Airline toilets recirculate sewage & chemicals
Recirculating toilets do exist in the marine and RV market...but they've never been very satisfactory. Danforth (same people who make anchors) tried one in the '60s...Monogram made the "Monomatic" until the late '70s...and Thetford currently has one on the market--their "Electromagic." Recirculating toilets on airplanes don't smell all that wonderful...scaling down to fit on a boat does NOT improve the situation! They all work pretty much the same way: an initial "charge" of 1-2 gallons of chemical and water that just recirculates through the bowl, adding sewage--just sewage, no additional flush water--to it till the system is full. Since they only hold a total of about 5 gallons including the "charge" anyway, you're far better off with a "pumpable" 5 gallon portapotty.