4 - Cold water systems

Abstract

Drink water supplies are not sterile at the point of entry, and the cold water supply into the building will contain a range of microorganisms, which occur naturally. Where the cold water temperature is below 20°C, these are likely to be in small numbers and will be dormant or only able to grow very slowly unless the building system provides the conditions that allow it to colonize and grow. Waterborne microorganisms are transported via the cold water supply to the hospital water system. Once in the building, the challenge is to prevent these microorganisms from multiplying in the cold water system. If the bacterial numbers can be controlled, then patients would be better protected. Unfortunately, many water systems provide idyllic conditions for microbial growth. In some cases, as soon as the bacteria enter the water system, they have access to a range of nutrients and will form biofilms on surfaces from which they will sequester further nutrients. Debris and sediment either from the supply itself or corrosion products will accumulate on the base of the storage tank and provide nutrients that support microbial growth. In addition, where cold water storage tanks are oversized, this will promote slow passage or stagnation of the water. This stagnation lengthens the time that the bacteria spend in the storage tanks and gives them greater opportunities to become established. The temperature of the cold water, e.g., below 20°C, should be sufficient to limit the growth of many microorganisms. However, the water storage tanks are often located in warm plant rooms, and the cold water pipes are often sited alongside the hot water pipes and not insulated. It is partly this transient heat transfer that results in warming of the cold water and in favorable conditions for the growth of waterborne pathogens, which leads to greater infection risks to vulnerable users.