Water that flows opposite to its normal direction is called backflow, a problem that can accidentally contaminate drinking water in your home and, possibly, the entire public water system. Backflow conditions are potentially dangerous, but preventable.
Common household hazards
Chemical spray applicators: Garden hoses carrying potable water can have a chemical spray applicator attached to dilute lawn and garden chemicals like pesticides, herbicides, and fertilizers. These can be toxic or even fatal if ingested. Should backflow occur while using this type of a product, it may contaminate the public water supply.
Submerged hoses: Hoses submerged in buckets or containers like pools or ponds present a common hazard in the event of a drop in water pressure. This is due to the possibility of chemicals and micro-biological contaminants being present in those containers.
Underground lawn irrigation systems: The puddles of water that often surround the bases of ground level sprinkler heads may contain micro-biological contaminants as well as chemicals from fertilizers and herbicides. These sprinkler heads are not designed to prevent backflow in the event of a drop in water pressure.
Eliminating the chance of backflow
The two easiest methods to prevent backflow at home are:
Maintain an air gap
An Air Gap is the physical separation of the potable and non-potable system by an air space. The vertical distance between the supply pipe and the flood level rim should be two times the diameter of the supply pipe, but never less than one inch.
Install a hose connection vacuum breaker
This is a small inexpensive attachment usually placed between the garden hose and your hose bib/spigot on the wall of the house. It prevents water from being back-siphoned into your home's internal plumbing system and potentially the public water system.
Commercial and multi-residential property owners
Plumbing codes have always prohibited any connection that could cause potable and non-potable water to mix.. Numerous applicable standards are in place to ensure backflow prevention devices meet acceptable standards; they include
CSA, AWWA, ASSE, ANSI, & IAPMO.