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Composting loo


Trilby Tim

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When I built my boat I fitted her with a Sun-Mar Excel composting loo. I've used it for 5 years with mixed experiences. The principle seems good but the practice disappointing, in particular the build quality of it. It feels plasticy and flimsy, basically a bit cheap and nasty. However, of course it isn't cheap, it's was actually very expensive!! Various bits have broken and fallen off it over the years. Most recently the back panel has cracked meaning that the gears to rotate the drum jumped when you tried to turn it, so you couldn't properly. So I'd had enough of it, it was time to replace it.

 

I looked again at what was on the market. The Enivrolet is similar to the Sun Mar in that it mixes lquids and solids. which I wanted to avoid after the Sun Mar. There's also been bad experiences of this reported on this forum, and without a UK importer there's no chance to see before you buy, so no way of knowing if it's any better constructed than the Sun Mar. And again it's really expensive.

I looked at the Airhead. This actually seems to be pretty well designed and built compared to everything else on the market. However it's really small, it has so little capacity that it'll need emptying frequently before things have had a chance to properly decompose. To be fair it's not designed for continuous use, would probably be fine if only used occasionally.

I also looked at the Villa. The principle was fine, I liked the trapdoor that opened when you sat on it and I liked the collecting bin that rotated by a small angle each time you sat on it. But again it feels cheap and flimsy (and at £600 plus options it ain't cheap!). Also I have a fairly large space available from where the Sun Mar stood, the Villa would only take up some of that space and would waste the spare space.

 

So I decided to build from scratch using the principles I most liked from the others but adapted to fit my application and properly engineered. So the design needed to be:

-Urine separating rather than mixing

-Large capacity to use the entire available space and maximise the time to compost before emptying

-Look, feel and actually be solid and robust!

-Use a standard toilet seat (not a flimsy plasticy one) and preferably a ceramic bowl.

 

A ceramic bowl was unfortunately unpractical to make (I don't have any experiences or facilities to work with ceramic). So instead I moulded one using a Separett plastic bowl as a pattern. I surfaced it with chemical resistant polyester resin blended with ground marble powder which gives a similar appearance to ceramic (like most baths, shower trays, etc) backed up with a thick layer of glass fibre/polyester.

 

My initial plan was to have 4 round collecting bins for solids sitting on a carousel. One would be in use at a time and would be rotated round by a small angle on each use (like the Villa). When one was full, the carousel would be rotated by a quarter of a turn to use the next one. When all four were full, the first would be emptied and used again and so on. However I decided that this was probably unnecessarily complex, wasted space between each bin and wasted height by having to have each bin on an indivdual turntable as well as the overall carousel. So I went for a simpler, more space efficient design with 4 quadrant shaped bins on the carousel. Each individual bin doesn't move on the carousel, you just fill one, and rotate to the next when all are full empty the first and carry on. It's large capacity, each one of the 4 bins is greater capacity than the single bin on the villa. The design looked like this:

Toilet CAD

Putting it into practice, I got the carousel and bins fabricated by a local company to my drawings. I had a coil of 8mm pipe to pump hot water though laid under the Sun Mar to heat it and aid evaporation so I kept that and put a steel plate over it to form the base of the new toliet. The walls became the insides of the new toilet. They shouldn't get damp in there, but I painted them with yacht varnish and sealed all gaps with silicone to be on the safe side. I then screwed some little wheels to the bottom of the carousel to allow it to turn:

Caroysel

I screwed a couple of fixed castors to the walls to act as guides for the carousel and put a shaft with another wheel to turn the carousel and a handle on the top of the shaft. The castors are spring loaded to press the carousel against the drive wheel. I've put in a 25 litre urine tank under the step, here's the workings of it:

Inside of toilet

 

I had some block board walnut worktop and birch ply left over from the bathroom units so used this to make the top, front and step. I designed a trap door mechanism that opens when you press down on the seat (e.g. sitting on it):

Back of bowl

 

Here's a picture of the finished unit (with a standard oak seat):

Finished toilet

 

And with the seat pressed down (trap open):

Toilet bowl

 

So it's just about done. It looks good and is built like a brick sh*thouse! Now needs a few years use to determine whether it works and is robust!

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No, there is still a fan, inside the chimney that runs up inside the cupboard behind. Even if you evaporate the liquids you still have to let the water vapour out. There's nothing that new here, besides possibly the carousel arrangement and the mechanism for the trapdoor, mostly it's just a case of putting ideas together and building it properly!

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No, there is still a fan, inside the chimney that runs up inside the cupboard behind. Even if you evaporate the liquids you still have to let the water vapour out. There's nothing that new here, besides possibly the carousel arrangement and the mechanism for the trapdoor, mostly it's just a case of putting ideas together and building it properly!

 

Hi Tim,

 

thanks for the posting of the story behind your motivation for building this toilet.

 

You probably have a job already, but if not, I think that if you would fancy producing more of these toilets (at a decent price) that there will surely be a market for them.

 

I think that it's a pretty clever system and it looks as if it will last better than the very expensive and more flimsy factory produced ones.

 

Cheers,

 

Peter.

 

For more on choosing/using compost loos on boats - join this Facebook group for lots of info/advice...

 

https://www.facebook.com/groups/compostingloos/

 

Hi colinlune and ditchcrawler,

 

thanks a lot for these interesting Facebook links.

 

Peter.

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Thanks for the comments. I do already have a job, and a lot of other projects taking up my time so don't have any interest in producing them. Quite happy to make it 'open source' though, happy to send the CAD on request (although the CAD was good for working out the general concept, the design changed quite a bit as I actually built it!). And if anyone wants the separett plastic bowl as a pattern for moulding from I'm happy to sell it on.

The urine can be disposed of down the Elsan, the solids can be buried.

The picture of the trapdoor is taken looking from the underside, it's upside down. The trapdoor falls open under gravity. It's pushed closed by an arm on each side (made out of clear polycarbonate plastic, so it doesn't show up that well in the photo!) that pushes on a pin that produces from the edge of the trap. The end of the arm that pushes on the pin is curved to a profile to allow the trapdoor to swing through a full arc and provide a sensible mechanical advantage, it took a bit of trial and error to get that curve profile right. The arm is pivoted on the pair of white blocks. The arm is sprung loaded on one side and pushed by a dowel on the other side. So when you sit on the seat, the seat pushes down the dowel (which passes throught he walnut worktop) and the dowel pushes the arm down (up looked at in the photo) out of the way of the trapdoor which swings open under gravity. When you get off the seat the springs are powerful enough to push the arms back up, pushing the trapdoor closed and the dowels back up. There's a matching arm mechanism on each side as one wasn't strong enough to hold the trap closed on it's own with the weight of the seat resting on it. Clear? wink.png

Edited by Trilby Tim
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