Circulating systems ensure water hygiene
The European Drinking Water Ordinance safety requirement has been valid since 1998. The guideline also sets out the legal responsibility of all parties involved in construction and operation.
An important aspect is to ensure the selection of correct materials and combinations for pipes, connectors and fittings for a new installation in accordance with the water condition.
Operating conditions that favor the growth of micro-organisms in the system can permanently cause more severe health hazards than the usual migration of materials during the initial weeks and months after commissioning.
This is especially the case with large distribution networks in hospitals, sports amenities, old people’s homes, hotels or apartment blocks housing several families. Due to long pipe sections and irregular water use, the risk of bacterial growth is especially high. The effect is greater when occupants are patients and older people whose immune system is weakened.
In many regions, permanent chemical disinfection is standard procedure for protecting water from bacterial infection. This is crucial in warmer regions where stagnation temperatures of above 25°C (77°F) are often unavoidable.
However, improved drinking water quality and tangible savings on maintenance and operating costs are available. This temperature guideline can be dispensed with if water quality at point of entry to the building – and specific rules for planning, construction and commissioning of the plant – are taken into consideration.
Tried and tested measures are presented here in a brief overview.
The danger of bacterial infection of the entire system is particularly high at constant temperatures between ~30°C and 50°C (~86°F and 122°F) if:
• Drinking water is stagnant for a long time in a pipe network and cold water is heated, for example, by adjacent heating pipework;
• Wastewater is fed into the pipe network due to a system fault, such as backflow.
For instance, Legionella pneumophila can cause legionnaire’s disease if steam is inhaled when showering. However, Pseudomonas aeroginosa (Illustration 2) are particularly feared in hospitals, as they can cause bed sores that are difficult to heal.
Taking regular samples from the pipe system – especially fittings that are rarely used – is the only way to determine whether the distribution network is largely free of germs (Illustration 3).
Chemical or thermal disinfection is a short-term solution for specific numbers of colonies, but generally the problem will be permanently treated only if the causes are identified and eradicated.
Future risks can be substantially reduced at the planning stage of a drinking water plant. By using needs-based pipework dimensions rather than reserves based, the required water exchange is favorably regulated in the piping network.
Exact manufacturer information should be incorporated in the calculation instead of standard values for individual resistance of press-connectors, valves or equipment.
Results for large-scale pipe networks often differ so substantially that total pipe widths can be one or two sizes smaller than calculated using recommended values (Illustration 4). Therefore, the entire water volume in the system is continually replaced quicker and the average cold-water temperature in the system is substantially reduced.
Microbial growth is thus reduced naturally.
Furthermore, regular water replacement in all parts of the plant is guaranteed if the consumer is involved in defining regular use – for example, hydraulically planning the WC tank at the end of a distribution pipe.
For the same reason, tapping points that are rarely used should be integrated into a series or closed circular piping system (Illustration 5). Branch lines should be exempt from this and should never contain more than 1L (2.1 US pints) of water.
In addition, hygiene-conscious planning of a drinking water network primarily includes safeguarding the cold-water pipe from heating. The danger mainly exists in shafts or lowered ceilings that are fitted with other pipework for reasons of space and cost.
Legionella multiply especially well between 30°C and 50°C (86°F and 122°F), so cold-water pipes should be as far as possible from heat sources and should be securely insulated.
For warm water, it is important to differentiate between circulating distribution pipes in the cellar and riser pipe area and the consumption pipes without circulation that are usual in a single story.
Circulating systems have to be hydraulically equalized so that constant temperatures, for instance, 55°C to 60°C (131°F to 140°F) can be guaranteed in all sections. But even this temperature level probably does not match the goals of energy saving and CO2 reduction.
Corresponding settings can be fixed at thermostatic regulating valves (Illustration 6), installed in each case at the return end of every branch line.
Distribution pipes on different floors are generally laid in the floor or walls. Due to construction conditions there is rarely any space for substantial insulation layers, such as for circulating systems in shafts or sloping ceilings.
Therefore, overall, the distribution network is to be planned so that these pipes contain only minimal water.
Short pipe lengths up to the circulating riser pipes and minimal cross-sections, will increase user comfort (hot water should reach the tap within 10 seconds) and will accelerate cooling of the water through the critical temperature range after shutting off the tap.
To install these plans, subject to construction conditions and a hygiene-conscious method, the use of aligned components is vital – such as pipes, press fittings and shut-off fittings.
Press connectors and fittings with a press connection that is certified as safe make it possible to dispense with welded and threaded connections. Some of the many advantages are:
• No solder and welding flux in the pipes – no hygiene problems, no heat treatment with copper pipes – no corrosion;
• ‘Forgotten’ press connections are guaranteed to be noted during a leakage test (Illustration 7). Major time saving compared with welding – many secondary tasks and maintenance time dispensed with;
• Threaded connections can be dispensed with, particularly if concealed in the building structure, facilitating a dry leakage test, which is proven to protect the system better from microbacterial contamination, especially during the period up to commissioning.
Large distribution networks in buildings should not be tested for leakage with water, but preferably with oil-free pressurized air or inert gas. The rationale: due to resulting construction work, these plants are often commissioned for regular operation only after several months.
In the case of ‘wet’ leakage testing, the system may be contaminated due to the testing water or microbiotic growth during stagnation of a system that has never been fully emptied –particularly in the case of ambient temperatures of >25°C (>77°F) during the construction phase.
Water remains standing in horizontal stretches of pipes, and bacteria may multiply. If the leakage test must be carried out using water it is important to test the building connection point. This should in all circumstances be flushed out with the appropriate water volume and water flow before connection to the supply network or installation of the water counter,
Depending on the length of pipework and stagnation period – often months or years – mircrobiotic testing is recommended in order to rule out any contamination of the building network via this route. Until that point, the building will be hygienically safeguarded.
In new residential blocks or larger-scale construction projects, closed-circuit pipes or branch lines may have insufficient throughput of water during the construction phase, so the necessary water exchange is not guaranteed.
For ‘dry leakage testing’ with oil-free pressurized air or inert gas, the leakage test is to be carried out using 110mbar (230lb/ft2) of pressure; load pressure with a maximum inspection pressure of 3bar (6,265lb/ft2) to DN50 or 1bar (2,088lb/ft2) for greater pipe diameters (Illustrations 8 and 9). If connections other than press fittings with welded, clamped or threaded connections a visual check is recommended to ensure their integrity, as well as a central leakage test.
The planning, construction and commissioning of a drinking water installation are only three of the important foundations for a trouble-free network.
Hygiene-conscious operation is just as important. This includes transferring the drinking water system (including all documents such as leakage test protocol, etc) to the operator, as well as information and instruction on which measures are needed for maintaining drinking water quality.
The information and instruction should be provided in case no specialist staff members are exclusively available to maintain the plant.
This maintenance involves, for example, regular filter maintenance in accordance with defined intervals, inspection of circulation temperatures, investigation of suspect cases of microbial contamination and testing relevant individual pipe sections or fittings.
For buildings that are not regularly used to the planned extent (eg: sports halls with shower rooms) or for which irregular use is expected (eg: large hotels) in addition to these measures, a flushing plan for regular water exchange should be included for little-used tapping points. Details should be confirmed with the local hygiene specialist.
Overview of hygiene-conscious planning and completion:
• Material selection in compliance, for example, with EN 12502;
• Use of certified (approved) products;
• Determination of minimum water volume – use pressure potential, accounting for actual values (manufacturer details) for real top volume flow for individual resistances of fittings, equipment, etc;
• Plan the maximum distance between drinking water (cold) pipes and heat sources;
• Provide sufficient insulation for drinking water (hot and cold) pipes in ducts and lowered ceilings;
• Do not install appliances for the secondary treatment of (cold) drinking water in rooms with a temperature of >25°C (eg: boiler rooms);
• Set temperature should be determined in the drinking water heater and manifold;
• Ensure hydraulic equalization in warm water circulation with permanent temperatures >55°C in any part of the piping system;
• Plan test extraction valves in public buildings;
• Choose singular locking for backflow prevention – guidelines such as acc. EN 1717 recommended;
• If possible, do not fit a membrane expansion container in DW systems – or use hygienic approved systems only;
• Measures to minimize stagnation – eg: bypass and drainage pipes, do not factor in reserves;
• Separate any unused lengths from existing systems;
• Separate fire extinguisher piping systems from drinking water systems;
• Dry leakage test recommended.
• For all measures of planning, construction and commissioning of drinking water piping systems, avoid long periods of stagnation in combination with continuous temperatures between 25°C and 55°C (77°F and 131°F).