The purpose of a dry toilet, or desiccating toilet, is to keep the mix dry and odor-free, it's not meant to carry its content to a full compost. A composting toilet is much larger, and small sailboats often cannot negotiate the power and space required by such a unit. A dry toilet on a boat eliminates the need for a holding tank and accompanying pipes, thru-hulls, proprietary gaskets, y-valves and macerators. The system is attractive to small boat owners because it is easy to maintain and to repair. On land, dry and composting toilets have the potential to save on water, and preserve the precious by-product that is compost.
See our dry toilet installation.
Medium. A tank for solids needs a medium to help process deposits. The choice of medium depends on budget, space and availability. Some mediums, like coconut coir(husks, byproduct of coconuts), are sold compacted. The easiest materials to source are wood shavings and sawdust, they have good wicking ability, cover up odors well, and can be obtained cheaply or for free.
Another good cover material is wood ash, a by-product of wood burning. Wood ash eliminates smells best, and is great at removing moisture from deposits. Using this medium makes sense if your boat also has a woodstove. The downside is that ash is volatile and can make a mess. It's recommended as an add-on, not used as the sole medium. Another good cover material for smells is coffee grounds, but it's important to let them dry thoroughly before use. A general rule is to ensure the ratio of medium to human waste is 1:1.
Non-churn systems have no agitator, and rely on the addition of more medium after every deposit. If enough medium is added there won't be any smells, even without a fan. This is the simplest system, but uses up more medium. Churn systems make use of an agitator, a handle which is fitted inside the solids tank. When turned, it aerates the mix while coating the latest deposit with the medium in the tank. A tank with new medium may smell a bit more in the beggining, but with a fan the smell will dissipate quickly. A few days and deposits later, the medium will develop a musty, earthy smell. Churn systems ought to have a fan.
Wire a small 12V 40x40x10 mm fan that vents outside. The vent helps for odors. Make sure the fan is on a fused circuit(use an inline fuse in a non-protected circuit), use the fuse that is no less than 2 amps to a maximum of 5 amps. The vent goes ought to vent on deck, and ought to have a green water shut off.
Emptying. How often you need to empty a solids tank depends on how big the tank is, how many people are using it, and how often. Full-time use doesn't allow for the deposits to fully decompose, so if the tank needs emptying it is important to wear gloves. Partnering with someone on land to make a humanure composting pile is the best thing to do. The alternative is to toss the mixture overboard when far enough from land (boating laws still apply). It's possible to bury the material, but considering that one deposit requires a hole that is 15-20 cm(6-8 in) deep, burying a week's worth requires considerable planning and effort.
So far, for full time use by two people, we empty the bin every 2 weeks. We stretched it to 3 weeks once as a test, and it was too heavy and awkward to empty.
Liquids are diverted into a separated tank by way or a urine diverter. Urine accounts for around 85% of the volume in a tank, having a diverter increases capacity for solids, which means not having to empty the tank as often.
We calculated that for two people for a full week we would need a 31 L tank. Of course, some days we don't urinate as much as other days, but that tank would allow for that odd week where we would exceed normal amounts. We found a 27 L (6 CDN Gal) polyethylene tank. Polyethylene tanks are CSA Certified to NSF/ANSI 61. The tank is smaller than we had planned, but because we overcalculated the first amount we think that this will work out fine. The tank has a fitting overtop so we can close the tank while underway.
To empty the tank we'll connect a foot pump to transfer the contents into a smaller 5 L carry container to dump in a public toilet, or in the sea when far from shore. When underway, we might just get a long hose and pump it directly overboard from the forward hatch, we are not sure yet. We could have kept a thru-hull to empty it out, but preferred to close it.
We bought a Forian porcelain urine diverter by Shit and Blossoms to keep the liquids and solids separate. We chose porcelain because of its durable and hygienic properties. Very few companies offer the option of a porcelain separator, it is easy to clean and will last for a long, long time.
We've been living without a holding tank for some time, instead using a sturdy 5 L jug while in marinas or busy harbors. We kept a funnel to help our aim (or rather, Rekka's aim). We would dump its contents in a public toilet once a day, depending on usage. All urine has an odor, we smell it when emptying the bottle and the big liquid tank will be no different.
Smells? Spritzing the diverter with a 5:1 water and vinegar mix using a spray bottle will help eliminate accumulations and smells. After many days the container itself may begin to smell, adding a few inches of vinegar to the bottle beforehand can help. Dissolving citric acid in water (5g per 500 ml) and adding it to the bottle it also a good solution (a tip by Drew Fry, from an article in Practical Sailor magazine). By lowering the pH, fermentation is slowed down, resulting in fewer smells. If building your own, choose a polyethylene tank. Polyethylene is less prone to retaining odors.
Emptying. When full, it’s possible to empty jugs of urine into public toilets. If there are no facilities nearby, capping it off to dispose of later, and replacing the jug is a good option. Urine is also a great fertilizer, but it must first be diluted with water at a ratio of 1:10 to balance out the pH and to make it less corrosive. Another option is to use mechanical filters to make it safe for fertilization (maybe possible if parterning with someone on land).
Some plants especially enjoy nitrogen, potassium and phosphorus found in urine: cabbage, beets, tomatoes and cucumbers. —Shit and Blossoms