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Water for Autoclaves

 
Purificazione dell'acqua
 
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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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