Hmm... well I did some more literature research on the solubility of struvite, and while it turns out that it might be technically possible to solute it in a acidic solvent, it would need a very strong sulfuric acid (0.5-1mol/l) to reach a strong enough salt gradient.
So due to the hazards and costs of that, I doubt that it is feasible to do with struvite in anything but a laboratory scale :-/

Urea, like used in the research paper above, might work, but most is to quickly converted to ammonia and it will be difficult to reach the needed concentrations with urine also...

So not sure if this is feasible... is there any other ammonia salt that can be relatively easily made with stale urine?

@CAGIEA: This is all there is right now... it was an idea I had when researching some other topics.
But it looks like I might do some small scale experiments in our lab next week to test if it is possible to make a stable gradient with struvite solved under acidic conditions.

@afolabibakare: Stuvite (MgNH4PO4+6H2O) is used to precipitate PO4 (and some NH4) from urine for example... you should find some interesting info about this on this forum easily.

Hallo

Am interested in the urine process but i could have missed the genesis of the topic can you kindly give the whole topic from the beginning


Thanks

Regards
Nabulime
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This is a very challenging concept.What is struvite? Can we have details of any of the research wor please?

Hmm yes, that might be the case, but how much of the Struvite does actually form without the addition of Mg?

Also one could acidify the bottom layer to keep the Stuvite in solution. In fact then it might make sense to produce as much soluted struvite as possible as it will probably form a strong gradient. And if the top was not acid enough for it to stay in solution, one would have an automatic concentration and replenishment of "brine" at the bottom via precipitation and re-solution...

But yes... I am not sure if this will really work. In general if it works with artificial fertilizer, it would be great if it could be done with natural also.

Anyways... thanks for the input... this is really a brainstorm topic at most

Edit: the dry feces could be maybe used as thermal isolation around the tank and thus as a side effect be nicely sterilized by the produced heat.

Hi,
I´m not sure it I got your idea completely, but if you pretend to use it as brine (?) (Sole - in German) than I guess the precipitation of Struvite will block everything fairly quick.

Christoph

For some time I have been interested in solar salt-gradient ponds ( www.solarponds.com/ ) as a cheap and low-tech way of collecting solar energy... however due to it's use of sea-salt I never really saw a direct connection of sanitation topics.

However recently I can across some interesting research from India which replaces the NaCl by fertilizer salts to make use of such systems outside of costal areas.
www.sciencedirect.com/science/article/pii/S0360544201000597

One very interesting research-paper explains the use of urea as a substance to create the convection suppressing gradient and tested it on a small scale:
www.sciencedirect.com/science/article/pii/0038092X9500096A

That got me thinking if it wasn't possible to directly use (human) urine to feed the bottom layer and abstract diluted (and heat-sterilized) urine as fertilizer from the top as needed anyways to keep up the gradient. Maybe the urine would have to be concentrated a bit, or supplemented with other fertilizer salts to get a sufficient gradient.

The produced heat could be used for many purposes including heat sterilization of feces.
The main problem is probably to prevent algae growth in the upper layers, but this seems to be controllable judging by the previous research.

Maybe this would be an interesting topic for research, as the urine needs to be stored anyways, and this would also give an incentive to collect it etc.