Drinking water hygiene

1. Flow rate

What does biofilm mean in the context of domestic water installations?

All wetted surfaces in domestic water installations are populated by biofilms. Drinking water biofilms are generally formed by the microflora naturally found in water, which has no direct effect on human health. However, they can provide a refuge for microbial pathogens. As a biofilm in domestic water installations is unavoidable, they should be kept as mechanically stable and compact as possible with sufficient flow. You can find more on the topic of biofilm here.

Sufficiently turbulent flow profile

Having sufficient flow in domestic water installations minimises the growth of microorganisms. In stagnation phases, bulky biofilms can form. These can be dissolved by water hammers, for example. The growth of a hygienically critical biofilm can be limited by a sufficiently turbulent, regular flow in all pipe sections of the domestic water installation.

Influencing biofilm through flow velocity

High flow velocities act upon the structure of the biofilm. Due to the considerable shear forces, a turbulent flow encourages the formation of a compact and mechanically stable biofilm that is highly resistance against dissolving.

Demand-oriented dimensioning

Sufficiently high flow velocities can be ensured by demand-oriented dimensioning and operation according to the provisions of the floor plan and materials catalogue. This encourages the water exchange required by the relevant standards and prevents stagnation.

Observing the floor plan and materials catalogue

When dimensioning the pipes, the simultaneity of use and the flow rates must mainly be used, taking into account the floor plan and materials catalogue. Only in this way can a demand-oriented pipe network be achieved with minimal pipe network parameters relevant to hygiene (nominal content, wetted pipe surfaces).

2. Temperature

Temperature is one of the most critical factors in relation to microbial growth. Determining temperature ranges that produce unfavourable living conditions for facultative microorganisms and therefore form the basis for preventative health protection is very difficult and depends on several factors. Cold water temperatures of <20°C and hot water temperatures of >55°C are optimal, as ensured by numerous studies.

Excerpt from DIN 1988-300

“When used as intended, the temperature of the cold drinking water must not exceed 25°C after the tapping point has been fully opened for a maximum of 30 s and the temperature in the hot drinking water must reach at least 55°C.”

Cold water temperature below 20°C

A temperature of <20°C is regarded as hygienically sound for cold water in, for example, the DVGW Water Information 90, technical publications of the German Environmental Agency, the RKI and many international drinking water guidelines and recommendations.

You can learn how cold water temperatures can be permanently kept below 20°C in a resource-friendly way on the cold water circulation page.

Excerpt from DIN 1988-300

“For hygienic reasons, a temperature of at least 60°C must be maintained at the outlet of the water heater with circulation. Circulating hot drinking water must not undergo a temperature drop greater than 5k.”

Hot water temperature above 55°C

In accordance with the generally accepted codes of practice, the temperature in the hot drinking water (PWH and PWH-C) must be at least 55°C at all points of circulating systems. The volume of hot water, which cannot be held at this temperature, must be minimised. If the volume of a flow path exceeds 3 litres, circulation must be provided in order to maintain temperature. You can learn more on the topic of hot water circulation on the hot water circulation page.

Growth rate of Legionella pneumophila

The diagram clearly shows that Legionella growth becomes critical at temperatures between 20°C and 55°C.

3. Water exchange

A major cause for microbial growth is stagnation. The cause of stagnation areas can be old, unused pipes or sections of pipes that are at times not used as intended.

Excerpt from standard DIN 1988-200

“The design must be such that, during intended use, water exchange sufficient for ensuring hygiene takes place.”

Regular water exchange

From a hygiene standpoint, domestic water installations must be designed and operated such that water exchange is ensured ideally every 3 days (VDI 6023), and at least every 7 days (DIN 1988). Water exchange is defined as a complete water exchange in the entire system (incl. containers and water heaters). So that stagnation can be avoided over the entire hydraulics cross section, a turbulent flow is recommended (see flow).

Stagnation and temperature interact

Depending on the ambient temperature and insulation of the pipe, the media temperature will gradually come to equal the ambient temperature. Therefore, it is important, especially in cold drinking water, to make sure that the structural design of the domestic water installation facilitates the water exchange. Floor and individual feed pipes must be regarded more closely in this case.

Avoid critical temperature maintenance

In the case of sufficient use, fresh drinking water will repeatedly flow, counteracting a critical temperature increase. In areas where the drinking water stagnates and no water flows, a critical temperature increase may occur in a very short amount of time.

Calculate water exchange with Flushtool

4. Nutrient supply

Low nutritional requirements of facultative microorganisms

Bacteria and other pathogenic germs are reliant on nutrients in order to grow and propagate. The factor of nutrients supply is very complex and very significant for the different types of bacteria and single-celled organisms. With the facultative microorganisms, we see types with very low nutritional requirements (e.g. Pseudomonas aeruginosa).

Selecting safe materials

It is particularly important to select suitable, hygienically sound materials for the whole area of the domestic water installation which do not facilitate the growth of biofilm. Stagnation can also considerably increase the concentration of available nutrients through migration from materials with water.

Selection of materials according to Drinking Water Directive

The selection of materials must be carried out such that the nutrients supply is reduced as much as technically possible and the predetermined chemical limits of the Drinking Water Directive are adhered to.

Suitability according to material positive lists

All materials must be tested for their suitability for drinking water usage. This “microbial suitability” is a basic requirement of Article 14 of the Drinking Water Directive and is applied by the German Environment Agency as an important criterion for creating material positive lists.

Flushing schedule - manual water exchange

Support for execution and form for documenting manual water exchange

Go to flushing schedule

Products to maintain drinking water hygiene

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