Engineering and waterborne bacteria in building water systems – Part 2
07 April 2015
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.
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:
- Health Protection Surveillance Centre, National Guidelines for the Control of Legionellosis in Ireland, 2009;
- UK HSE document HSG 274 –The control of legionella bacteria in water systems, 2014;
- Water Supply (Water Fittings) Regulations (NI) 2009 BS 8588: 2011 – Design, installation testing and maintenance of services supplying water for domestic use within building and their curtilages;
- Health Technical Memorandum 04-01: The control of Legionella, hygiene, “safe” hot water, cold water and drinking water systems. Part A: Design, installation and testing. Part B: Operational management.
In general, these documents outline the principal standards required for cold-water storage tanks, including the following:
Tanks should be constructed of suitable materials. Materials which will support corrosion in the cold-water storage tank are not to be used. Typical modern tanks are glass-reinforced plastic (GRP), plastic, fibreglass or lined concrete or steel tanks for large volume storage. We no longer utilise galvanised or lead-lined tanks but, whether through carelessness or cost saving, it can happen that fixtures and fittings used can be unsuitable such as outlet flange connections and submerged bolts. Where these are a steel-type fitting, they corrode over time and this corrosion will compromise the tank integrity and potentially promote bacterial growth.
- Water tanks should be accessible for maintenance and cleaning. In many cases, cleaning is completed on an annual basis and the hatch into the tank should be accessible to allow equipment and personnel enter the tank. In recent years, space has become a premium and cold-water storage tanks are often positioned in tight spaces with minimal access.
- The storage capacity of water tanks must reflect the system demands and storage must be a maximum of 24 hours system water demand. Over storage of cold water results in stagnation and warming of the water, which can allow bacterial growth to occur before the water is distributed through the systems.
- Water tanks should be completely sealed to prevent the ingress of contamination such as rodents, birds, insects or anything else that may be inadvertently introduced into the stored water. Airflow is required to prevent condensation build up, which would occur in a sealed tank and therefore a screened lid vent should be installed to allow air circulation.
- Water tanks are fitted with an overflow pipe and this should have a permanent, secure screen fitted to prevent the ingress of rodents and insects.
- Inlet and outlet pipework should be installed at opposite sides of the tank to promote crossflow of water and reduce the possibility of areas of stagnation forming within the cold-water storage tank.
- All water tanks should be completely insulated to prevent heat gain of the stored water.
In the previous article, the issue of proper balancing was highlighted and this can also be an issue with cold-water storage tanks. Many modern installations consist of a water tank split into two sections. These split tanks enable cleaning and maintenance operations to be completed on one side, which is drained down without affecting the water supply to the building as one side of the tank remains live.
During normal operation, stagnation is very often evident in one section of split water tanks as a result of an unequal balance between the sections. This bias in demand from one section over another can result where the split tank sections are unequal in volume or where the pipework from the sections differs one section can dominate and one can stagnate. Stagnation can result in an increase in water temperature and promote the growth of bacteria present in low numbers initially in the mains water.
Problems with hot-water storage
Hot water is generally stored in calorifiers. These are water-storage vessels such as the copper cylinder found in most homes. These can be heated from a range of possible sources such as boiler heating systems, gas or oil fired burners, electrical elements or solar heat source coils. The potential risk arising with hot-water systems and bacterial growth is very apparent in that water is heated above 20˚C.
In general, the principal standards required for hot-water storage calorifiers include the following:
- The calorifier should be constructed of suitable materials such as copper or stainless steel. If a mild steel vessel is utilised, then internal glass lining or an appropriate and durable internal coating should create a barrier between the stored water and the steel vessel or corrosion will occur.
- Lined vessels should be inspected periodically to ensure the integrity of the lining or coating is maintained. To facilitate inspection, calorifiers should be fitted with inspection hatches.
- Large calorifiers should be fitted with a circulation pumps to ensure an even temperature distribution of temperature across the cylinder.
- The calorifier must achieve a stored water temperature >60°C at all times.
- Hot-water systems should not vent into cold-water storage tanks. Expansion pipes commonly terminate over cold-water storage tanks in traditional installations. In the event of expansion from the hot-water system, hot water is discharged into the cold-water storage tank, which warms the stored cold water and can contaminate the water with any elevated bacterial levels from the hot-water system. Venting should be directly to drain or into heating system/condensate tanks or similar.
The levels of waterborne pathogenic bacteria can increase when suitable conditions for growth are present within the building water systems. Factors which will increase the potential for pathogenic bacteria like Legionella to grow and multiply in building water systems include:
- temperatures in the 25°C – 45°C range;
- stagnation in water systems;
- materials which promote bacterial growth;
- dirty water systems.
Water storage occurs in the hot- and cold-water distribution systems. Poorly designed or specified calorifiers or cold-water storage tanks can significantly contribute to some of the risk factors outlined above, resulting in potentially poor quality water delivery downstream in the distribution system. Where water storage is unavoidable, it must be ensured that the appropriate standards of design are applied, the storage capacity must be appropriate to the system demands and the equipment installed must meet all appropriate standards and guidelines.
Building water systems must be operated in a safe manner while not creating a risk to employees, visitors, residents and members of the public. Exposure of the building users to the water systems through ingestion or inhalation is unavoidable. There is a legal obligation on building owners and managers to operate their premises in accordance with appropriate guidelines to maintain safe water systems.
This is achieved by monitoring the performance and operation of the building water systems and rectifying defects as they arise. Storage of hot and cold water is a hazard which increases the potential risk of bacterial growth within the water systems. It is essential, therefore, that the storage element of the water systems is carefully considered, designed and engineered.
Colum Ó Bric is the technical manager with Environmental Services Ireland. He is responsible for water quality and Legionella awareness training, Legionella risk assessment, consultation on new builds prior to handover and the creation of water management policy documents for healthcare facilities. Ó Bric also participates in healthcare environmental monitoring committees as an external consultant, and he interprets and advises with regard to bacterial sampling results. He also offers assessment and risk management for other non-domestic water systems. Ó Bric has lectured on ‘Water Quality in the Built Environment’ for students of UCC’s Environmental Science & Social Policy course and for students of Facilities in Portobello College. Ó Bric holds a BSc Hons in Microbiology and a Diploma in Environmental Science & Social Policy (both from UCC).http://www.engineersjournal.ie/2015/04/07/engineering-waterborne-bacteria-building-water-systems-part-2/http://www.engineersjournal.ie/wp-content/uploads/2015/04/Legionellen-3d-rendered-illustration.jpghttp://www.engineersjournal.ie/wp-content/uploads/2015/04/Legionellen-3d-rendered-illustration-300x300.jpgChembacteria,water