There are several good discussions on the Airhead composting toilet archived on this forum; e.g.,
http://www.tugnuts.com/viewtopic.php?f=2&t=1599&hilit=airhead
To review my experiences along with two other Ranger owners, except for problems as the result of incorrect installation (like putting the fan in backwards which blew air through the toilet INTO the cabin-Duh!) we've all been happy. Household composting toilets are huge because they allow urine and feces to mix, which results in the gross stuff you get pumped out of your septic tank. When the "solids" are separated in the Airhead, agitated with a half turn of the hand crank and then left in the presence of a gentle venting breeze they quickly decompose without odor. Amazingly, you get about 70 uses of the solid tank which translates to a full-time month aboard for two people who each do "you know what" once a day.
I met a couple cruising full time on a catamaran; the captain would dump the contents of the airhead solids tank once a month into a five gallon bucket with a loosely fitted top he strapped it to his forward deck. In another month he could dump it out as 3-4 couple of coffee can loads of compost onto a little garden his girlfriend also had on his huge foredeck. I have a spare solids tank, so I can pull out the full tank add a few grams of enzyme to quicken the final decomposition and a week or two later shake out the contents on my lawn which has zero smell and looks like dry, granulated peat moss. I do this twice a year since I only use the boat weekends or for a few weeks at a time in the Keys. Yup, twice a year I empty my tank! I don't dump it overboard because it's loaded with nutrients which is bad for sea life. On Cape Cod we have many dead kettle ponds, too much nutrient (e.g., from septic or fertilizer overflow) in a pond = algae which depletes the oxygen killing the fish.
Now, the liquid tank is another story. Urine doesn't break down, so it adds up fast with two people aboard, even with low beer consumption you'll need to disconnect the liquid tank, put its cap on, and carry it up to a marina's toilet and empty it every 2-3 days. You can empty if overboard–more about that later. There is some smell, of course, which oddly can be abated by pouring a few ounces of sugar into the empty tank before use. Any way you look at it, there is some hassle with the liquid tank. I had removed the hoses to the old standard head (capped the sea water inlet at the strainer) and clipped off the hose from the macerator that went to an overboard through-hull fitting and left it dangle. I attached the outbound hose fitting on the macerator to a length of hose which I can direct to the fresh water tank's inlet hull plate. I use the macerator to pump clean water stored in the ex-holding tank to the main fresh water tank, when needed. I fill the the ex-holding tank by pouring water into the sewer discharge fitting (on the transom of an R25) by simply setting the Y-valve correctly. Sort of in reverse. Of course, you do this modification only on a new boat where the holding tank has, hopefully, never been used.
Now to my point. How to zero-out the hassle of dealing with the urine tank which needs emptying every couple of days. Sure you can keep a spare tank on board, and every 4-5 days head down the dock with a tank of sloshing pee in either hand yelling out "Gangway!" or "Bringing out the dead!" Having done this, I recently figured out a way while chatting with Mike (Knotflying) how to make this easier. Geoffrey, the sailor who invented the Airhead installed a female nipple at the bottom of my spare liquid tank for very low money. Its made of thick plastic. I affixed a half of a plastic two-part quick release coupling to that nipple. The other half of the quick release coupling had a hose barb which I connected to 5/8" plastic tubing, of which I left a few feet hanging under the head's sink, ran through a hole in the floor past the R25's water heater, down along the starboard deck lazarette, turning the corner at the transom and then snaked the tubing (dousing it with WD-40) through that dangling old macerator hose which led the tubing neatly past the batteries until it came out the other end of the hose which I had detached from the macerator through-hull plastic fitting. A brass tubing barb to a standard garden hose fitting connected the tube to the through-hull fitting. I could have added an electric pump to periodically evacuate the liquid tank, but urine does a job on neoprene seals... Also, more electric stuff I do not need. Instead, every few days I disconnect the liquid bottle, open the under sink lazarette and pull out the tubing, set the bottle on top of the toilet, connect the quick disconnecting coupling and let gravity drain the bottle right out the macerator's discharge fitting on the port side of the boat.
Now, before anyone gets upset about how gross this is remember there are no nutrients in urine. Field medics and surgeons all know to use it as a sterile solution in preference to contaminated water. Peeing overboard is only dangerous to the moron standing on the swim platform. When I visited with the chief harbor master in Key West, FL (the Keys are PARANOID when it comes to holding tanks and the clarity of their water) and told him I had a composting toilet, he suggested that I should just pour the urine component overboard
in his marina to avoid the long walk to the head. Folks are getting more sophisticated, now.
Anyway, I figure this is about as hassle free as it gets regarding boat head maintenance. No tubes or fittings to foul or eventually loosen and smell up the engine compartment or bilge areas. No hauling 30 gallons of smelly gook around and looking for a pump out guy every several days. No posting onTugNuts about how to know if your tank is too full when the plastic turns brownish black and you can't see the level in the holding tank. No having to lift the engine compartment lid to try to see if the tank is full. Gain back a couple of knots of speed by eliminating 250 pounds of waste (if you don't need the extra fresh water) and leave it empty. Or, carry 30 ga of extra diesel, instead of water, in your old waste tank for that trip to the Bahamas. No hassles with the coasties when they see your Y-valve goes to the fresh water tank, or just open the engine compartment and show them a pristine "never used" holding tank.
The $1000 Electro Scan unit can also eliminate your holding tank. But, you need space for it and it requires a 60 amp fuse-think about what that means in power draw. It runs 2.5 minutes and averages about 1-2 amp-hours per flush. Two people times five flushes per day each is about 20 amp-hours used per day... Keep your generator fired up! Oh, you don't have a generator, then you have a problem. And, you have to clean it out with acid every 6 months, change the plates (for $700) every 3000 flushes. Now you have another electrical/mechanical system onboard to maintain.
It's hard to wrap your mind around it, but the Airhead is as simple as it gets (basically a bucket with a fan) and for a one-time investment of about $1000 you are done. There is no maintenance, unless the tiny fan (I think it draws 0.2 amps to compensate for I keep a cheap 15 watt solar panel plugged into a cigarette lighter) fails. The sail boat guys have used them for years, and I think most of us have to admit (having been there in my "yut") they know a thing or two. The only hassle is the urine tank, and that can be overcome as explained above.
