Towards a better tinyhouse

Inventing to freedom?

environmentally friendly flush toilets off grid inexpensively

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Night soil, as you may know, is raw humanure. In China, as explained in the humanure handbook, it is used as a fertilizer, and makes a very good one. What he doesn’t mention is that it is almost undoubtedly pasteurized before being spread on the fields, thereby killing all pathogens to the extent that the pasteurization was done correctly, which is not hard, but mistakes happen.

And by the way, the humanure approach is no more immune to the potential failure to follow the pathogen killing procedure. If for any reason there is a mix up and it is not store long enough or whatever, there you go. In fact the pathogen killing is in large part the same, with heat (also time, though). Just so that is out of the way.

He points out that it smells terrible. This may be only because of the compounds in feces that stink, or a the majority of the smell might be caused by compound produced by bacteria after it leaves the human, because substantial anaerobic decomposition was allowed to occur somewhere along the lines. If it is stored in the outhouse or whatever for more than a day presumably this is the case. The bacteria will produce hydrogen sulfide, ammonia, and other nasty smelling substances that will still be present after the bacteria are killed by pasteurizing.

I don’t know exactly why it smells and to what extent, but there is a thing called an Aerobic Treatment Unit ATU, which are sometimes used where septic tanks cannot be installed, and the output from them does not stink (a slight musty odor is reported). It is reported to be a must sort of odor. Also, aerobic treatment involves putting the sewage in contact with air (or other wise adding oxygen but almost always air) to get some of the oxygen to dissolve, so that it is available to the bacteria for respiration. Clearly not all of the air can dissolve, so this involves an output stream of air that has been very well exposed to the wastewater which is in the process of being treated.

As far as I have been able to find out this air does not stink. What does this mean? Well, where are the odiferous compounds that were in the fecal matter going? The have to go somewhere. It is possible that they are diluted to an extent they are no longer detectable to the human nose, or they are no longer offensive. There is a certain threshold below which you cannot smell a substance.

Or maybe it is one of the first things to be digested by the bacteria during the anaerobic treatment process. Either way there seems to be a way of dealing with it in an effective way that ATUs are taking advantage of.

In case it’s not clear where I’m going with this, if we mix humanure with some water that we had anyway like leftover greywater, that does not destroy it’s value as a fertilizer – after the pathogens and the smell is removed, that diluted night soil can just be put on a lawn or garden just like greywater, as it is just fertilizer…. almost. ( it might make more sense to keep the amount of water it is diluted with to a minimum, so just the flush water then it would be easier to treat, and then after treatment dilute it.)

There is only one problem that I know of which could be hitch, and maybe a minor one, which is the “ulitmate BOD” or ultimate biochemical oxygen demand, of the water. This is a measure of how much stuff in the water can be digested by bacteria, measured in grams of oxygen aerobic bacteria would take to fully digest it all. By the way there is another measure, which is BOD5 which refers to how much oxygen a sample requires to sustain microbial growth for 5 days. Basically you just put the sample in a sealed container and wait 5 days then measure the amount of oxygen that was removed from the air. I don’t know if they give it a standard inocculant too, but I’m pretty sure they do not, so it’s whatever bacteria are present in the water that are acting, which come to think of it would be a funny and potentially inaccurate way to measure… hm, anyway they are related, and the BOD5 is what is used for regulatory purposes, do decide if the water is clean enough to release into a stream or whatever, but there is a lot of criticism of this because ultimate bod can be several times the bod5, or it could be equal to it, depending entirely on the substances present in the water.

The reason it matters is that if you dump stuff with too high bod into a natural ecosystem like a stream or soil, bacteria will much away on it and use up all th oxygen nearby, taking it away form fish or other bacteria nearby, changing and damaging the ecosystem. However, the lower the BOD is , the slower bacteria grow, so get it low enough and you are okay, because oxygen can still get into the water fast enough to keep the oxygen levels from getting too low.

Also, if all the oxygen is used up, anaerobic decomposition kicks in, which as I said stinks, and some of the organisms are pathogenic. Also, for respiration nh4 and nitrate and nitrite ions are used, converting the nitrogen into N2 so you loose the nitrogen as a fertilizer, although the process is very slow compared with aerobic decomp.

To prevent this from happening the rate at which oxygen gets in just has to be high enough to balance the metabolic usage rate of the bacteria, whatever that may be. That means if we spread the water out in a very thin sheet, or mix it with something like soil so it forms a thin over a very large number of very small particles, with air in between the particles, then we might be okay.
Edit: wordpress is deleting some of my words for some reason. I am editing it, and I see “thin sheet” above, but the post when viewed shows just “thin”. Whatever, so the sloppiness of this post is not all me being lazy (mostly though).

This is what happens with the greywater systems that put the stuff onto the lawn or garden. But put too much water in one place, and the situation changes, oxygen can’t get in fast enough, and aner decomp kicks in. You will see this mentioned if yo uread up about these systems. By the way there are various ways to get O2 dissolved into the water, but basically here somewhere along the line it has to diffuse in by the same mechanism that a gas permeates through a thin plastic sheet.

Well we know it works okay with greywater, which has a bod of I think 128 to 500 mg/liter bod5 (no idea what the ultimate is). It is much lower if you don’t put food down the sink, which is obviously a very digestible substance.

The amount of ultimate BOD in feces and urine and toilet paper combined is reported to be anywhere form 50 to 90 grams per person, per day, and the BOD5 is IIRC about 34 grams. So we dilute that with 60 liters of greywater said person also produces and then add in the bod of the greywater then we are not that high above 500. Hopefully that may be low enough, I would have to double check things, just check what is acceptable for a greywater system.

If it is not low enough, the bod5 may be reduced by particle filtering, which removes toilet paper and other stuff that can be digested by bacteria but is not soluble in water, so it could be in the right range, although then you would have to do something with that stuff you filtered out.

So there may be hope for this sort of thing, just pasteurizing or otherwise killing the pathogens, then a brief aerobic decomp possibly with a Rotating biological contactor, then out to the lawn.

If you filter the particles out anyway, then you could also maybe skip the aerobic decomp and then maybe activated carbon could be used to absorb the smell compounds, it depends on what the compounds are and the isotherms, which tell you how well they are absorbed b the AC. If you can transfer the compounds to the air, you could also adsorb them with AC, but in that cas it is probably easier to highly dilute it, I would think. If you google “zeocarbon” you can find a company selling a gas phase adsorbent stuff (zeolite is also and adsorbent) that is used for septic tank outputs, so I dunno. Might take some experimenting.

There are surely other ways too of dealing with the odor causing compounds, I don’t know. Ozonation?

An MBR could do both particle filtering and the anaerobic stage and some disinfection. When I say some disinfection the problem is that the membrane might ge ta hole in it at some point, letting harmful bacteria through, but I have posted on this blog some bioreactors for greywater which seem to have sufficiently durable membranes. They are pretty amazing these days, so maybe you could skip the pasteurizaton. If you used pasteurization obviously you would want a regenerator to reduce energy consumption.

BTW there are ideas for inexpensive chlorination, pasteurization, membrane bioreactors and membranes for them elsewhere on this blog.


Written by gregor

February 4, 2011 at 13:37

Posted in Uncategorized

7 Responses

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  1. Hey Gregor,

    i just got done ordering a bluetooth earpiece/ microphoe…my dell mini 9 has built in bluetooth, and so does my cell phone, so a 17 dollars well spent i imagine…

    lot of good stuff to think about in this post…currently, i am using peat moss in my natures head, though as soon as i get the bio-char retort up and running i will very enthusiastically be changing over to bio-char…

    like you mentioned at one of the cons, it is very similar to activated carbon…though more porous, or sponge like, it will serve as a sort of “moisture battery” in addition to other benefits to the composting process…

    i can produce it myself, and the carbon negative process also yields the low cost power storage option of tanked syngas.



    February 6, 2011 at 12:45

  2. oh, and if anyone studies retorts, you will see most to be crude with no net gain…the idea with the retort i am working on is to optimize pyrolysis efficientcy and insulate the unit very well…

    net gains are thus possible, and as simple as having your gassous pump triggered by a temp-sensor when pyrolysis temps exceed 502C pulling the syngass out of the manifold though an earth coil and compressing in into an underground tank..where a pressure switch will open the pump circuit above the max preset pressure level.



    February 6, 2011 at 13:00

  3. oh, and here is a link to sun-mar’s low flush units ( for those who prefer a flush system…

    they require a bi-level type construction, although benefit could be found in a kitchen counter accessory port for kitchen scraps…

    Tom and i have discussed the possibility of low flush systems for his Spheres, where a low flush toilet in the Sphere is ducted to an outdoor kitchen, at the base of the spiral stair, that conceals the composting unit…



    February 6, 2011 at 13:21

    • Yeah I have seen the sun- mar and the microflush toilets, and the toilets might be useful. You could even potentially have a dehydrate-pasteurize-compost arrangement if there was only 0.5 liters per flush, with the humanure being dried by evaporation, pasteurized in a handy chamber such as a slow cooker, then emptied only once a month to the compost pile, sans pathogens.

      It would be even easier if people would give up the demand for flushies, though. Unfortunately they are strongly associated with wealth and civilization in most peoples minds.

      The composter though is grossly overpriced.


      February 6, 2011 at 15:20

      • well the price played a big roll in my selection of the natures head…$850 isn’t cheap either, ( unless you compare that with the cost of a mound system, and extra water consuption ), but it really is a nice unit, and is one of my most treasured possessions….

        the sun-mars are allot more complicated, with the rotateing compost drum, and probably a more complex air flow scheme, not to mention flush capability…but they do have a 1″ excess liquid over flow that might work in with some of your ideas…

        of course if you come up with a better topology that uses a minimal amount of water to some better end, i’m sure you will find a market..


        February 6, 2011 at 16:33

  4. I absolutely love your site.. Great colors & theme.
    Did you make this web site yourself? Please reply back as I’m
    looking to create my own personal blog and would like to know where you got this from or exactly what the theme is named.

    Many thanks!

    November 28, 2013 at 03:20

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