Towards a better tinyhouse

Inventing to freedom?

Practical total water recycling system for a tinyhouse

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Suppose you could get by with 70 liters of water for potable use, per person, per day, and 24 liters for the toilet. That’s wouldn’t require any lifestyle compromises, that’s enough for a (fairly quick) shower, doing dishes, some laundry. The average Japanese apparently uses about 200l of water a day, while the american apparently uses more than 300. Drinking water is less than 2 liters per day, so let’s just ignore than for now.

An interesting design I can think of is the forward osmosis membrane bioreactor, as I started to describe in a previous post. The membrane needs to pass more than 2.9 liters an hour to really work. You’d might have to end up using reverse osmosis to concentrate the draw solution, but maybe not. It could consume about 0.7 Kwh or so per 70 liters, after the power requirement of the pressure pump and the air pump for the bioreactor, I figure, with almost all of that for the reverse osmosis pressure pump.

That’s not that much water per hour, so you could use a reverse osmosis membrane, either with a flat plate “pillow” type membrane (really a membrane module) for the FO, or by modifying a spiral wound module (see the elimelech file for details), or just by buying the membrane material itself and making your own sort of module. Whether to use CA or TFC material, I don’t know yet. I don’t see how CTA does not get digested by bacteria, but if it would last you could go with the HTI membrane material by buying some of their products or you could get them to sell you some of the membrane material. There would still be some chlorine in the water when it got flushed, so CTA might actually be the only option, because TFC would get damaged. If you can get the hti material cheaply enough, you could just replace the membrane every couple months, even if it didn’t last. I wonder if laser transparencies or something would work.

The FO membrane could be placed in several different configurations, with the goal being to try to prevent excessive pressure from damaging it at any point. I don’t know if that would be a real problem, though, the membrane might be able to withstand a fair bit of pressure in terms of water column for practical purposes. Of course the pressures involved with the RO system could not be allowed to get to the membrane, but you could just have an intermediate chamber, open to the atmosphere, into which the output of the RO system goes, and have the RO system draw water similarly from a tank open to the atmosphere.

You don’t have to use RO. It could be electrosorption or electrodialysis, for instance. I’ve also read about a sort of self- replenishing ion exchange system that uses electric current to produce an acid and base to regenerate the ion exchange material, but was different from electrodialysis. I don’t know why they don’t try using it on boats, I have read about it used in industrial wastewater treatment, though. I think you could make your own unit. Same deal for electrosorption – with some carbon gel electrodes, maybe even activated carbon, I don’t know exactly how much salt you could get out, but you basically just run current through the electrodes. Then turn it off and reverse the water flow to remove the salts from the electrodes. Electrodialysis can get you very pure water.

If you did use RO, you might be able to get away with a cheaper pump and membrane than are used for seawater desalination, because you get to pick your solute. Why use corrosive, nasty sodium chloride? You can use glucose just as well (might want some sort of antibacterial stuff in there, though). You might be able to get the parts relatively cheaply off ebay, and/or could use relatively low pressures with relatively dilute draw solution. After looking into PEG, I’m not sure that it’s practical without doing a fair bit of experimenting first.

Apparently there is a naturally occurring material that forms non soluble magnetic nanoparticles, mentioned in the elimelech paper. I would be a bit concerned about abrading the membrane, and also you can’t beat the thermodynamics involved here – you need to provide a good deal of energy, and that might entail, in this case, a very strong magnetic field to remove the particles.

Conceivably, you could rig up a system that could regenerate ion exchange beds with acid/base generated onsite from salt, e.g. HCl and NaOH.

There are also other ways, of course, multifiltration, packed bed reactors, UF or MF MBRs, and I’m sure others I’ve never heard of….

Anyway, here’s an example design, in my next post.


Written by gregor

August 15, 2010 at 00:48

Posted in Uncategorized

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