Beating the bugs

Hospital Development, Aug 1998 by Revans, Martin

Martin Revans believes that chlorine dioxide is the best way of controlling legionella in hot and cold water systems.

Storage and delivery of clean, fresh water has always presented healthcare estates managers with a significant Health & Safety issue, because static water which is not subject to a disinfection regime is an ideal medium for bacterial growth. Left untreated, internal pipe and tank surfaces become coated with biofilm, a complex, slimy matrix of organic material, corrosion products and breeding bacteria, which can infect the water in contact with it and seed further growth elsewhere in the system.

Biofilms cause health problems. Legionella bacteria are known to favour an environment where biofilms exist, where they can be ingested by amoebae which 'graze' on biofilms. Inside amoebae, legionella can grow in a low hazard environment and become more virulent. Legionella bacteria, or infested amoebae, can be carried in water droplets and breathed deep into the human lung, which can lead directly to Legionnaires' Disease. This explains why hot and cold water services account for the majority of identified cases of Legionnaires' Disease in the UK (source: Building Services Research and Information Association (BSRIA)).

Biofilms also cause engineering problems. Their presence impairs heat transfer owing to their very low thermal conductivity. They raise pumping costs by increasing frictional drag in pipework, and encourage corrosion by providing an environment for problem microbes like anaerobic sulphate-reducing bacteria.

THE REGULATORY FRAMEWORK

In 1992, two key documents were published and their recommendations became law: the Health and Safety Commission's The Prevention or Control of Legionellosis (including Legionnaires ' Disease): Approved Code of Practice (known as ACOP), and the Health and Safety Executive's The Control of Legionellosis, including Legionnaires' Disease: Health and Safety Booklet (HS(G)70). In 1993, owing to concerns in the healthcare sector as to how these general guidelines were to be specifically applied in a hospital environment, a technical memorandum, HTM 2040, was published by NHS Estates. Compliance with the guidance given in HTM 2040 would then satisfy the ACOP requirements.

Very little in HTM 2040 actually precludes the use of unspecified means of disinfecting stored water, but the onus is clearly placed upon the estates manager to verify that any alternative method will be as good as, if not better than, those methods stated. This stance has discouraged an innovative approach to alternative treatment methods, which effectively become excluded from selection.

Since the publication of HTM 2040 five years ago, much progress has been made in understanding how legionellae may colonise hot and cold water systems which apparently satisfy all the temperature requirements of ACOP and HS(G)70. The building services industry has become increasingly aware that a temperature regime is not always achievable and in some cases not effective in controlling the problem. Because of this new awareness, and the development of innovative new methods of disinfection control, HTM 2040 is currently under full review. NHS Estates will eventually publish the revised document, in partnership with the Health & Safety Executive and the Drinking Water Inspectorate.

CURRENT METHODS OF DISINFECTION

Chlorine is the usual method for disinfecting piped water systems in a hospital environment. It is an excellent and fast-acting biocide, the primary disinfectant used in public water supplies. However, there are a number of drawbacks:

Chlorine cannot penetrate or disperse biofilms, so it is essential that slime and debris are removed by thorough cleansing before chlorine is used, otherwise micro-organisms which are protected by biofilms will survive disinfection.

Chlorine is a highly reactive chemical and can combine with organic matter, ammonium compounds and any oxidisable materials (for example ferrous and manganous salts) present in the water system. These reactions will greatly reduce the disinfecting power, or even neutralise it completely. They may also produce bad tasting and carcinogenic chlorinated organic compounds, such as trihalomethanes.

Unless they are known to be compatible, other biocides should not be used with chlorine, since they may neutralise each other.

The effectiveness of chlorine as a disinfectant falls off dramatically above pH value of 7.5.

Other disinfection methods include ultraviolet light, copper/silver ionisation and chlorine dioxide, but the only alternative disinfectant to chlorine currently approved by the DWI is chlorine dioxide.

ALTERNATIVE METHODS OF DISINFECTION

Ultraviolet

Ultraviolet can only be implemented at the point where water is used, so the number of outlets dictates the number of UV devices. Stored water in tanks cannot be protected by conventional UV apparatus, because the effective range of conventionally powered UV as a disinfectant in water is between 1 to 2 inches.

Ionisation BSRIA carried out a research project on ionisation in 1994 (published as Ionisation water treatment for hot and cold water services - TN 6/96). The aim of the experiment was to assess the effectiveness of ionisation using copper and silver at reduced water temperatures against a temperature regime for microbiological contaminants, including legionella bacteria, in hot and cold water systems.