Suppose a dog was roaming in your yard and did his business on top of a sprinkler. That evening it rained and part of the doggy-do washed into the sprinkler. Without a backflow prevention device, that contamination could be siphoned into your household water and into the city water main. So, what do you think you should do? Not only should you have one, it should be certified annually to make certain it is still functioning properly. Case in point. My city requires annual inspection and certification. (Failure to do so results in the water being shut off) When I had my double check valve backflow preventer inspected this spring, the inspector found both check valve pistons had worn to the point they would no longer hold the prescribed pressure. Total tab for inspection and repair was $50.