Water is a reservoir and a mode of distribution for a wide range of bacteria. The vast majority of species are harmless, but some are pathogenic i.e. have the potential to cause illness. These species are a serious consideration in building water systems. Waterborne bacteria can cause illness or infection directly through ingestion (e.g. Escherichia coli), direct contact (e.g. Pseudomonas aeruginosa) inhalation (e.g. Legionella pneumophila).
A proliferation of bacteria in building water systems can have significant cost implications in terms of time, management, repair, remediation and monitoring – to say nothing of the potential health risks.
Modern design and engineering aims to improve performance, durability and function, but it must take account of our understanding of the operation of equipment and systems from a practical perspective as we use these buildings into the future. Design and engineering must now incorporate our knowledge and experience of building operation in relation to maintenance, water chemistry and microbiology and try to engineer out the potential problems that can come to light during the lifetime of a building and its water systems.
A typical building is generally supplied by treated mains water that achieves the standards outlined in SI 122 of 2014 European Union (Drinking Water) Regulations 2014.This does not mean that the water is sterile, merely that the bacteria and microorganisms that would be directly harmful, such as Escherichia coli and Cryptosporidium, are removed. The water still has a level of bacteria and microorganisms present, but these are in small numbers which are generally not immediately harmful.
From the point of entry, we use the water in many ways such as for drinking, hygiene and washing purposes, heating, cooling and in our process systems. In the case of some mains drinking water systems, and most hot-and-cold water systems, the first step in the system distribution is the storage of water. In this article, we look at the typical conditions of cold and hot water storage, the associated standards and guidelines, engineering considerations and some of the issues that arise in relation to the prevention of conditions that might promote the growth of waterborne pathogenic bacteria within our systems.
Risks of cold-water storage tanks
Cold water is generally stored in water storage tanks. It is becoming more common (in the case of some councils, it is even prescribed in the planning conditions) to have mains water stored in mains water ‘break’ tanks. Commonly, even the smallest water systems are installed with cold-water storage tanks. This is in contrast with many of our European counterparts, who do not use building cold water storage tanks but rely on mains water and non-storage pressure boosting systems to distribute cold water throughout buildings both large and small.
In Ireland, an apparent concern over security of mains water supply means we install cold-water storage and, in many cases, the desire is to store as much water as possible to ensure that security of supply. In reality, these concerns are unfounded as our mains water network is rarely disrupted and supplies water at a consistent pressure to most buildings. Waterborne bacteria become a real consideration with poorly considered cold-water storage.
In our experience of Legionella control, cold-water systems generally account for approximately 40% positive Legionella results – therefore, a building that stores too large a volume of water leading to heat gain or stagnation is immediately creating conditions that may allow bacterial growth before the water ever reaches the distribution system.
[caption id="attachment_19396" align="alignright" width="300"] Dirty cold-water storage tank[/caption]
Cold-water storage tanks should be considered a hazard in any appraisal of risk completed on building water systems in relation to bacterial growth. The first step is the mitigation of risk is to remove the hazard and, in the case of cold-water storage tanks (particularly for small systems), mains-fed systems should always be considered as the first preference by design engineers.
If this is not considered as practical, then the installation and operation of the cold water storage tanks should be in accordance with appropriate standards such as: