To the best of my knowledge all plumbing codes within the United States will require a trap in the drain line leading to your processor. This will prevent sewer gas from entering habited space. The water in the trap will prevent some back flow resistance to you your washer backing up into the processor. Additionally there is a "flapper type check valve" that is usually installed in the outlet line of a sewage pump. The ones that I have experienced are usually 2 in IPS. They work very well insofar as back flow but they do present some minimal resistance to the gravity flow from your processor.
Originally Posted by Jan Brittenson