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He already has antisyphon valves, and if they are properly located, he has protection superior to what a DCVA can provide. If you can feed a mortuary table through a vacuum breaker, that speaks well of its ability to protect a water supply. But, since he has changed the story to now want fertilizer injection, nothing less than an RPZ will do. That goes in no matter what the city wants. If the city demands a DCVA, or else, then you give up on fertilizer injection, or go to the extreme of installing both a DCVA and an RPZ............I disagree with boots and highly recommend you get a backflow preventor into your system , what you choose is your choice and the codes in your area. ( Such as the double check, Reduced pressure back flow, Pressure Vacuum Breaker, Spill Resistant vacuum breaker. Anti-siphon valves are not to code on proper back flow protection. The main reason for this is because they are Non - Testable devices so you never know if it's truly working or not.