Our boat, a 42-foot Grand Banks built in 1980, has a relatively large, factory-built holding tank of about 65 gallons installed between the engine stringers, right over the center bilge. With one exception, this setup has worked well for us, particularly since we use a VacuFlush head. I figure we have at least a one-month holding capacity.
The flaw in the system became obvious when we started rafting with another boat on our starboard side, where the holding tank vents. By its nature, a VacuFlush pumps a bit of air into the tank with each flush. This resulted in a foul effluvium coming from the vent, with the attendant coughing, waving of hands in front of the face and jokes about the quality and quantity of our boat's flatulence.
The blue liquid tank treatment, a mixture of formaldehyde and methanol, didn't really help much. If the tank was almost empty and freshly charged with the stuff, the odor wasn't too bad, but it got worse quickly.
I had thought for some time that aeration of the tank was the answer, because this is what our aboveground septic system does at home, and it never smells. This also is how large municipal sewage treatment plants work. The question was how to get the air into the tank without doing something drastic like cutting a hole in it. Fortunately, Murphy took a holiday and luck intervened-the dip tube into the tank rotted off.
The original manifold on the forward end of the tank containing the vent, pumpout and forward head discharge was made of bronze, and it finally corroded through. I had a new one fabricated from stainless steel, and added a 1-inch port- simply a threaded coupling-to the manifold. Now I had my air port! The rest was easy.
I visited the local pet store and purchased the largest, quietest aquarium air pump I could find, together with a bubbler tube about 1 foot in length (also called an air curtain), a couple of feet of hose and a non-return valve (important). I had a close nipple and PVC cap for the new holding tank port, so it was a simple matter to drill a hole in the cap for the air tubing; insert the air curtain with hose attached into the tank so it sat on the bottom; hook the whole works up; and plug in the air pump. I am fortunate enough to have a 110VAC outlet in the engine room.
How does it work? After a full summer's cruise, plus a few weeks in the fall, I am happy to report that it exceeds my wildest hopes- there is virtually no holding tank odor at all. I add nothing else to the tank-no deodorant, no blue liquid, no enzymes, nothing. I leave the air pump on all the time except at night at anchor, when I turn the inverter off. There are 12-volt air pumps available if 110VAC power is not available. They can be run continuously, and the power draw is negligible.
It turns out that great minds think alike. Groco has come out with a similar system called "Sweet Tank," which uses a small air pump and diffuser. The principle is quite simple: Noxious odors are caused by anaerobic bacterial decomposition of the sewage in the absence of oxygen. The bugs give off smelly gasses such as hydrogen sulfide and methane, the main ingredients in-er-flatulence.
By simply adding oxygen through aeration, the balance shifts to aerobic decomposition, which does not produce noxious gasses at all. If a chemical such as formaldehyde is added, all decomposition stops, and the odors return. Letting in a little fresh air works wonders!
Ken and his wife Harriet live in Nevada but spend summers cruising along the west side of Vancouver Island in British Columbia aboard their GB, Fourth of July.