Friday, July 9, 2021

The Desicating Head

 My Stiletto 27 had a portable toilet that stunk. Use the right chemical and it stinks less. Every time it is used a laborious haul/dump/clean process is triggered.

My PDQ 32/34 had a nice holding tank system that I had engineered to perfection. No odor, nearly like home. But it required space and weighed a good bit. Perfect for the PDQ, but totally unsuitable for the F-24.

We really only day sail the PDQ, and the head is only for emergencies. It didn't get used for years at a time, so we switched to WAG bags. Never used them.

 I'd been poo-hooing composting toilets for years. I'd experienced some nasty ones in cabins and some friends had a bad experience with a Natures Head. In fact, both failures probably included elements of design and operator error.

I was asked to investigate the topic, and so I did. There was a long-winded article in Practical Sailor last month, covering both toilet design, absorbants, and anti-odor additives for the urine tank.

 There are two keys to function:

  • Separate the urine from the solids. First, the smell is greatly reduced, and second, the solids side it much drier, preventing it from getting wet and aerobic.
  • Dry the solids. Like any animal dropping, once it is dry on the outside, there is little odor. That is the function of the absorbent; to speed drying, filter the air, and cover. It is NOT to cause composting. This is a desiccating toilet. 

There simply is not room on a boat for true composting toilet. The process tanks months, requires temperature and moisture control, and continuous mixing and ventilation.  Since you cannot effectively compost, then stop pretending and dry the waste instead. You can then take it home and compost it if you like, or double bag it and dispose of it like litter box scoopings. Your choice. The urine is odor-free once treated (see below) and can be disposed of easily.

I built a test version from a storage tub, a bucket and scraps to test the absorbents and additives. It was a crude thing, but in fact I used it in the basement bathroom for two months, during which it proved to be amazingly odor-free and easily to deal with. I was stunned. It was time to eat a large helping of crow. But I was happy to eat the crow, because the result was a truly user friendly head solution for my F-24.

The final version was based on molded parts from Separatte, a fiberglass tank I modified to fit the available space, an under seat baffle I cut from fiberglass, and yes, a bucket and jug. The top hinges up for service, which is a simple matter of lifting out a bag and replacing it; a clean, contact-free process. The absorbent of choice, at any price (I tested many), is millwork sawdust and shavings I get for free (aspen bet bedding is also very good). The urine treatment is citric acid, though vinegar and Nilodor are also very good.

I no longer see any point in conventional portable toilets at all. A desiccating toilet is better in every way. Regrettably, you must either pay a king's ransom or build your own.


  1. Do you mean to say that Nature's head is in fact a dessicating toilet, and that we've been calling it wrong? Or instead that a dessicating toilet is a better alternative to the Nature's Head and other composting toilets?

  2. Natures Head is somewhere in between composing and desiccating, depending on how it is operated.

    A true composting toilet, as defined by NSF/ANSI standard 41, Non-Liquid Saturated Treatment Systems, provides actual composting and will reduce coliform bacteria to <200 MPN/100ml. This takes months of holding capacity, venting, controlled temperature, moisture, aeration, and mixing, and a lot more space and power than any boat can afford.

    If you have only one user and the waste is in the bin for a month or more, a good bit of composting takes place, but it will still take months to finish the process and the waste is still infectious. If there is a full crew and it fills up in a week or so, it's really just semi-dried poop. Of course, so are diapers.

    As a practical matter, all three approaches (composting, semi-composting, desiccating) can work. But calling the latter two composting toilets is not accurate and can lead to sub-optimum designs. By understand that the smaller ones function primarily by desiccation, we can design and operate them better.

  3. We have been using a C-Head for 2 and a half years. Its great and the price is affordable. Its not a kings ransom and copying the C-head would cost more in materials and a very low labor rate. We use peat moss. We tried shavings but could not find a reliable source. We get 8 changes from a 50 liter bag. We live aboard and only use the C-Head. When it gets difficult to churn, its time to change. For us its about every 5 days. We don't have a fan. We like the folks at C-Head, they are honest and call it a desiccating toilet. I don't understand how some folks claim weeks between changes on composting toilets for live aboards. If you measure the volume of waste generated by 2 people defecating once a day plus a drying agent and the toilet volume it just does not compute.

  4. Coconut husks are better than sawdust. The exhaust smells faintly of the Pacific.

  5. Funny how tastes in absorbents vary.

    Sawdust is not recommended by anyone, but aspen shavings, readily available as small animal bedding through pet stores everywhere, is C-Head's recommendation (I spoke with the designer at length). I tested a bunch of absorbents, side-by-side, including coconut coir peat moss, and shavings (as distinct from sawdust) were my top choice as well. Peat most was one of the poorest performers, and I have heard the same from others; try aspen shavings from a pet store. I've been using millwork shavings, which are basically aspen shavings without the sawdust removed. They're free for me, and the trace of sawdust actually makes it even more effective than shavings alone, or any other material (technically wood ash is better, but messy).

    The two day figure (from a Practical Sailor Article) was for a small conventional portable toilet, not a desiccating toilet. As you point out, a desiccating toilet goes MUCH longer before emptying. In the article I gave 5 days as a very conservative minimum for a compact 3-gallon size (not the standard size), assuming lots of TP, lots of absorbent, and not completely full when emptied. I imaging a couple could go as long as two weeks with a full size unit and some care. Note also that I was describing a non-churn toilet in that article, which increases absorbent use. So your 5 days is also very conservative; I'm sure you could go longer. We did go as long as two weeks on the test model, though a few uses were elsewhere in that time period... but they normally would be. Much depends on how much absorbent you use and how much TP is included.