There are several makes and models, but the principle is the same. Solid waste, including toilet paper, drops down into a lower chamber beneath the toilet bowl. A trap door keeps the contents hidden between uses. In the lower chamber, waste is mixed with a composting medium, such as coconut fiber. The user turns a handle on the side of the toilet, to mix everything together after each use. This speeds up the composting process.
The key to the success of marine composting toilets is the separation of urine and solids.
When placed together in one tank, urine and feces produce an intensely bad odour. Anyone who has used a porta-john or outhouse knows this only too well. When the urine is diverted away from the feces, odour is reduced dramatically. To be blunt, wet poop stinks and dry poop does not. By separating urine from feces, most of the odour problems surrounding boating sewage are eliminated.
Urine is typically diverted to a 2 gallon bottle, which is, in effect, a small holding tank. This bottle will have about 2-4 days capacity for two people. Some boaters carry an extra urine bottle to extend this capacity.
Urine does not pose a health risk, and can be easily disposed of ashore in any toilet or pump out station. As a last resort, urine can be discharged legally, the proper distance from land. While there is no significant bacteria in urine, it does contain concentrated nitrogen – which can cause algae blooms in stagnant water, so it should never be dumped in a bay or any anchorage under any circumstances.
The fan is the critical component in any marine composting toilet installation.
There is usually a small 12 volt fan built in to the toilet. This pulls air into the toilet from the bathroom area, and exhausts it outside through a vent. The purpose of this is two-fold. First, air moving through the composting chamber removes excess moisture. The solid material shrinks dramatically as it dries out (just like a pile of grass in your backyard composter). Because of this shrinkage, the toilet can hold a surprising amount – estimated as 60-80 solid uses. Two people living on their boat full time would need to empty the toilet only about once a month or so. If the boat is only used for weekends and holidays, you might easily go the entire summer without emptying it at all.
Secondly, the fan eliminates odour. Air is being pulled in to the toilet from the boat, rather than wafting up into the boat. Even with the trap door open, air (and smell) is not escaping into the boat. It is going outside.
Boaters are understandably concerned about power consumption. A marine composting toilet uses a small computer fan that draws about 2 watts. If this amount of electricity usage is a problem, you can install a solar vent. This is a small vent with a solar panel and battery built in. It works even on cloudy days to charge the battery, and the fan will go 24/7 without draining your main batteries. Another solution would be to install a small solar panel to make up for the electricity used by the toilet.
When it comes time to finally empty the toilet, you undo two clasps and remove the upper section. Place a compostable ‘plastic’ bag over the top of the lower bin, and dump the lower bin contents into the compostable bag. The whole process takes about 3 minutes.
The most recent additions to the toilet will not have fully composted, so it is vital to take care of the solid material properly. You can place the compostable bag in any outdoor pit style toilet in a marine park. Or you can take the material home and place it in your composter, where it should sit for several months. It is then safe to put the fully composted material on non-edible plants. Remember, you only have to deal with the solid waste very infrequently – in sharp contrast to the holding tank approach.
But is it legal?
Yes. Composting toilets are legal marine sanitation devices in coastal (salt) waters in Canada, and all waters in the USA (including fresh water).
Composting toilets are not inexpensive, at about $1200-$1500. However, they are comparable in cost to a more typical marine sanitation system, with a marine toilet, holding tank, two sea cocks, pump, hoses, and a Y valve. As a bonus, a composting toilet will allow you to remove your old holding tank, gaining precious storage space. They are ecologically benign, producing no waste for treatment, and require almost no maintenance. Above all, composting toilets are far more convenient to use, compared to carrying around a sewage tank and planning your travels around pump out stations.