Understanding water quality - water quality in dental practice

Various forms of water are used in the dental practice. The previous article, ‘Dental unit waterlines’, focussed on waterline biofilm issues. In this second article, Professor Laurence Walsh summarises key aspects of water quality that impact on instrument reprocessing and sterilisation.

WATER FOR ULTRASONIC CLEANERS

Tap water has dissolved oxygen and other atmospheric gases. Aerators on taps slow the flow to reduce splash, but they increase the dissolved gases. Degassing of water is essential to remove dissolved gases before undertaking ultrasonic cleaning of items.

WATER FOR CLEANING ITEMS AND FOR RINSING INSTRUMENTS

Water used in instrument cleaning should have low levels of viable bacteria and bacterial products such as endotoxin, and be free of heavy metals and other contaminants. A key property of water that is used with detergents is its hardness. In hard water, there are high levels of calcium and magnesium salts, particularly carbonates, bicarbonates, sulphates and chlorides. This causes faster build-up of limescale, which can foul plumbing in the water distribution system, as well as major problems with detergents, such as low foam height and reduced surfactancy. The reason for this is that calcium or magnesium ions form insoluble salts with detergents, reducing their surfactant effects. Instead, they create a coating of insoluble stearates on the surface of instruments. This is equivalent to ‘soap scum’ seen in a shower recess.

Water softening creates soft water by replacing the calcium and magnesium ions in the water with sodium or potassium ions, so that no insoluble products are created by the reactions with detergents. Typically, water softening uses an ion-exchange resin. When all the available sodium or potassium ions in the resin have been replaced with calcium or magnesium ions, the resin must be re-charged by eluting the bound calcium and magnesium ions off the resin using a very strong solution of domestic salt (sodium chloride).

The hardness of ground water varies considerably around Australia, and water hardness problems are often encountered. This is why some detergents used for instrument cleaning include chelating agents such as citric acid or EDTA, which act as softening agents.

IS RAIN WATER SUITABLE FOR INSTRUMENT CLEANING OR FOR FEEDING STERILISERS?

In short, the answer is no. Normally, rainwater collected into polyethylene tanks is soft as it has low levels of calcium and magnesium ions and so forms a strong lather with detergents. Rainwater stored in concrete-lined tanks becomes hard due to dissolved calcium hydroxide that has been released from Portland cement into the water. Overall, rainwater is not well suited for use in a sterilising room. It is neither sterile nor is it free of anions and cations. Rainwater picks up multiple contaminants from the atmosphere (dust, pollutant gases) and from where it has been collected (off a roof). Normally, rainwater is acidic due to the presence of dissolved carbon dioxide (carbonic acid) as well as containing traces of sulphuric acid and nitric acid, both of which are derived from natural atmospheric sources and from industrial activity

WATER FOR STEAM GENERATION

Demineralised water is used for steam generation in steam sterilisers. Tap water is not suitable because this contains many cations (calcium, magnesium, aluminium, sodium, potassium, iron, and copper ions) and anions (phosphates, carbonates, sulphates, silicates, chlorides, fluorides). Precipitation reactions will cause scale build-up on water heating elements in steam sterilisers, in the same manner as occurs in water distillers, kettles, clothes irons and steam cleaners. In water distillers, regular removal of this white or yellowish mineral scale left on the heating elements of the distiller must be undertaken with an appropriate agent such as citric acid or dilute sulphuric acid.

Water may be demineralised, i.e. rendered free of cations and anions, by one of three different processes – distillation, ion exchange in a deioniser resin, and reverse osmosis (RO). While these processes remove ions, they do not remove uncharged materials including organic compounds and microorganisms. Deionised, distilled and RO water are not sterile.

When water is boiled the condensate is collected, resulting in distilled water free of inorganic minerals, and so is suitable for use in a steam steriliser. Distilled water does not have a neutral pH (7.0), but rather tends to have a low pH, caused by carbon dioxide from the atmosphere dissolving back into the water, forming a dilute solution of carbonic acid. When distilled water is made, the collection container is normally not sterile and so distilled water almost always contains some viable bacteria. To avoid this, additional treatment (such as ozonation or short wavelength ultraviolet light treatment) would be needed. The same holds true for deionised and reverse osmosis water; as with distilled water these are not sterile.

Many dental practices use water deioniser cartridges to generate water for use in a steam steriliser, as the process is easier, less expensive and more energy efficient than distillation or reverse osmosis. In a water deioniser unit, an ion-exchange resin exchanges dissolved cations and anions for hydrogen ions and hydroxide ions. A water deioniser does not remove uncharged organic molecules or bacteria, except by incidental trapping in the resin. As mentioned above for water softening resins, the ion exchange resin beds in a water deioniser unit have a limited capacity and need to be regenerated or replaced periodically. In many cases, this is on a regular schedule (e.g. three monthly) based on the water flow through the cartridge. Often deioniser cartridges are fed by sediment and carbon filters, and will have their own replacement sequence. It is essential to follow the supplier’s advice on when the different cartridges in a multi-cartridge system should be changed. Commercial test strips and meters to check water quality are readily available for monitoring ion content.

The process of RO uses high pressure to force water through a semipermeable membrane to remove ions, molecules, and larger particles including some bacteria. Tap water pressures of around 40 psi can drive a small RO unit. An RO system must have pre-treatment filters to stop membranes clogging. As with water deionisers, RO filter cartridges need to be changed on a regular basis as per the supplier’s instructions. RO water tends to have a neutral pH, but can be too soft and thus corrosive to pipes and control blocks which is why RO water is not approved for use in many types of dental chairs. Similar issues can occur when using distilled or deionised water, since these are mineral-free and can be very aggressive in reacting with some types of dental unit control blocks.

An additional issue with an RO system fed from mains pressure water is that they tend to have low recovery (5-15%) and so will use a lot of water that will be discharged as waste water into the drains. Surprisingly, this can lead to high water usage bills.

To achieve sterility, the steam in a steriliser must condense to transfer the latent heat of condensation onto the instruments or items. This raises an important problem when the water for steam generation is condensed and then re-used in later cycles, rather than being dumped. In re-used water, contaminants (such as handpiece lubricants) build up progressively over time, leading to problems with superheated steam that is too dry and will not condense. For steam sterilisers that condense and re-use water in multiple cycles, demineralised water must be completely replaced every week, and not just topped up.

 

Disclaimer

Insurance issued by Guild Insurance Limited ABN 55 004 538 863, AFS License No. 233791 and subject to terms, conditions and exclusions. This article contains information of a general nature only, and is not intended to constitute the provision of legal advice.  Republished from the ADA News Bulletin, December 2017 No 471 and February 2018 No 472 with the kind permission of the Australian Dental Association. Guild Insurance supports ADA through the payment of referral fees. Please refer to the policy wording and policy schedule for details. For more information call 1800 810 213.

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