Have you seen the Sun Frost composting toilet design?

Design, build and install a low cost batch feed composting toilet.


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Shawn Tisdell
Joined:Tue Apr 11, 2006 4:41 pm
Have you seen the Sun Frost composting toilet design?

Post by Shawn Tisdell » Tue Apr 11, 2006 5:01 pm


This design has several similar features and some differences that may be worth discussion. I like the liquid recirculation concept, but am still concerned about the ability to evaporate all urine if a diverter is not used. But perhaps if it does not evap. all liquid, this way the liquid could be removed from the top without cutting a hole in the bottom of the barrel. I talked with the owner and he mentioned that if additional evap. is needed just add flour to enhance the composting temperature.

Although this composting system is illustrated as sitting right on the drum, it is possible to locate the drum directly beneath the toilet, just as the Biorealis toilet.

I am not quite sure how the mixer works, but from my bucket toilet experience, leveling and mixing would be helpful.

I also like that a vented room is not required, each drum is attached to the ventilation system.

The unit is insulated so perhaps that helps enhance the thermophilic composting action.

The asking price for the system is $750 (per phone quote on 4/11/06). During the middle of June I will be able to view the unit at our nearby Midwest Renewable Energy Fair.

What do you think?
Compost-real alchemy-waste into gold

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Joined:Tue Jul 03, 2001 11:01 pm
Location:Willow, Alaska USA

Post by Bob » Wed Apr 12, 2006 12:19 pm

Hi Shawn,

That looks very interesting. Thanks for the heads up.

Without getting my hands on one, or at least seeing more specific details about how it is made and how it works, it's hard to offer an opinion.

But I agree with your comment about urine separation. There are a couple of other reasons why I ended up going that direction -- besides the basic problem of how to deal with excess liquid. Additional considerations include optimum C/N ratio (feces w/o the urine is a better mix), best use for the nitrogen in the urine (composting wastes it to atmosphere), and salt content (kills earthworms).

My design should also--at least theoretically--require less energy (the latent heat of evaporation for water is approx 1,000 btu/lb). In a warmer climate that may not matter. In a colder climate it might. Also in a cold climate, depending on the installation, the extra humidity in the exhaust air itself adds some complication -- lowering the dewpont and producing more frost build up on cold surfaces (like the HRV heat exchanger.)

A healthy earthworm population (made possible by diverting the urine) eliminates the requirement for (human) mixing and leveling. If they do all that work, you can reduce/eliminate some moving parts--like a recirc pump and stirring mechanisms.

But, all that said, it does seem to be a nice, well thought out design. There is a lot to be said for simplicity. Kudos!

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