Infection Control Today - Instrumental Knowledge - Water Quality and Reprocessing Instruments

Instrumental Knowledge

Water Quality and Reprocessing Instruments
by Herbert J. Kaiser, PhD; Gerald E. McDonnell, PhD; Jason F. Tirey, MA; and Daniel A. Klein, BS

While being one of the most important components of instrument reprocessing and sterilization, water is also one of the most ignored aspects in this entire process. Water is involved as the principal component in the cleaning, rinsing, and steam sterilization of stainless steel surgical instruments. It is very easy to determine the quality of the water used at a hospital or instrument reprocessing facility.

Before proceeding, general water descriptions need to be defined. Make-up water is defined as the water being used to make dilutions of detergents. For example, if a 1-ounce per gallon solution of a detergent was prepared, the make-up water is the water that is used to prepare that solution. Make-up water is also the water used to make up lost water in the boiler. Final rinse water is defined as the water used to rinse instruments prior to the instruments being dried. Make-up water and final rinse water should be sampled as close to the point of use as possible when an analysis is being done.

Boiler water is defined as the water that is in the boiler in liquid form. Boiler water contains a wide variety of chemicals that have been added to it. These chemicals are added to prevent the boiler and steam lines from corroding, becoming scaled, foaming, etc. Boilers are operated by licensed personnel who go through extensive training in boiler maintenance and operation. The importance of CS personnel establishing a good relationship with the boiler operators cannot be stressed enough. Boiler operators take a great pride in the quality of their operations and the significant impact their services may have on their facility and the services they provide. With this in mind, care should be taken when addressing potential problems with boiler water. Sampling boiler water must only be done by the boiler operator. Samples of boiler water should be taken anywhere below the surface of the water in the boiler. Ideally, samples should be taken immediately below the water level in the boiler and at the bottom if the boiler has these ports.

Condensate return water is defined as the steam that has condensed and is being returned to the boiler. Steam leaves the boiler in a facility and travels through steam lines, performing whatever required functions it needs to. While it is traveling through the steam lines, it eventually cools and condenses. This is why it is called condensate return water. The water that condenses in the steam lines is returned to the boiler. This water should be sampled before the water reenters the boiler. If the steam lines in the facility are equipped with steam coolers, samples should be taken as close to the point of use as possible. Sampling of condensate return water or steam must only be done by the boiler operator.

Carryover is defined as water and boiler chemicals that are transferred from the boiler to the steam system through foaming (also known as priming) or other processes. The amount of carryover is what is being determined when condensate return water is analyzed. Blow down is the process by which solids in a boiler are controlled. Boiler operators use blow down to drain part of the resident water in the boiler on a periodic basis. If this were not done on a regular basis, the solids level in a boiler would increase to a point where the water in the boiler would foam. When the boiler foams (or primes), boiler chemicals can be transferred into the steam lines and subsequently deposited onto instruments in the sterilizers.

To understand water quality issues, a few concentration definitions need to be established. Most components in water are described in terms of milligrams per liter (mg/L) or parts per million (ppm). Hardness and alkalinity concentrations are typically expressed as an equivalent calcium carbonate (CaCO3) concentration. For example, while water hardness is primarily composed of calcium and magnesium carbonate, the magnesium carbonate portion is expressed as calcium carbonate. This is done for ease of comparison among other analyses. Sometimes, water hardness will be described in terms of grains. One grain is equivalent to 17.1 ppm. In some water reports, the term "


Water quality reports often present water indexes. These indexes are measurements of various water tendencies. One such index is the Calcium Saturation Index. This is a calculated value that indicates whether calcium carbonate will come out of solution (a value greater than zero) or stay in solution (a value less than zero). Based on various parameters of the water, it can be predicted whether or not calcium carbonate hardness will form in the water. The Calcium Saturation Index helps to calculate the probability of calcium carbonate precipitating from various waters rather than as an experimental determination.

Another water quality index is the Aggressive Index. This is a calculated value that gives an indication of the aggressive nature of water. A value of less than 10 means that the water is highly aggressive. A value between 10 and 12 indicates that the water is moderately aggressive and a value greater than 12 indicates that the water is nonagressive. This index allows us to predict the corrosive nature of the water.

There are many definitions of various hardness qualities. In this article, we define distilled, deionized, or reverse osmosis water as water having a total hardness value of less than 1 ppm. Soft water is defined as having a total hardness value of less than 2.2 ppm. Water is considered to have a low hardness if its total hardness concentration is less than 75 ppm. Medium hardness water is defined as water having a total hardness of 76-150 ppm. High hardness water is defined as water having a total hardness value greater than 150 ppm. These values vary widely depending upon the source of the water. They will also often vary from season to season. Some changes can be dramatic when communities change their water source during the year. For example, some communities receive their water from a reservoir for part of a year and then from a river during another part of the year. Communities located between two rivers sometimes switch between rivers as their water source. Sometimes differences in water quality exist within the same facility. This occurs because part of the facility may use municipal water while another part of the facility uses well water.

Water Purification

There are many methods of purifying water. These include reverse osmosis, deionization, and softening. Generally speaking, the purity of water produced through various processes can be considered to follow this trend: reverse osmosis> deionized water> softened water> tap.

In a water softener, only calcium and magnesium ions are removed from the water. Water softening simply removes the water hardness. It does not remove any other metal ions or organic contaminants. A water softener functions by passing water through a resin bed. As the water flows through the resin bed, any calcium or magnesium in the water exchanges with sodium that is attached to the resin bed. Two sodium ions are exchanged for each calcium and magnesium ion in the water. Eventually, the resin bed of the water softener becomes exhausted. Instead of purchasing a new resin bed, water softeners are regenerated. This is done by passing a highly concentrated solution of salt through the resin bed. The sodium ions from the salt exchange with the calcium and magnesium ions that have been absorbed on the resin bed. An important step in the regenerating process is to rinse the resin before the water is used. This rinsing step removes the chloride from the resin bed. This is especially important in a facility that does instrument reprocessing and sterilization because chloride is one of the most harmful chemicals to which surgical instruments can be exposed.

Deionization removes all ionic species, not only the water hardness. In the deionization process, ionic impurities are exchanged for hydrogen ions (H+) or hydroxide ions (HO-). Deionization by itself does not remove organic contaminants. Typically, facilities will obtain deionization cartridges from a reliable water treatment company. Deionization columns are typically not regenerated at the facilities.

Osmosis is the process by which solvent molecules (in this case water) pass through a semi-permeable membrane. This occurs when a solution on one side of the membrane has a higher ionic strength than the solution on the other side of the membrane. The solution having the lower ionic strength tends to pass water molecules through the membrane to dilute the solution having the higher ionic strength. Given a suitable membrane, this osmosis occurs naturally. In reverse osmosis, as the name implies, the reverse occurs. The solvent molecules or water from the solution with a high ionic strength passes through the membrane producing pure water. The passage of the water through the membrane leaves the ionic impurities on the far side of the membrane. This process occurs due to pressure being applied that is greater than the osmotic pressure that would occur naturally. Reverse osmosis removes nearly all of the ionic and organic impurities originally present in the water. It is typical that prior to entering the reverse osmosis unit, the water is first deionized and passed through a variety of filters (to prevent clogging on the reverse osmosis membrane).

What quality of water is good enough? The answer to this question depends on the water's intended use. If the water is being used as make-up water, any quality of water is sufficient if and only if the detergent that is used to clean the instruments can sufficiently chelate the water hardness and metals that are present. A chelating agent essentially reacts with water hardness and metals in the water preventing them from interfering with the cleaning process. Many commercially available detergents contain chelating agents. The ideal water for use as a final rinse for stainless steel surgical instruments is reverse osmosis water. If reverse osmosis water is not available, then deionized water or at least softened water should be used as a final rinse. Untreated tap water should never be used as a final rinse for stainless steel surgical instruments that will be steam or gas sterilized.


There are many signs of insufficient chelation during the cleaning process. The primary signs are white spots or a film on the instruments and also inside the washer itself. These white spots or film are either the water hardness precipitating out of solution or a combination of the water hardness with the detergent being used. Other signs are metal deposits on either the instruments or the inside of the washer. These metal deposits can form due to high levels of iron or copper present in the water.

There are also many signs when a poor quality rinse water is used. Again, white deposits can form on the instruments prior to sterilization. These white deposits occur simply because of dry down of the water and concentration of the hardness content on the surface of the instruments. Rust deposits may also appear on the instruments prior to sterilization. This can occur due to poor quality rinse water corroding the instruments. Gold tints on instruments after sterilization typically mean that the instruments were exposed to high chloride levels, which can come from improper regeneration of a water softener or carryover of boiler chemicals into the steam system. Pitting of instruments after sterilization can also mean that the instruments were exposed to high levels of chloride either in the rinse water or the steam.

In addition, sterilization may be compromised. Experiments have been conducted to investigate this. Both Bacillus subtilis and Bacillus stearothermophilus were individually mixed with various soils, and kill curves were determined for steam sterilization. It was shown in our laboratories that crystals of both iron oxide/hydroxide (rust) and calcium carbonate (water hardness) significantly inhibited steam sterilization. These results can be linked to possible clinical situations.

Spore Survival Experiments

Many previous efforts have explored the use of agents to indicate the effective sterilization of a given technique. A more recent study showed that B. stearothermophilus could survive within 18 different crystals for over seven years.3 These attempts showed that the occlusion of spores within crystals did significantly inhibit sterilization in all types of apparatus. However, crystals were thought to be unrepresentative of the clinical environment. This is simply not the case for calcium carbonate and iron oxide/hydroxide occluded spores that can be readily related to clinical experiences. This was demonstrated in the lab.

Calcium carbonate and iron oxide/ hydroxide occluded spores were prepared by suspending the spores in a 10% solution of either calcium chloride or iron chloride. The final concentrations of the spores in the solutions were 108 cfu per mL. A solution of 10% sodium carbonate (for calcium) or sodium hydroxide (for iron) was added to the spore suspension to cause precipitation. The precipitate/solution was vortexed for five minutes to aid the suspension of the spores within the forming crystals. After precipitation was complete, the crystals were centrifuged. The supernatant was poured off, and the crystals were rinsed with sterile water. The crystals were centrifuged again, and then suspended in sterile water. Stainless steel coupons were inoculated with 10µL of the crystal suspension. The coupons were dried overnight at 90ºC.

The coupons were placed in a small glass petri dish with a lid. The petri dish was placed in the center of the autoclave, and the autoclave was run for the desired amount of time at 121ºC with a five minute drying time. The temperature build-up time was neither monitored nor controlled. The petri dishes were removed from the autoclave immediately following the completion of the cycle. After cooling, the coupons were recovered in five mL of distilled water. The samples were sonicated for five minutes to disrupt the crystals and vortexed for at least 30 seconds prior to plating. The pour plate method was used with tryptic soy agar. Incubation was done at 35ºC for B. subtilis, and 57ºC for B. stearothermophilus.

The resistance of the spores in crystals dried at 90ºC differed from those dried at 37ºC. This is because the drying of the samples at 37ºC, and at room temperature, does not remove all the water from the sample. When this sample is placed in the autoclave, and the temperature is raised, the water remaining in the sample and the spores is also heated. The presence of the water, now at a high temperature, within the sample kills the spores more rapidly. Drying at 90ºC significantly changes the environment of the spores in the crystals. This drying increases the resistance to steam sterilization. Drying at 90ºC removes most, if not all, of the unbound water (along with its heat content) present in the crystals. This means that the steam must pass through and rehydrate the soils before it can reach the actual spore and kill it.

This presents a potentially serious problem within the clinical setting. In many, if not all, of the automated washers used in CS units in hospitals, there is a drying cycle at the end of the washing cycle. The temperatures in these drying cycles are often somewhere between 100ºC and 140ºC. If the final rinse of the instruments is done with hard water, hard water deposits could form on the instruments. Calcium carbonate (water hardness) displays inverse solubility. Therefore, as the temperature increases, calcium carbonate becomes less soluble. In extreme hard water areas, thermal rinses could cause precipitation of hardness crystals. If spores were to be trapped within highly concentrated hard water deposits, it is possible that those spores could survive steam or gas sterilization.

Prevention of this scenario is fairly easy. First, all rinse cycles should be done with either reverse osmosis, deionized, or at least softened water. Secondly, inspection of the instruments should be done before use. Any hard water deposits or rusing should disqualify an instrument from use in an invasive procedure. These instruments should then be reprocessed using appropriate water conditions. Thirdly, the use of detergents containing agents to prevent precipitation of water hardness onto the instruments is recommended.

Steam quality is a measurement of the amount of moisture in steam. Steam used in sterilization processes is typically a mixture of steam and moisture. The steam content should be greater than or equal to 95% but less than 100%. This means that steam should contain less than 5% liquid. It is important for steam to contain a slight amount of moisture to be effective as a sterilant. Steam purity is a measurement of the amount of contaminants in steam. This measurement is usually that of the chemicals being used in the boiler. For example, if there is moisture present in the steam, that moisture will contain those chemicals found in the boiler. An excessive amount of these chemicals (carryover) in the steam will cause staining, spotting, and corrosion of surgical instruments. There are many signs of carryover. Stained wraps occur when boiler chemicals are deposited on the wraps as the steam penetrates the wraps to reach the instruments within. A purple rainbow color on the instruments is also an indication of carryover. This is typically caused by having excessive neutralizing amines in the steam. Black deposits on instruments or wraps are typically caused by high sulfate or sulfite content in the steam. Again, the sulfate/sulfite is a result of carryover from the boiler. A heavily stained sterilizer chamber is a good indication of a problem with carryover. Pitting of instruments is typically caused by high chloride levels in the steam due to carryover.

The Association for the Advancement of Medical Instrumentation (AAMI) indicates that there is no approval process for boiler water additives intended for use in steam sterilization. However, AAMI references the lists in 21 CFR 173.310 and 21 CFR 100.11 as appropriate chemicals for use in boilers. These lists include compounds that are allowed for use in steam where food contact will occur.

Water impurities, such as alkali metal, metal, and chloride ions adversely affect surgical instruments, both in their appearance and functionality. Sending a sample of your water to a testing facility can help you get the results you need to troubleshoot problems that may be occurring in your process. It can also help you in maintaining the quality of your process by knowing the quality of the water that is being used.

For a list of references, click here.

Herbert J. Kaiser, Ph.D., Jason F. Tirey, M.A., Gerald McDonnell, Ph.D., and Daniel A. Klein, B.S., are employed at STERIS (St. Louis, Mo).

For a complete list of references click here

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