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Why Water Quality is Important When Using a Steam Autoclave

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Why Water Quality is Important When Using a Steam Autoclave

Water is the lifeblood of a laboratory autoclave.  The quality of water used will drastically impact the lifespan of the autoclave chamber and steam generator, as well as the types of loads that can be sterilized. To understand why, let’s address four common questions about water quality and ways to remedy some of the issues you may be facing with your autoclave.

Can I Use Tap Water in My Autoclave?

It depends on the quality (e.g. hardness, minerals, chlorides, etc.) of your water.  While many laboratories use tap water with their steam autoclaves, this practice can lead to costly downtime and expensive repairs over time, if the quality is not adequate. Tap water is certainly a practical source for creating steam, but it can also be destructive to your autoclave.

What Causes Tap Water to Damage My Autoclave?

Tap water contains a variety of dissolved minerals and salts. Depending on your geographic location and water source (ground well, lake, river, etc.) the amount of this dissolved content can be very high.  The more minerals water contains, the “harder” it is.  When hard water is boiled into pressurized steam or used to cool the waste, it leaves behind salt and mineral deposits within the steam generator, pipes, and valves.  These deposits build up over time like layers of paint.  As they build up, they decrease the efficiency and functionality of the steam generator in addition to clogging up pipes and valves. Consolidated requires that any water that is harder than 5 grains (85 mg/L) should be treated.  The hardness of your tap water can be found in your municipal sewage report or by having a water quality test performed.

How Can I Purify My Water?

There are 3 ways to remove impurities from tap water:

  1. Reverse Osmosis (RO) – RO systems filter water by forcing it through a thin permeable membrane that removes most solid contaminants and dissolved minerals but does not remove biological particles, such as bacteria or viruses.
  2. Deionization (DI) – DI systems remove the ions from the water and produce a very high purity level.  It does not affect uncharged molecules, viruses, or bacteria.
  3. Distillation – Distillation removes virtually all impurities from water by boiling it and then condensing the steam into a clean container. This process leaves nearly all contaminants behind.

For a carbon steel steam generator, often the easiest and least expensive water treatment solution is to install a Type III RO filter on your water line and install an “automatic generator blow-down” feature on your electric steam generator. The RO filter will retain the majority of the contaminants while the blow-down feature will flush away any minerals that do make it through, thereby greatly extending the life of the heating elements in the steam generator.

How Pure Does My Water Need To Be? 

Water between 0.1 megohm·cm to 1.0 megohm·cm (purity achieved with a Type III RO filter) would be appropriate for the vast majority of research lab sterilizer loads such bio-hazardous waste, clothing, cages, glassware, media, or general lab equipment. On the other hand, items very sensitive to mineral contamination, such as tissue samples or items used in cGMP processes, should only be sterilized by steam generated from water greater than 1 megohm·cm. Water of this purity is generally only achieved from DI and distilled purification. Keep in mind, when high purity water (greater than 1 megohm·cm) is used as the source for steam generation, the autoclave must to be constructed from stainless steel, specifically the chamber, jacket, steam generator and process piping. High purity water lacks ions or minerals and will try to leach impurities from everything it touches, including glass, steel, and copper. This could cause continuous corrosion and premature failure of non-stainless steel components.

Impact of Water

Steam sterilization uses saturated steam to heat glassware, solutions or surgical instruments to achieve sterilization. The steam comes into contact with the items placed inside the autoclave. In the case of packed surgical instruments, for example, the steam must permeate the pack wrappings and heat the instruments inside the packs. For this reason, steam must be free from impurities in order to avoid staining or corroding the items being autoclaved.

The quality of the water used in autoclaves can also affect their performance and lifetime. The steam produced by a facility’s in-house boiler system may not be appropriate for use with autoclaves, and dedicated steam generators may be preferred. Poor steam quality may impair the sterilization process. For this reason, various norms are now suggesting maximum impurity levels for steam feed water of autoclaves and sterilizer used in the medical field. In the case of benchtop autoclaves (e.g. dental autoclaves), care must be taken to clean the water reservoir regularly, and refill it with freshly purified water.

The following water contaminants may have a deleterious effect on autoclaves:

  • Ions
    Calcium and magnesium salts are less soluble in hot water than in cold water. This leads to scaling, which is the main concern for autoclaves. Carbonate deposits are usually porous, while sulfate deposits tend to be much harder and dense. Scale deposits inside hot water pipes, boilers and heat exchangers restrict water flow and reduce heat exchange efficacy. This may lead to increased heating and maintenance cost.
  • Metals
    Iron, copper and manganese may also form deposits in the steam generator and reduce heat transfer. In addition, they can generate discoloration and stains of the instruments being autoclaved.
  • Particles
    They may form deposits in the steam generator or in the instrument and alter their functioning. They may also contaminate the solutions or instruments being autoclaved. 
  • Bacteria and their by-products
    Levels of bacteria and endotoxins should be minimized in the steam feed water of autoclaves, as there is a risk that they may be deposited onto the items being autoclaved. Although steam sterilization inactivates bacteria, it does not fully inactivate their by-products, such as endotoxins. Endotoxins may affect experiments (such as cell culture, etc.) or patient care (by causing inflammation and delaying wound healing). 
  • Chlorides
    They lower steam efficiency. When present in large quantities, they lead to uneven steam delivery and may provoke foaming in the steam generator. 

Note: It is important to follow autoclave manufacturer guidelines and recommendations concerning feed water quality.

The European standard EN 285:2006 and the United States standard ANSI/AAMI ST79:2006 both suggest maximum water impurity levels for steam generation:

Parameters
AAMI ST79
EN 285

Evaporation residue

≤ 15 mg/L

≤ 10 mg/L

Silica

≤ 2 mg/L

≤ 1 mg/L

Iron

≤ 0.2 mg/L

≤ 0.2 mg/L

Cadmium

≤ 0.005 mg/L

≤ 0.005 mg/L

Lead

≤ 0.05 mg/L

≤ 0.05 mg/L

Other heavy metals

≤ 0.1 mg/L

≤ 0.1 mg/L

Chloride

≤ 3 mg/L

≤ 2 mg/L

Phosphate

≤ 0.5 mg/L

≤ 0.5 mg/L

Conductivity (at 25°C)

≤ 50 µS/cm

≤ 5 µS/cm

pH

6.5 to 8

5 to 7.5

Appearance

Clean, colorless, no sediment

Clean, colorless, no sediment

Hardness

≤ 0.1 mmol/L

≤ 0.02 mmol/L

NOTE : The values reported in this table are for information only. It is the responsibility of the user to verify the values that apply in his/her specific case.

Table 1. Recommended limiting values of impurities in steam feed water (Information from ANSI/AAMI ST79:2006, “Comprehensive guide to steam sterilization and sterility assurance in health care facilities.” and EN 285:2006,
“Sterilization – Steam sterilizers – Large sterilizers” Appendix B).

 

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  • Christine Miller