The Effects of Water Purity on Sterilization

Water quality can affect the life of the autoclave, the types of loads sterilized, as well as the autoclave’s materials of construction. It is important to match the sterilizer with your load requirements as well as the water quality.

What are the types of water?

In order of increasing water quality:

  • Tap water can be hard or soft depending on many factors such as geographic location and water source (i.e. ground well, lake, river etc.). Tap water almost always has a variety of salts, minerals and other chemicals dissolved in it or added to it by water treatment facilities.
  • Softened water results when water has been processed to exchange most of the dissolved calcium with either magnesium or sodium.
  • Reverse Osmosis (RO) filtrated water is produced by forcing the water through a thin permeable membrane which removes most solid contaminants and dissolved minerals. RO is often the first step during production of Deionized or Distilled water. RO filtration does not remove biological particles such as bacteria or viruses.
  • Deionized water (DI water) is water that has had its ions removed. Because the majority of water impurities are dissolved salts, deionization produces high purity water that’s generally similar to distilled water. However, deionization does not affect uncharged molecules, viruses or bacteria.
  • Distilled water has virtually all of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container, leaving nearly all of the solid contaminants behind. Distillation produces very pure water but also leaves behind a leftover white or yellowish mineral scale, which requires that the distillation apparatus be frequently cleaned.

Types of Water Scale



RO filtered water and softened water are typically under 1 MΩ•cm. There are various grades of deionized and distilled but they are typically over 2 MΩ•cm.

Water hardness is a measure of how many minerals such as calcium and magnesium are dissolved in the water. The harder the water, the more minerals it contains and the more damaging it can be to equipment. Information on local water quality values, including hardness can often be found in municipal sewage reports.

Water Hardness Scale


What water type should I use to sterilize my items?

Generally, water between 0.1 MΩ•cm to 1.0 MΩ•cm would be appropriate for sterilizing bio-hazardous waste, clothing, cages, glassware, media or lab equipment. However, unless you have water that is softer than 5 grains, Consolidated recommends using an RO filtration system.

High purity water, water greater than 2 MΩ•cm, should be used when sterilizing items sensitive to mineral contamination such as biological tissue samples or items used in cGMP processes. For these applications Consolidated recommends the use of DI or Distilled water.

Water Feed Requirements, Carbon Steel Steam Generators

Carbon Steel Steam Generators (1) General Vacuum Device & Quench
Characteristic Recommended Condition Maximum Condition Recommended Condition Maximum Condition
Temperature [°F (°C)] As Supplied 140 (60) 40-60 (4-16) 70 (21)
Total Hardness (mg/L) 17 85 10-85 171
Alkalinity (mg/L) 50-180 350 50-180 350
Total Dissolved Solids (mg/L) 50-150 250 50-200 500
pH 7.5-8.5 7.5-9.0 6.8-7.5 6.5-9.0
Total Silica (mg/L) 0.1-1.0 2.5 0.1-1.0 2.5
Resistivity (Ω•cm) (2) 2,000-6,000 26,000 2,000-26,000 500,000

(1) Stainless-steel generators require deionized water with resistivity ≥ 1MΩ•cm.
(2) If water supplied is greater than 26,000 Ω•cm contact Consolidated for recommendation.

How does the water affect my autoclave?

When the steam generator boils water, the salts and minerals from the water are left behind. These minerals will coat the inside of the generator like paint and stay there until they are cleaned off. These mineral deposits will continue to build up and decrease the functionality of the generator until the generator stops working. This problem is very common when the generator is fed using tap water. The easiest and least expensive solution is to install an RO filter on your water line coupled with an automatic generator blow-down. The RO filter will retain the majority of the minerals while the automatic blow-down feature will flush away any minerals that make it through, thereby greatly prolonging the life of your heating elements.

If you are using RO water, we recommend a resistivity between 100,000 Ω/cm to 250,000 Ω/cm because this water is pure enough to help prevent harm to the generator. Additionally, this purity is low enough that the carbon steel will reduce the water’s resistivity to a range acceptable for generator control circuitry.

If you are using DI or Distilled water, then your autoclave, steam generator and piping MUST be all stainless steel. This type of water is very pure and will try to leach impurities from everything it touches including glass, steel and copper. DI and Distilled water will continuously corrode non-stainless steel parts until the parts fail. If you sterilizing tissue cultures or items for a cGMP process that require the use of high purity water, your autoclave should be constructed from all stainless steel; this includes the generator and all process piping.

Where can I get water purification systems?

Consolidated can provide either Reverse Osmosis or Deionized Water Purification Systems for use with your autoclave or for general laboratory use. These systems are designed to seamlessly integrate with all sterilizers and other equipment that requires purified water. Additionally, the water purification systems have a low cost of ownership. Either type of system is available in purification capacities ranging from 30 liters/hr up to 120 liters/hr. Technical support is provided by Consolidated’s staff of experienced water system engineers.

For more information please contact your local Consolidated representative or contact Consolidated directly.