Keystones of potable water hygiene
At least three basic requirements are to be applied to potable water supply systems:
• Usage comfort – water quantity, temperature and sound protection
• System operation and value preservation – safe, sustainable, energy-efficient
• Maintaining water quality
The first two requirements are the classical goals, while the last one only came into focus in recent years. As a foodstuff, potable water has something like an ‘expiry date’ – that is the point in time when its quality at a given extraction point no longer satisfies the requirements of potable water. Therefore planning specialists and trade professionals must know the major influencing factors for maintaining water quality and consider them in their respective areas of responsibility.
Essentially, this applies to all performance items up to and including handing over the necessary documents and provision of instruction regarding the plant’s functions. Comprehensive and clear documentation explains how the plant is to be operated and maintained properly and also clarifies the legal situation for specialist planners and trade professionals if situations should arise involving compensation claims.
To rule out critical propagation of legionella bacteria in potable water supply systems, the latter should be planned, implemented and operated in such a way that the following thermal and hydraulic conditions in accordance with the Interdependency triangle - potable water quality. This applies to all individual stages.
Temperature
The temperature is a critical parameter for the hygiene of potable water. It is important to avoid the temperature range of 25 - 55°C, which is particularly favourable for numerous pathogenic microorganisms, in order to prevent their proliferation/growth.
Cold potable water (PWC) must not heat up to a temperature in excess of 25°C. The temperature in the entire hot water system (PWH) must not fall below 55 °C. The temperature of the water must be kept at a minimum.
Circulation systems for potable water hot (PWH-C) are to be operated in such a way that temperatures of at least 55 °C are maintained in all individual sections. However, the exit temperature at the potable water heater must be at least 60 °C.
Water exchange rate
In accordance with DIN EN 806-5 (European Union), proper operation of a potable water supply systems requires an exchange of water at least within 7 days (≥ 1x / 7 days). This refers to complete exchange of the water in all partial sections and in the water heater.
Series piping and ring systems to frequently used sampling points can be used to ensure the required water exchange even for little-used fittings, while maintaining economic viability. Under these conditions it is sufficient in the event of interruptions of use to schedule manual flushing or flushing by a flushing system.
According to the German guideline VDI / DVGW 6023, potable water supply systems are to be planned to ensure that the water is exchanged at least every three days.
Turbulent flow
Another important keystone from a potable water hygiene viewpoint is the dynamics of water movement in the potable water supply system, which is characterised by water exchange and flow-through (flow velocity). Even under less favourable environmental conditions with regard to temperature and nutrient supply, a correspondingly slow microbial growth can occur if enough time is available, i.e. if the water movement is low and/or the water in the pipeline is partially stagnated. Especially in oversized pipes, there is a risk that there will be a laminar flow only in the centre of the pipe, and thus the required water exchange cannot be guaranteed at the pipe walls.
In the interest of potable water hygiene, it is therefore important to dimension the pipes as small in diameter as possible and to combine them with parts, fittings, and wall plates with low Zeta values (smallest pressure losses). Zeta values are pressure drop values that should be given by manufacturers after being determined in accordance with recognized standards (see local legal requirements); i.e. in Germany the calculation method is given by DVGW W 575, in Italy by UNI/TS 11589. Pressure drop values are directly connected to the shape, geometry, material and design of piping components; Furthermore, the pipe network should be designed as straight-forward and clearly structured as possible. Any pipe section that is no longer used shall be cut off at the T-piece with regular flow; replacing the T-piece by a straight pipe section is recommended.
For the dynamics of water movement, the pipe network should be designed as straight-forward and clearly structured as possible.
Precautionary principle
To avoid contamination within a potable water supply system, specific procedures must be followed. This is particularly important in facilities housing people with weakened immune systems, such as hospitals, retirement homes and care homes.
In order to minimise the expense required to remove the contaminants from the outset, it is necessary to avoid the introduction of impurities as far as possible during installation or repair work. To do this, all system components comprising the potable water supply system are to be transported and stored in such a way that internal contamination from earth, mud, dirty water, pathogens, small animals, etc. can be reliably avoided during installation.
Additional requirements or measures:
- All pipelines, fittings and appliances are to be delivered with protective caps during transport.
- Press connectors are to be delivered in shrink-wrapped transport bags. After the bags are opened, they are to be stored in airtight containers (also in the material container).
- Fittings must be sealed with protective plugs or caps to ensure they remain airtight until installation.
- Despite the previously described measures having been taken, any fittings, appliances, pipelines or press connectors which show any visible residues or deposits are, under no circumstances to be installed, but are to be cleaned and disinfect or removed from the construction site.
- Furthermore, it must be ensured during assembly that all pipe ends and fittings are sealed with protective caps during all breaks in work.
Potable water is the most important asset. This should also remain the case when operating the potable water supply system, i.e. the operator must keep the potable water fresh by ensuring the regular operation and use of the potable water supply system. This begins as early as the filling of the potable water supply system. From the outset, it is essential to avoid a lack of water exchange in unused pipelines (e.g. guest toilet, outside tap), otherwise there is an increased risk of microbiological contamination.
Special-use buildings, such as hospitals, retirement homes, kindergartens, schools, also require a hygiene plan. This hygiene plan must contain information and instructions for the increased requirements of the maintenance measures and actions to be taken in the event of faults and incidents.
Not every technical abnormality leads to a defect that requires remediation. However, in the case of technical abnormalities, an assessment of the abnormality must be carried out even if there is no microbial abnormality. Immediate remedy of technical defects is indicated in order to avoid a difficult to remedy bacterial contamination as a result of the defect.