Martin Baxter wrote:

I think you could make an argument for a trap if you are using a holding tank,
I'd rather have some of the nasty gasses that form in a holding tank choose
to exit the system via an outboard vent than the interior sink drains.

If you had a straight gravity flow into th eholding tank with no valves or
anything, you could make a case for a trap. Personally I would think at least a
check valve ought to be in there.

DSK