Composting Toilets on Boats: How They Work, the Rules, Emptying and Winter Care

Make sure the unit is bolted down with its brackets so it can't move in a seaway, the trap-door lever and crank turn freely, and the vent hose runs to the outside without dips or kinks where moisture can collect. Many owners vent through a deck or hull vent, or into a ventilated anchor locker; the vent must not end in a closed cabin space. If you replaced a holding tank, double-check that all the old seacocks are removed or closed and plugged, and that the old deck pump-out fitting is sealed.

The fan is what keeps odors out of the cabin, and both makers say to run it continuously while the head is in use. It draws very little: Nature's Head says under 1.8 amp-hours a day and Air Head about 1.4 amp-hours a day for their standard fans. Wire it to the house bank through a fuse or breaker sized per the maker's instructions, or use the maker's solar or AC options. Check that it spins and that you feel air at the outside vent.

Use the medium your maker recommends: organic sphagnum peat moss or coconut coir. Coir usually comes as a compressed brick that you crumble and soak in water until it fluffs up, then wring out so it's damp but not wet. Fill the solids bin to the level shown in your manual, usually just below the agitator shaft. Too much medium makes the crank hard to turn; too little leaves solids wet.

Every visit is a sit-down visit, because urine has to reach the drain holes at the front. Keep the trap door closed when only urinating, open it for solids, and close it again. A coffee filter on the closed trap door before a solids deposit keeps the door clean. Nature's Head notes that some users put used toilet paper in a small lidded bin rather than the toilet, which slows how fast the solids bin fills; either way is fine. Nothing else goes in: no wipes, tampons, or food scraps.

After each solids deposit, close the trap door and give the crank a few turns to bury the solids in the medium. Don't force it if it's stiff: the bin may be too full, too dry (add a little water) or frozen.

Urine smells worse the longer it sits and can stain the bottle. Air Head estimates that a couple using the head full time will empty the small bottle every day or two. Cap it tightly, carry it ashore and empty it into a toilet at the marina, a pump-out station's toilet dump if it has one, or a shore sewer connection. Rinse it and refit. Carrying a spare bottle with a sealed cap lets you swap and empty later. Never pour it over the side inside US waters, and don't route it to an overboard through-hull.

Makers estimate a bin lasts around 60 to 80 uses, roughly a month for two people living aboard, or a full season for weekend use. Nature's Head suggests waiting several hours after the last use; for a weekend boat, many owners empty it once a season. To empty:

• Wear gloves and a dust mask, and work in a ventilated spot.
• Remove the bin as your manual shows and tip the contents into a heavy-duty bag or compostable liner, then close it tightly.
• Take it ashore. A home compost pile used only for non-edible plants is one option; otherwise bag it for the trash if your marina or local trash service allows it. Ask first: some marinas don't accept human waste in their dumpsters.
• Leave a little old material in the bin if your manual suggests it, then add fresh medium.

Nature's Head recommends spraying a 1:1 vinegar and water mix in the bowl and down the urine holes after use, which stops scale building up in the urine trap. Avoid bleach, products containing bleach, abrasive cleaners, scrub pads and toilet-bowl chemicals; Nature's Head warns that they harm the composting process and can damage the bowl's coating. Keep toilet paper out of the urine holes.

A composting head should smell only faintly earthy. If it smells:

• Ammonia: the liquids bottle is too full or overdue, or urine is getting into the solids bin. Empty and rinse the bottle, and check that the urine drain is clear.
• Sewage: the solids are too wet. Add dry medium, keep the fan running, and make sure crew close the trap door when only urinating.
• Musty or stale: the fan has stopped or the vent hose is kinked, blocked or too long. Check the fuse, the fan and the outside vent.

Small flies can appear in warm weather when solids are wet. Keep the bin on the dry side, keep the fan running (it also helps keep insects away), put insect screen over the outside vent, and empty and restart the bin if they take hold.

Cold doesn't hurt the toilet, but it changes the routine:

• Below about 55°F, solids mostly dry rather than compost. That's fine; expect to empty more often.
• If the bin freezes, the crank may not turn. Don't force it.
• Freezing urine expands and can crack the bottle. Empty the liquids bottle before any freeze. Air Head suggests that if you use the boat now and then in winter, a little non-toxic antifreeze in the urine bottle helps.
• For layup, empty the bottle and either empty the bin or leave a dry bin to finish over winter, as your manual suggests, then empty it at spring commissioning. Nature's Head notes that if the fan runs for months with no use, the bin can dry out and the crank can get hard to turn, so some owners unplug it for winter storage.

Note medium changes, bin empties and fan replacements in BoatMend, so you know how long a bin lasts with your crew and when the fan is due.