Might I also suggest that it needs to be properly situated away from your water supply.
This is a point brought out in "Cottage Water Systems" by Max Burns. This guy is a little off-the-wall in some regards, but his advice about properly situating the components of the system made me realize that I needed to rethink my system out at the farm.
Keeping you outhouse far from your well is something which should not even have to be said, but I recall reading that American soldiers in the revolution became healthier when a German general showed them you need to put the latrine on the opposite side of camp from the kitchen.