This issue has come up before. Pasted in below are our prior comments:
"My two cents is that you may want considering plumbing
your liquids tank to the holding tank, instead of dumping the liquids tank fairly often (depending on how much you use the head).
We live aboard our boat all summer at a mooring
in a crowded harbor. Dumping the liquids tank overboard
may or may not be illegal, but is, at the very least, rude to ones neighbors. Hence, we plumbed our Airhead liquids tank directly to our holding tank and vented the composter thru a small mushroom vent in the head
. We like this arrangement for we only have to pump out the holding tank liquids every three weeks.
Just a thought. We're not the first composter owners who have done this.
Cheers, and enjoy the new composter. Once you get the hang of using it, you'll never want to go back to a conventional head with its finicky plumbing