Using an open outlet to separate process water from mains drinking water

Clean water is essential to human life. That is why many countries take special measures to protect the public drinking water supply. Once water has been extracted from the mains water system, it can become exposed to contamination or colonization by microbes. For this reason, it is absolutely essential to prevent water from systems connected to the mains from flowing back into the mains supply.

An "open outlet" is the most reliable method of preventing backflow. According to the relevant technical standard, an "open outlet" is an "unhindered flow path (air gap)", i.e. the water falls freely downwards into a buffer tank from which it cannot flow back into the mains system.
Since the water pressure decreases sharply as it passes through the open outlet, a pump is installed in the buffer tank to generate the necessary system pressure. The volume of the buffer tank, the pump type and the piping cross sections are carefully selected to ensure that water is delivered at the required pressure and flow rate.

Throughout Europe, the general requirements of devices to prevent pollution by backflow are defined by the standard EN 1717. It has been adopted into German law as DIN EN 1717 (i) and DIN 1988-100 (ii). EN 1717 divides fluids which might come into contact with drinking water into categories according to their potential danger to humans – the higher the danger, the higher the category number. These fluid categories are numbered from 1 to 5. Category 1 refers to water suitable for human consumption. Fluids that present a human health hazard due to the presence of microbiological or viral elements fall into category 5.

The standard prescribes appropriate safety devices depending on the potential hazard. These are divided into types and identified by a combination of two letters.

According to the European standard, category 5 fluids must be separated from the drinking water supply by safety devices with an open outlet of type AA, AB, AD or (subject to restrictions) type DC. The German standard gives specific examples of applications involving fluids that fall into this category, including, for example:

  • Active carbon filters in chemical apparatus
  • Vessel filling, e.g. tank trucks
  • Sprinkler systems, underfloor systems
  • Fish tanks
  • Meat- and fish-processing machines
  • Electroplating equipment
  • Glass washers, e.g. at bars
  • Commercial cooking appliances, water baths, cooking kettles, convection ovens
  • Potato peeling machines
  • Cooling circuits, cooling towers
  • Rainwater harvesting equipment
  • Swimming and bathing pools (filling and topping up)
  • Animal water troughs

(i) DIN EN 1717:2011-08, Deutsches Institut für Normung e. V. (German Institute for Standardisation)

(ii) DIN EN -100:2011-08, Deutsches Institut für Normung e. V. (German Institute for Standardisation)