Additives for Grey Water Holding Tank

We don't smell any odor inside the TT from the grey water tank, but when I dump it, there is a noticeable odor, even though I have been using the Grey Water Odor Control (lemon smell or some such). The odor when dumping kind of surprises me, as 1) the wife doesn't put greasy water or water with a lot of food debris down the sink, and 2) we always dump the tank right after camping for 4 or 5 nights.