It is fundamental to keep the potable water in the water supply systems uncontaminated
It is very important to keep the potable water in the water supply system uncontaminated. A cross-contamination between potentially contaminated water and public water supply systems can occur when the pressure in the supply system is interrupted by a water main break, power failure or some other disruptions in the water service.
When there is a drop in water supply pressure, water can flow back into the public water supply system - so called "back-siphonage" or "backflow". If the backflow water is contaminated, the public water supply system will be contaminated.
Typical sources for cross-contamination:
- a hose connected to the water supply system is submerged in polluted water
- a boiler with chemical treated water is connected directly to the water supply system
- tanks, baths, swimming pools, cooling towers or similar - are connected directly to the water supply system
- irrigation systems connected to the water supply system
Cross-contamination can be prevented by avoiding direct contact between contaminated water and the water supply by in using
- vacuum breakers in pipe lines or hoses
- air gaps between fixtures and potentially contaminated water surfaces
- check valves
Back flow of potentially contaminated water to a supply system can occur due to siphoning if the pressure in the system suddenly drops to a lower level. A vacuum breaker installed in the piping or in connection with the hoses will siphon air from the surroundings when pressure drops below atmospheric pressure, preventing contaminants from entering the system.
A physical separation like an air gap between a fixture and a open water surface in a sink or similar prevents contamination entering the fixture outlet and the water supply system.
The air gap should in general be larger for larger fixtures and closed surfaces (surface with walls) than for smaller fixtures and open surfaces. It is common that
- the air gap for a fixture of size 1/2" should be at least 1" for an open surface, and at least 1 1/2" for a closed surface.
- the air gap for a fixture of size 1" should be at least 2" for an open surface, and at least 3" for a closed surface.
A check valve will prevent water from flowing back in the wrong direction.