The 1.5 gallon injector is approximately 13" x 10" x 8", and is usually placed below ground in a second casing next to your drip system valve unit.
It is installed between your water supply line and the existing irrigation system valve unit. The pressure of the water into the valves draws the liquid fertilizer from the injector unit into the irrigate flow.
You must have a Back Flow Preventor or a Double Check Valve on the incoming water line to prevent the possibility of back flow into your main water supply system. Pools and ponds with auto fill systems must not be connected to the irrigation system for the same reason. A Double Check Valve works best, as it can be placed in a valve box below ground level. It will not leak, it is out of sight, the water district considers it safer than the back flow preventor.
However a Back Flow Preventor, according to code, should be at least 12 inches above the highest sprinkler in the yard. Thus if you have a raised flower bed in your back yard and you have a system that waters this flower bed you must raise the Back Flow Preventer 12 inches above that, placing the Back Flow Preventor at or above the windows of a single story home. Also Back Flow Preventors are not typically sold at "home improvement" stores, but may be found at some nurseries and sprinkler stores.
We cannot install an injector if you have an Anti-siphon Valve - example on the left. On the right you can see that someone replaced a Back Flow Preventor with an Anti-siphon Valve. You can tell it was replaced by the way the pipe goes out on the left. Not only is this illegal, but can be dangerous.
All homes that were built with a Back Flow Preventor must either have this or a Double Check to meet code.