, Central Valley, Monterey Bay, LA, San Diego), I have measured conductivity of over 600 µS/cm in the municipal water supplies. Also in Philadelphia. A new facility in the Denver, Colorado, area had conductivity of over 500 µS/cm. These are just examples of cases I have done. You should assume that the water for your facility is noncompliant until proven compliant.

Note that you don’t want to be on the edge of compliance. You want to have a safety margin.

Water softener

ST108 (and TIR34) are fairly clear on the subject of TDS and conductivity levels. Some of the dissolved ions can bind to the surface of instruments and cause corrosion sites. We would like to remove them. The first line of defense here is a water softener. It replaces heavier ions like calcium with sodium, which does not bind to anything in a typical SPD processing load. As long as the utility water is not too soft (which can cause problems with copper from the piping), softer is better.

If the TDS is due to iron or manganese, you need to install green sand filters in the water supply. These capture iron and manganese ions, removing them from the water supply and ensuring they don’t compromise or corrode the surface layers of stainless steel.

These measures can be taken on the water supply specifically for the SPD, so as to not add the cost of the softening and filtration to the rest of the healthcare institution’s supply. Again, these aren’t expensive, although more expensive than sand filters, but if the EPA safe drinking water standards are otherwise met for the facility’s water, there is no need to provide water for the rest of the facility that is good enough for instrument processing.


The next most critical issue is high chloride ion levels in the water. Chloride eats stainless steel for breakfast, lunch, dinner, high tea at 4:00, midnight snack, etc. It is a very bad actor relative to your stainless steel instruments. The EPA standard, TIR34, and ST108 all state that chloride levels up to 250 mg/l are okay. I disagree but did not win that argument in the process of creating ST108. In my experience, any level above 100 mg/l is too high, and 50 mg/l is preferred.

How do you get there? Some available methods are to admix critical water with the utility water to lower the level of chloride. This requires a lot of critical water. Other methods are anion exchange columns. You are best served on this issue by consulting your water chemistry supplier, usually the same people who provide the chemistry for house steam boiler protections.

This is a start. This isn’t the whole story, but there will be more in future articles. See you next time!

Flowchart for analysis:

SPD water supply solutions