Talk to a licensed plumber, he'll advise where the valve should go to protect your whole system. We have a great plumber we trust, so he knew our traps and lines and recommended a backflow valve out at the end of the sewer line, and we included a new box in the ground to make cleaning and maintenance simple (home repair). I did the digging and put in the concrete box, Dale tapped the line and put in the valve. There are a variety of valves to consider too, including ones that you can manually switch to block all flow, or those that just prevent backflow up the sewer line. We chose a version that has a bit of both, including a manual block if we knew something was coming and had time to turn the plug at the box, and pretty solid backflow coverage as well. Where we live isn't ordinarily subject to sewage backflow problems, but if the Green River had flood, millions of gallons of water would have exerted enough pressure on the sewer line to cause just about anyone to flood. Most of that flood risk is now abated, the Army Corp of Engineers is testing the Howard Hansen Dam as we speak to assess whether repairs the past two seasons have done the trick. I've cleaned up houses after floods, I sure didn't want to deal with any raw sewage in mine - a couple hundred bucks for peace of mind.