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Take care with the cold water line: avoiding Legionella and Pseudomonas aeruginosa 

Drinking water is essential for life – and yet it can harbour risks if drinking water installations are not planned, installed and operated to professional standards. Two activities are essential for maintaining drinking water quality and avoiding the excessive growth of harmful bacteria such as Legionella or Pseudomonas aeruginosa: first, a regular exchange of water after no more than 72 hours; second, keeping potable water hot (PWH) and potable water cold (PWC) within their respective temperature limits. In recent years, excessively warm cold water lines have been increasingly responsible for hygiene risks in drinking water installations. However, with carefully targeted structural and organisational measures, planners and building operators can prevent excessive PwC heating and avoid expensive renovation work.

Legionella and Pseudomonas aeruginosa: pathogen profiles
Potential reasons for cold water lines becoming contaminated
Tips for planners and building operators to prevent contamination of drinking water
With the right strategies, PWC overheating and hygienic challenges can be effectively prevented

Legionella and Pseudomonas aeruginosa: pathogen profiles 

Legionella spp. and Pseudomonas aeruginosa are rod-shaped bacteria that can occur naturally in drinking water at extremely low concentrations. At higher concentrations, however, they can cause serious problems for human health. Vulnerable individuals and people with pre-existing conditions are especially at risk. Accordingly, these bacteria are viewed as key indicator microorganisms for drinking water hygiene in the health sector.

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Legionella 

Warm water left to stagnate for long periods of time offers ideal growth conditions for Legionella. Temperatures from 25 °C to 45 °C are especially problematical. Above 60 °C, Legionella dies off and its growth slows to a standstill below 25 °C. A Legionella infection is caused by inhaling tiny water droplets (aerosols) during showering or at the wash basin, for example. Individuals may then develop the serious and often life-threatening pneumonia-like symptoms of Legionnaires’ disease or Pontiac fever (like having a bad case of ’flu).

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Pseudomonas aeruginosa 

While Legionella can spread rapidly in warm to moderately hot drinking water, the bacterium Pseudomonas aeruginosa is only ever found in the PWC line. Pseudomonas reproduces faster at 20 °C but starts to die off at only 45 °C. From empirical studies, we know that Pseudomonas is most commonly detected in the summer months, as temperatures start to rise above 25 °C. The bacterium is highly resistant to disinfection practices as well as antibiotics, and can form biofilms to colonise piping, fittings or water storage tanks. These biofilms protect it against eradication by chemical agents. In people with weak immune systems, Pseudomonas aeruginosa can cause severe pneumonia-like symptoms, sepsis and (wound) infections, although it is not a risk to individuals in good health.

Legionella and Pseudomonas aeruginosa – a side-by-side comparison 

The tables shown below offer a comparative overview of the most relevant properties of Pseudomonas aeruginosa and Legionella spp.:

Comparison of properties and significance of Pseudomonas aeruginosa and Legionella spp.
Pseudomonas aeruginosa Legionella pneumophila Can spread even at 10 °C to 15 °C, but only slowly Almost no growth below 25 °C and above 55 °C, slow growth up to 27 °C Accelerated growth between 25 °C and 40 °C Accelerated growth between 35 °C and 45 °C Is often a primary coloniser as it has few competitors, especially at low temperatures Not a primary coloniser, as it multiplies almost exclusively in amoebas and amoebas need other bacteria to survive Forms biofilms that offer protection against disinfectants Does not form a biofilm Parts contaminated at the manufacturer are an important source, but only for testing with water Often detected in waste trap samples Not detected in new parts Often occurs even if the generally recognised codes of practice for drinking water installation have been properly followed Does not occur in excessive numbers if the generally recognised codes of practice for drinking water installation are properly followed Can be suppressed by high rates of water exchange and local bacteria, as it is sensitive to competition (explains its common status as a primary coloniser) Can be effectively prevented by maintaining proper temperatures (<25 °C and ≥55 °C). A regular exchange of water is also necessary. Drinking water testing as required by the Federal Environment Agency ‘Recommendation for required testing for Pseudomonas aeruginosa, for risk assessment and for detection measures in drinking water’ Drinking water testing as per section 14b of the German Drinking Water Regulation
Comparison of primarily technical properties of Pseudomonas aeruginosa and Legionella spp.
Pseudomonas aeruginosa Legionella pneumophila Rod-shaped bacterium Rod-shaped bacterium Significant pathogen for nosocomial infections. Via invasive foreign object systems, if these come into contact with drinking water: catheters, ventilation tubes, contact lenses, implants. Important pathogen for acute/chronic wounds and burn injuries. Significant pathogen for systemic diseases like cystic fibrosis. Significant pathogen for equipment that produces aerosols: a/c units, cooling towers, water treatment systems in breweries, showers, etc. Low nutrient requirements: survives on airborne carbon High nutrient requirements – spreads in specific amoebas Multiple resistance to antibiotics (approx. 16% of isolates in 2009) Can only be treated with specific antibiotics Rapid doubling time: approx. 20 min (optimal) Slow doubling time: approx. 2 to 4 h (optimal) Pneumonia, sepsis, (wound) infections, etc. Produces blue-green pus with a lime blossom scent Atypical pneumonia-like disease with 10–15% lethality, Pontiac fever (influenza-like) Second most common pathogen for nosocomial pneumonia-like symptoms, third for urinary tract infections, in eighth place for sepsis (approx. 10% of fatalities = 7,000 deaths/year in Germany) Incidence without healthcare-associated cases: 18 to 36 per 100,000 people (calculated), approx. 30,000 cases per year, around 3,000 fatalities

Causes of contamination for cold water lines 

Over the last ten years or so, PWC piping has overtaken PWH piping as more likely to be affected by Legionella. There are two key reasons here: first, building envelopes are becoming denser, with ‘luxury installations’ featuring a growing volume of increasingly complex pipework routing; second, ‘heat exchanger installations’ with PWC in a ring and PWH circulating at the same time in pre-walls. 

Technical weak points in the drinking water installation 

To keep the drinking water installation hygienically safe, the temperature in potable water hot (PWH) must be at least 55 °C after tapping 3 l from each tapping point and below 25 °C in potable water cold (PWC), also after tapping 3 l. In practice, however, there are many structural factors that result in PWC becoming excessively warm, which can therefore lead to a risk of contamination. 

  • Modern construction methods: Oversized drinking water installations with significant hydraulic complexity and excessive numbers of tapping points. Compared with modern T-piece installations, these installations can have up to 29% more surface area, which (unnecessarily) absorbs more heat (PWC) and stores this heat in a water volume that is up to 25% larger. This creates hygienic risks. 
  • Lack of thermal separation of PWH and PWC (e.g. shared riser shafts with hot- and cold-temperature lines, excessively high temperatures in technical rooms) leads to undesirable input of heat into the cold water piping. 
  • Complex drinking water installations with hygienically risky and avoidable circulation lines in the pre-walls – even though a 3 l rule applies to individual supply lines. This results in excessive temperatures in PWC and increased thermal losses from circulation. 
  • Contaminated parts (e.g. from leak-tightness testing with water in the factory) can result in the drinking water installation becoming colonised with microorganisms – especially Pseudomonas aeruginosa – even before commissioning.

Influence of operation and usage 

Alongside structural aspects, specified normal operation is extremely important and, in this context, the regular use of all tapping points has a decisive role to play in maintaining the quality of drinking water in buildings. Accordingly, drinking water installations must be used regularly and every 72 hours as a minimum, across all tapping points. This usage should also comply with the operating conditions and safe temperature ranges for PWH and PWC, as established during installation planning and setup. Every tapping point not used regularly will become a ‘dead leg’ yet full of life – regardless of the specific piping layout chosen. Regular testing of all parts for functionality is also needed, and the required maintenance tasks must be completed to ensure the operationally safe condition of all parts. These tasks also form part of specified normal operation, which must not be reduced under any circumstances to merely the regular and full exchange of water.

Duties of building operators 

In order to maintain specified normal operation, and thus prevent the excessive spread of bacteria, the water in the installation must be exchanged in full and across all tapping points on a regular basis and at least every 72 hours. The legal basis for these duties for operators of drinking water installations is section 13 of the German Drinking Water Regulation. Compliance with generally accepted codes of practice is also a minimum requirement for operations. These codes (as published by the DVGW and VDI) require the regular and full exchange of water across all tapping points. This is the only sure way to maintain water quality in buildings and cannot be supplanted by any particular type of piping layout or an exchange of water that only occurs via terminal flushing stations. Flushing stations are at best an ancillary measure.

Legionella testing duties in the cold water line 

If overheating of the cold water line is suspected, then this line must also be tested for Legionella (DVGW W 551 (A)). Overheating is simple to check: if, after drawing off 3 l of cold water, the temperature of this water exceeds the stated 25 °C – measured in a volume of 250 ml (VDI 6023 Part 1), then this suspicion has been confirmed. Although DIN 1988-200 specifies a ‘30-second tapping rule’, this should no longer be used, due to the much more recent and more precise testing conditions given above.

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Steps to prevent excessive heating of the cold water line 

Alongside health risks, excessive temperatures in the cold water line also have economic consequences if extensive remediation work becomes necessary. However, these costs can be avoided if a professional and goal-oriented approach is taken during planning and installation. A distinction is made here between structural/planning measures (passive measures) and operational measures (active measures). In short, the aim is to ensure the spatial separation of hot- and cold-temperature piping sections as far as possible. Nor is insulation a reliable substitute for such separation: any insulation merely slows temperature equalisation but does not fully prevent it (DIN 1988-200).

Passive measures Maintaining drinking water quality starts with the planning of the installation 

To avoid the risk of excessive Legionella growth, the input of heat into the cold water line should be minimised as far as is possible. To this end, planners and tradespersons should keep a number of aspects in mind during the planning and design of drinking water installations. 

  • Route the hot and cold water lines in two separate installation shafts: This prevents the transfer of heat into the cold water line. However, this will only be successful if the developer instructs their architect accordingly. The ROI here is a safe drinking water installation instead of expensive and intractable problems.
  • Keeping cold water lines out of heated rooms: Water mains connections and water softening equipment, etc. should be installed in separate, unheated technical rooms and not installed in one room with remote heating connections or heating equipment. 
  • Ensuring the regular and complete exchange of water across all tapping points by optimising piping routing to keep sections as short as possible and piping volume as low as possible (prioritisation of T-piece installations). 
  • Minimising crossover points between the cold and hot water lines: At these kinds of hotspots, colonies of Legionella can form that can then contaminate the entire drinking water installation. 
  • Never routing cold water lines beneath underfloor heating – but always at a minimum distance to these systems (CEN/TR 16355). 
  • Never routing hot water circulation lines up to the tapping points in pre-walls, but always working with stubs instead: as before, max. 3 l stubs are permitted to maintain good hygiene. With a piping diameter of 15 mm, a 1.5 l volume is equivalent to a connection length of more than 10 metres. And this is already fully exchanged once with a usage lasting 20 seconds.

Active measures Safeguarding drinking water hygiene during operation 

Even the most thorough and hygiene-friendly planning of a drinking water installation is of no use if subsequent operations fail to ensure the regular and full exchange of water across all tapping points. Tip: Landlords who rent out commercial or residential properties should always inform their tenants about the need to ensure the regular use of all tapping points and also include a corresponding clause in the rental agreement. Beyond ensuring normal usage, the following measures are also necessary to provide optimum support for maintaining drinking water quality: 

  • Regular stagnation flushes during interruptions in use lasting more than 72 h: these are completed by hand using manual flushing plans or – much more efficient – automated with the help of a water management system like SWS from SCHELL. 
  • Monitoring of temperatures in potable water cold and hot, with the help of temperature sensors: SWS from SCHELL can be used to schedule stagnation flushes or trigger them based on temperature changes. In the latter case, SWS can react to temperature readings that are above or below set limits.
  • Regular completion of maintenance work (inspection, servicing, improvements, etc.) as par manufacturer specifications and DIN EN 806-5. 

A comprehensive set of recommendations, plus other details about passive and active measures to keep temperatures to the levels required for drinking water by general and hygiene standards, is offered in a practical guide from the Federal Technical Building Services Industry Association (BTGA): ‘How to keep cold water cold’.

Maintains water quality and temperatures by ensuring a regular and full exchange of water: SCHELL SWS 

Maintaining drinking water quality is a key responsibility for planners and operators of drinking water installations. Particular attention should be given to the cold water lines here, as the risk of an excessive proliferation of Legionella and especially Pseudomonas aeruginosa is particularly high here. Optimising the planning and design of drinking water installations (passive measures), and ensuring hygiene-friendly operation (active measures) is an effective strategy for preventing the excessive heating of potable water cold and therefore avoiding contamination. For operational use, water management systems like SWS from SCHELL are a highly efficient and reliable solution for the scheduled and temperature-controlled execution of stagnation flushes.

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