Information for Businesses

Backflow Causes

Backflow is the undesired reversal of water flow between the city's supply-system and a private-system. This can cause contaminants to enter into the drinking water supply system. There are two causes for backflow: Back-pressure and Back-siphonage.

Back-siphonage is backflow that results from a negative or reduced-pressure in the water-supply system. This occurs when four conditions exist simultaneously in a water supply system:

  • Pressure in the water distribution system falls below that which is found in a private plumbing system;
  • An open water supply valve;
  • The outlet of the open water supply valve is immersed in a contamination source (e.g. chemical mixing tank in an industrial building);
  • Absence of protection from Back-siphonage (this includes the malfunctioning of an existing device);

Some typical scenarios that can cause Back-siphonage are:

  • Breaks in a water main;
  • Drainage of the water supply system for repairs;
  • Shut-off of high-raised water supply;
  • Nearby fire fighting;

 

Back-pressure occurs when four conditions exist simultaneously in a water supply system:

  • The pressure in a private water system rises higher than the water supply pressure.
  • An open water supply valve;
  • The portable water supply is connected to a device or system that contains a contamination source;
  • No backflow prevention device installed (this includes the malfunctioning of an existing device);

Some typical scenarios that can cause back-pressure are:

  • Water pumps/boost pumps (e.g. ship docks, marinas, industrial plants, etc.);
  • Boilers with increased temperature;
  • A private water supply system inter-connected with public water supply systems;
  • Elevated tanks or other water pressure altering scenarios;