Holding tank
These installations run from simple, just a holding tank which is emptied via pump out through a deck mounted fitting to more complicated which in addition to the deck fitting also have an overboard discharge.If the bottom of the tank is below the waterline, a pump must be fitted to allow overboard discharge. If the bottom of the tank is above the waterline, gravity can carry the effluent overboard.If you can get hold of a West Marine master catalog, you'll find diagrams of several different installations as well as the parts and materials needed for each.