I got to thinking about building houses lately, and started costing out a geodesic dome. As an aside, I don't think there is much of a cost saving in building a dome over a long-house style greenhouse, but that's a topic for another day. To make a long story short. I discovered that a 20x100' long house at the back of four rowhouse units would be a rather clever dual use of foundation walls and would allow for the back wall of the greenhouse to be planted with strawberry towers or a folkewall style structure. It would also, depending on the cost of glazing, be less expensive than a geodesic dome... and a hell of a lot easier to build. Sorry, dome guys, I'm just not converted.
So, my main interest in this folkewall/strawberry tower thing would be the treatment of greywater. As you may or may not know, greywater is wastewater with little to no organics in it. It comes from washing machines, dishwashers, sinks, and showers, to name a few places. It's basically water that you would normally let wash down a drain that hasn't got any pee-pee or poo-poo in it. Earthships use in-house greywater filtration based on living machines. I envision a similar indoor greywater trough filled with air-cleaning plants along the south-facing wall of each unit, through which all greywater will filter. Soaps and their ilk will be cleaned out of the water by the micro-organisms around the plants' roots. The then cleaner greywater can be mixed with urine and run through the folkewall in a dilute form. But how to get the urine separate from the faeces?
Enter the low-flush urine diverting toilet. As you can see by following all the links, the idea is not a rarity anymore, and it appears there is even an economy of scale - the highest priced unit is around 800 bucks. These units can be hooked into a simple reservoir for urine, and then that urine can be pumped in a measured way into the greywater that's cleared the first filtration trough. The key to using urine in this way is that it needs to be diluted, and since it, well, smells like pee, you want to keep it in a sealed container... and I think applying it to the folkewall through vermiculite laced with biochar would probably be a good way to keep the place not smelling to Hliðskjálf . Luckily, the WHO has already thought about the safety of just such a proposition, and they explain how best to handle human waste in their (aptly named) "WHO GUIDELINES FOR THE SAFE USE OF WASTEWATER, EXCRETA AND GREYWATER" publication. Convenient. Plus, other people think about this stuff, too. You can see a whole host of links to urine in agriculture here.
Since urine is pretty pathogen-free, its direct use (after minimal treatment) is reasonably safe and efficient. The problem is the pathogen-laden faeces. Luckily, those diverting toilets can lead to numerous different local treatment systems. The low-volume flush means less water to become leachate, and allows for better composting of the waste in composting units. As you can see, composting toilets are now more or less mainstream; they've got commercially available systems for automatic composting of humanure. There are other systems that produce useful byproducts: the Anaerobic Baffled Reactor, Methane Digester, and Biogas Settler all produce biogas for use in stuff like gas ranges. I have noted previously that bubbling biogas through an algae bioreactor would make it sweeter by pulling some of the CO2 out of the mix.
The matured compost can be spread on fields, but it would probably be better to run it through several other digestive processes before doing so. Putting it through a vermiculture operation would be a no-brainer, as that should provide further bacteria and fungi to break down the already pretty broken down organic solids. By putting both urine and faeces back into the food production cycle, the macro and micronutrients that are normally flushed down the toilet go back into the soil and food system. That's a good thing, and it means we go from a linear eat-crap-flush system to a renewable cyclical system. That's good. That's pee-pee and poo-poo.