Fresh Water Flush--a Miniature Water Closet?

Seawater contains bacteria and sulfate. Flush with seawater, deprive the bacteria of oxygen for long, and facilitative (those that can go aerobic or anaerobic) start using sulfate (SO4-) as an electron source. The result is H2S (rotten eggs) and the head stinks. The cure? Either use the head every 8 hours (thus, smell is not a problem when living aboard or actively cruising) or flush with fresh water.

But you can just plug the freshwater pipe into a manual head; water will blow through, flooding the compartment, and bugs can swim up-stream. The water closet, introduced a century ago, prevents both by employing a float valve and creating an air gap (bacteria can't fly, as a rule).

What about a miniature water closet? $38 from McMaster Carr

Made for commercial ice machines, this unit holds enough to flush 3' of 1.5 inch line--not enough for most manual heads--and refills at 0.33 gpm, or about 1:20 seconds per full flush (12 strokes or so). Clearly, a larger size, holding about 60 cubic inches, is what is needed. It could also be expanded by mounting a 1-3' length of 3" pipe under it to serve as a reservoir (4' of hose can be flushed for every 1' of 3" pipe). Mount it to a bulkhead somewhere handy and you should be good to go. A pair of valves would allow switching from seawater and isolation.

But I haven't tried it. I'm happy enough with sea flush and a rinse with potable using the shower head before going home.