Best Backflow Preventer Certification in Washington DC

Called them to do irrigation system blowout which was fine but the tech didn't instill any confidence with his expertise - first he was video chatting with someone asking some question when at the backflow device location (I have a very standard setup) and second he wanted to let me know that since I had already taken out the backflow device, I shouldn't use the system as the irrigation water could flow back into the city supply contamination the water (yeah, the water will jump through the giant gap where the backflow device was). Undecided if I want to use them for future needs.