Towards a better tinyhouse

Inventing to freedom?

Dehydrate-pasteurize Toilet

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The TinyhouseBlog has a post up about a composting toilet by Walt Barrett.

So I’m spurred to take a draft post out and finish it. In that “toilet options” post I put up a while back I mentioned that a dehydrate-pasteurize toilet could be a safe and pretty practical option. The idea is that, instead of leaving the humanure to sit for a year or more, during which it must be stored somewhere, taking up considerable space, it can be given a short (e.g. 5 minute) heat treatment at 70 deg c. or so. The information in the humanure handbook indicates this should be more than sufficient to kill all pathogens.

A big used slow cooker would make an inexpensive and effective heat treatment device. Get a big one and you could simply place it under the toilet seat, have a negative pressure ventilation thing to prevent any smell from entering the house, and you’re almost there for a safe toilet solution.

There are 2 problems with that, though, one is that while you could periodically heat-treat the humanure to kill all bacteria, if the urine just sits there there is some potential for anaerobic bacteria to grow, some of which are themselves pathogenic (but the diseases they cause are not contagious), and which release a variety of foul smelling substances during the decomposition process.

Secondly, it takes up a lot of space if you just tried to store it. About 1.5 liters a day for urine, and the feces are actually quite small, about 150ml a day, plus TP is very small. Still, that’s too much. Your slow cooker pasteurizing chamber is probably 15 liters or so. You don’t want to have to empty it every couple days, and a quantity of liquid humanure would be hard to handle (though remember hygiene with regards to bacteria is no longer a problem at this stage, after heat treatment).

But if the water can be removed, urine is 95% water, so 7.5 grams of solids from there, plus I forget how much faeces is water but I remember it was surprisingly high, suppose 60%, so you would have 60 grams of solids from feces. So that would only be 67.5 grams of solids every day. Unfortunately there would be a lot of air mixed in, but you can see a 15 liter container should take a long time to fill up even so.

It could be lined with a plastic bag, such as those slow cooker liners available at grocery stores (or better yet if you could get something biodegradable) making emptying a snap.

Drying the material takes a significant amount of energy, but this is what existing commercial composting toilets do with the liquid so you are using no more energy than existing solutions, anyway. Those heaters in e.g. the biolet? They consume a lot of power and when you suck air out of the dwelling, into the toilet then vent it outside in the winter you must be sucking cold air in somewhere else, and that is a substantial contribution to the power consumption. No matter what you do, you have to at least provide the latent heat energy to evaporate the water (636 W-hr/kg). In an off grid situation this could be provided as heat energy much more cheaply than using solar panels to provide the electrical energy the biolet design apparently requires. In summer of course this is not an issue, all you need is airflow.

Similar deal with the smell going out the vent stack, it will not be as bad as a regular commercial composting toilet (in fact if you add salt to the urine chamber as mentioned below, I would think much less).

The dried stuff can then be easily transported somewhere else to put on a compost pile, as all the nutrients are still there, and it is now completely safe to handle. No need to worry about pathogens escaping the compost pile. It could also be tilled directly into the soil, this would be much like the “night soil” approach mentioned in the humanure handbook except that this would be dry material so it wouldn’t smell much (and the smell fades soon after application anyway). Actually maybe the dried stuff would not smell much if at all to begin with. And you don’t need to worry about spreading pathogens, at least to the extent you remembered to do the quick pasteurization.

One issue I can see here is that it might be better to separate the urine and toilet paper. The urine and feces could probably be mixed okay, thought I guess that might increase the amount of smell going out the vent stack. With TP, though I assume a wet mass of TP takes a looooong time to dry, and anaerobic decomp would probably kick in before it was done.

So a urine-separating receptacle sounds like the way to go, then put a vertical divider in the slow cooker, and the bag can go over top, giving you 2 chambers. Walt’s toilet is simple but I like the idea of a receptacle made of teflon (the metal coated with non stick stuff used on cookware etc. would of course be great but I don’t think you can buy that as sheets anywhere, maybe it’s worth checking). In the absence of flush mechanism and trapdoors, to maintain a hygenic appearance you could just putting a suitably wide, twisted tube after the respective holes in the two bowls to block the view to the chamber below and any splashing.

Next, the airflow going into the toilet used to prevent smell entering the dwelling can be reused to evaporate the water.

So now we have the good quality urine separating receptacle, the 2 separate chambers in which the urine and faeces+TP rest and through which air flows, evaporating the water.

You might want to add salt to the urine chamber to prevent any bacterial growth, the amount required is small enough that does not affect the utility of it as fertilizer, I think. Actually, as the concentration of urine solids went up over time I assume the salt naturally present would eventually be enough to prevent growth. Only if you wanted to suppress bacteria when it is first starting to be used would you add a bit of salt.

The size of the urine chamber should be at least 1.5 liters multiplied by the number of people that are to be using the toilet full time, and the airflow high enough that it can evaporate that much water from the urine each day, so it never overflows.

Every couple of months you just turn on the slow cooker to a low level to finish evaporating all water, use a twist ties to close the bag (containing both urine and feces), turn the pasteurizer up to 70, put the lid on, and leave it for 5 minutes, then remove, put a new bag in, and take the old bag to the nearest compost pile. Or you may be able to legally put it in the municipal garbage, you’re still being way more environmentally friendly than a normal flush toilet.

So that’s it. Cheap, safe, easy, low maintenance, very environmentally friendly, and works.

This post is a post from the archives I selected to update and repost. Since few if any of the current readers have read it, I thought it would be just as good as a newly written post.


Written by gregor

March 29, 2011 at 17:09

Posted in Uncategorized

One Response

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  1. and to smear a couple of thoughts together, for toughs running flushies, you could set up your shower drain with a pump to feed your flushie, hence recycling the shower water..

    just a thought,



    March 29, 2011 at 17:43

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