A recent customer raised the question about whether they should or should not install a P trap on the outlet piping of their trench drain. The answer varies depending on where the trench drain is located. If the trench drain is outside the building and carries storm water then the answer is NO. You do not want a trap on a storm water trench drain. The reason for a P trap is to stop foul air from coming back up the pipe. This is not a problem outside the building because the downstream end is usually open to air and you are not confined inside a building. A P trap on a trench drain in this application will only cause clogging and maintenance issues.
On the other hand, if the trench drain is inside a building then ALL outlets need P traps. The trench drain outlet piping inside a building is connected to a sanitary sewer system and these pipes have sewer gasses that will back up into the building and create foul odors. The P trap is a water seal that keeps the sewer gasses out while allowing water to pass. It should also be noted that most if not all plumbing codes will mandate a P trap on a trench drain (or any drain for that matter) outlet inside a building. Note that when you add a P-trap you will also want to add a vent and a clean out (check plumbing codes for your area).