Safety valves guidebook

Maintaining drinking water quality

Connections to third-party systems

In most cases, a drinking water installation comprises several connections to other systems with more or less hazardous media. For example, the connection between hot and cold drinking water or the connection between the drinking water and heating system. Furthermore, there are delivery points, such as drinking troughs, which have special requirements for the connection to the drinking water installation. The Drinking Water Directive states that a drinking water installation must not be connected to installations in which non-potable water may be suspected “without a safety device that complies with the generally accepted codes of practice”. The technical measures for protecting our “number one food” are defined clearly in the guidelines. In daily practice, the requirements specified in said guidelines are applied to the local conditions, which frequently raises uncertainties. There exists no safety device that is suitable for every installation with requirements for water quality specific to local conditions and that is suitable for all tapping points. An expert must analyse and evaluate the potential risk of the specific installation situation and select a safety device that fulfils the relevant requirements. This may be an anti-pollution check valve, an anti-vacuum valve, an RPZ backflow preventer or a break tank station .

DIN EN 1717: Classifying risks

The nature of the fluid whose backflow the drinking water is to be protected from should be considered an important criterion when selecting a safety device. The nature of fluids is classified by DIN EN 1717 and its supplementary standard DIN 1988-100 across five categories.

The following definition of the fluid categories in accordance with DIN EN 1717 describes the risk of the contamination of the drinking water, which increases as the categories ascend.

Human consumption or health hazard?

Fluids intended for human consumption and water that may have only been modified with regards to taste, odour, colour or temperature should be assigned to category 1 or 2. Category 1 and 2 fluids present no risk to human health. Hazardous fluids belong to categories 3, 4 and 5.

Boundary between fluid categories 3 and 4

Hazards presented by substances (generally chemicals) are assigned to categories 3 and 4. The boundary between categories 3 and 4 is defined on the basis of the median lethal dose of LD50 and the median lethal concentration of LC50. These values are determined for chemicals on the basis of animal testing and reported by the manufacturers in safety data sheets. If there is uncertainty regarding the assignment of a fluid to category 3 or 4, it is advised that the safety device that provides protection against the higher category 4 be selected. Radioactive, mutagenic and carcinogenic substances explicitly belong to category 4. Category 5 fluids present a health hazard due to microbial or viral pathogens. The health hazard presented by these fluids must be viewed as critically as hazards presented by category 3 and 4 fluids.

Fluid category 5

The higher classification of category 5 is justified where there is growth of microorganisms. This is because microorganisms are capable of multiplying exponentially under favourable growth conditions. This means that a hazard presented by a category 5 fluid can increase over time. In contrast, the hazard presented by category 3 and 4 fluids remains the same or increases as water is diluted. The decisive aspect in differentiating categories 3 and 4 from category 5 is that, through their multiplication, microorganisms in drinking water installations propagate in retrograde fashion, that is, against the direction of flow. This sets special requirements for protecting drinking water from category 5 fluids. The higher classification of category 5 does not directly indicate a higher health risk, but rather the risk of impacting the drinking water quality against the direction of flow.

What must be considered when classifying?

When classifying a fluid, it must be taken into account that external conditions such as increased ambient temperatures, unfavourable material characteristics and irregular flow can provide growth conditions for microorganisms. During on-going operation, conditions may arise against which some safety devices do not provide sufficient protection, despite an original risk not being microbial in origin. Therefore, it is vital to enquire about the local use and operating conditions and consider the information when selecting the safety device.

Defining the scope of the Drinking Water Directive

When using a safety device to protect against category 3, 4 and 5 fluids, the water on the outlet side of the safety device does not fall under the scope of the Drinking Water Directive. Therefore, according to the following figure, no requirements or limits for the quality of the water are set.

Despite this, the water on the outlet side of an RPZ backflow preventer, for example, can still be used as food. However, this means that it is mandatory that components also be used on the outlet side of the safety device that are suitable for use in drinking water installations with regards to the material characteristics and intended use is maintained.

Operating principles for providing protection

Fluid categories 2, 3 or 4

Safety devices that are approved for providing protection in fluid categories 2, 3 or 4 work according to the principle of mechanical separation, which may be complemented by an atmospheric separation. In the principle of mechanical separation, the output side of the safety device must always be connected to its input side. Sealing surfaces or mechanical components are intended to prevent backflow from the output side to the input side. However, as mentioned above, microorganisms are able to propagate in a retrograde manner, meaning that mechanical separations can be overcome. Safety devices that exclusively ensure a mechanical separation are therefore not suitable for providing protection against category 5 fluids.

Fluid category 5

To provide protection against fluids that present a health hazard to humans through diseases that can be transmitted through microbial or viral pathogens, complete atmospheric separation from the drinking water must be ensured. Type AB break tank stations for providing protection in category 5 meet this requirement. Atmospheric separation in an open container ensures that drinking water and category 5 non-potable water do not come into contact with each other. Due to a constant entry of oxygen, the irregular water exchange and irregular flow into these containers, the drinking water quality does not need to be maintained in accordance with the Drinking Water Directive[1]. Such safety devices are not permitted for use with tapping points at which water is intended to be used, e.g. for cleaning the body.

Most common safety devices

The most common safety devices in DIN EN 1717 are type EA anti-pollution check valves, type CA backflow preventers, type BA RPZ backflow preventers and type AA/AB free drains.

Maintaining safety devices

Nothing lasts forever! Certainly not without maintenance!

Regular inspection and maintenance is necessary to ensure long-term functional capability of technical devices. This is just as true of safety devices in drinking water installations as it is of cars and heating systems. Inspection and maintenance measures for the different safety devices with corresponding intervals are specified as is mandatory in DIN EN 806-5[5]. The main point of these measures is to check the proper functioning of the safety device. Furthermore, additional installation parts, such as dirt traps, must be cleaned and the ambient conditions of the safety device regulated. This is because, in addition to a negative impact caused in the protection against backflow, critical operating states may also arise due to external influences. A clear example of this is safety devices with dried out anti-siphon traps. These present a risk of bacteria growth due to microorganisms which enter from the wastewater system via channel gases. Recurrent inspection of safety devices by the expert technician will eliminate such risks. Otherwise, safety devices which have no direct influence on criteria for comfort and are not noticed by the operator on a daily basis are forgotten about. Therefore, the designer or plumber must make the operator of the installation aware of the necessity for inspecting and maintaining the drinking water installation and in particular the safety devices. There is little room for interpretation for the operator due to the clearly defined measures and intervals.