Urban greywater biodigester plus rainwater harvest plus composting toilet - are we overcomplicating?

Hello folks,
My husband and I are building a house in an urban area of southern Mexico. We want to make it as sustainable as possible and have hired earth-builder specialist archictects... but they're a bit on the young side and I sometimes wonder if they've thought through all the options. Perhaps the Permies hive-mind can help. My question is: do you see a way to simplify our greywater use, instead of our current plan which is to clean it in a biodigester and then channel it into a concrete soakaway pit.
Our plot is 300 m2, (ca 11 m wide by 24 m long) on a steep slope downhill from the road. The house with a footprint of about 100m2 will have two levels, "staggered" to fit the slope. We've built a 30 m3 (30,000 liters) ferroconcrete rainwater catchment cistern at the bottom of the plot, with a smaller (5000 l?) separate cistern for using (scarce) neighbourhood mains water when the rainwater runs out in the dry season (6 months in winter). Because the cisterns are 5 m below the bedrooms, for our water use, we'll need to pump water from the cisterns into a plastic tank (known here as "tinacos") in a tower above the bedrooms and upstairs bathroom,  in order to get enough water pressure in the shower. The house sits towards the bottom end of the plot, with a chunk of steeply terraced garden above, and at the base we only have a 1m-wide strip of space between the cisterns and the house.

The current grey- and blackwater plan is as follows:
- Greywater will come from the kitchen, shower, washing machine, sinks.
- We'll have one dry composting toilet (separating pee and poo) on the ground floor where we hope to do most of our "contributions of biomass". Poo falls into drums stored in a chamber under the toilet (where we'll let them decompose in peace and haul the compost out to a plot in the countryside, or on the fruit trees), pee goes through a tube into its own little soakaway.  
- The complication comes from the upstairs toilet next to the bedrooms. We'll probably use this mostly for night- and morning pee, but because it's on the second floor we couldn't work out how to make it a dry toilet. (We also think that if we want to sell the house later, the lack of a WC might put buyers off.) So this pee will go into water, which will join the greywater, turning it into pee-water. (we can put in a "popbox" upstairs to poo in a bucket and cover with decomposed sawdust and ashes - we already do that in our rented house.)  
- The greywater + upstairs pee-water is destined for a ready-made biodigester with 300 l capacity. The water coming out of this will be directed into a soakaway ("pozo de absorción") in the downhill corner of the plot. The builders say that it will be a massive engineering project, 4 m deep, made of breezeblocks, concrete, etc, very expensive... that sounded surprising.
So what's the view of Permies?

Constraints:

- The downhill corner where we're planning the soakaway pit is itself right above to the downhill neighbours' property and on the uphill side of a 2m retaining wall - structural issues! The neighbours haven't built anything there yet, it's fruit trees and rubble, but who knows in future. Otherwise we'd be very happy to plant one of those banana grove things.
- The soil is basically clay.
- Why not use the cleaned-up greywater for irrigation? We don't have physical space for another cistern especially for the treated greywater coming out of the biodigestor. Could we contemplate using the rainwater tank for post-biodigestor greywater in the dry season? Would that screw it up for the next lot of rainwater?
- Drinking water. I'm not sure if  post-biodigestor greywater would be clean enough to drink... (Our town is second only to Calcutta in salmonella diversity.) Bit we are planning to install an ozone drinking-water purifier in the kitchen in any case. When we run out of captured rainwater and switch to municipal water, could we also start using post-biodigester greywater in the house?
- In the rainy season (May to September) it rains heavily every day, so we need a way to drain the rain that falls on our plot (although the water that falls on the roof will go into the catchment tanks).
- At the moment we're not planning to connect to municipal sewerage at all, because those pipes are at street level, five meters above our house, and we'd need another pump.

Building codes here are subject to a gleeful anarchy which we hope will let us get away with unorthodox solutions.
I'd be grateful for suggestions on how to simplify this and make better use of our resources!

Very interesting! i hope to do something like this in the future. i wonder, do you plan to have a food forest and some animals (chickens, ducks, rabbits?). if you reserve an area for composting (specially the chicken/duck bedding; that even when using deep litter, you may need to compost)  then the diverted urine could be piped to go into composing pile; thus adding nitrogen and speeding up the composting process. As far as the humanure, you may want to look into bokasi? i'm learning about it myself.

Is the house too big for the land area you have?

Emilia here with the update on "how it went". The house is now up and running and funcioning as mentioned in my original post. For the greywater filtration we ended up with the following, "left side" and "right side" systems:
- Left side drains only the downstairs bathroom shower and sink. Since this shower doesn't have much water pressure and is far from the bedroom we don't use it much. The water goes into three concrete settling tanks (the architect needed to keep the builders busy for a week and invented this massive concrete structure much to our annoyance). From there it goes into an artificial wetland with false papyrus, from there into a tank we can open and scoop out water with a bucket for watering plants. From there any extra runs into the soakaway pit.
This "left side" bathroom contains the dry toilet, in use during the days. The urine from the dry toilet drains (via a simple hosepipe) into the "solids trap" of the "right side" WC. The dry toilet poo (with a mix of sawdust, ash and earth) falls into large barrels that we let decompose and then move to our forested plot outside town (a considerable inconvenience). So there's not a huge amount of flow here, mostly just frmo washing our hands in the sink. The false papyrus seems fine regardless.
 
- The right side system drains the upstairs bathroom shower, sink, WC, kitchen sink, laundry room sink and washing machine. The greywater goes into a similarly excessive three-step tanks system. We added mulch to the middle chamber to try to put the solids to good compost use, remains to see how that works. From there the greywater drains into a 300-litre pre-built septic tank. This also receives the flush straight from the WC. There's a "solids trap" with a lid before the septic tank; a cuboid concrete box with a diagonal mesh across that you can lift (made from the same stuff as a chainlink fence, only a bit smaller calibre). It's supposed to be for catching non-biodegradable things like condoms or nappies that unaware guests might flush. However, in practice it blocks the poo from flowing into the septic tank and is generally annoying. The "solids" chamber was built according to the digestor's specs, but we'll have to ask the builders to come back and make it less cuboid and more funnel-like and get rid of the mesh that catches everything. When the dog takes a dump in the back garden it's handy to collect the poo and chuck it straight into this section.

Then both grey and black water gets biodigested in the septic tank, where bacteria should eat solids and crowd out pathogens - we assume. The manufacturers sell the bacteria "pucks" that cost an arm and a leg. I'm actually on the forum right now to find out about alternatives.
From the biodigestor/septic tank the water flows into another artificial wetland, with papyrus and other plants - we'll see which ones get established the best. The "wetlands" are brick troughs lined with thick plastic membrane (to stop water seepage), filled with sand and plants. The water level is supposed to stay under the surface of the sand to stop mosquitoes from breeding. At the moment this works in the "left side"  wetland but not in the "right side" one, which needs more sand or fine gravel - you can see water in it. It also smells a bit sewagey which makes me wonder just how well the biodigestor/septic tank works.
Still on the right side system: after the reed bed there's another well with a lid where we can choose to collect the filtered water (for watering non-edible plants), or open the valve and let it run into the soakaway pit - in theory. In practice it leaks where the walls meet the outflow pipe, something else that needs fixing.

So that's the kind of system we ended up with - overengineered and overcomplicated (for which I blame the architect and her need to keep builders working during a slack week). But it works, we think. We still have to see how it goes during our first rainy season in the new house.

Hope that was useful for someone!
Emilia

John C Daley wrote:Is the house too big for the land area you have?

Well, it's the size it is and the same goes for the land... it's an urban plot so we don't have e.g. an orchard for a leach field. Most of the back garden is 30 cm of soil on top of concrete rainwater catchment cisterns.
All the best!
Emilia

Vanessa Alarcon wrote:do you plan to have a food forest and some animals (chickens, ducks, rabbits?). ... As far as the humanure, you may want to look into bokasi? i'm learning about it myself.

Hi Vanessa! My husband does have a food forest but it's outside town and he doesn't manage to go there very often, so it's planted with a law of the jungle approach: whatever survives the neglect gets to flourish come the rains (little tasty animals would definitely not make it). But it's good for burying the decomposed humanure.

We tried making a ferment for the humanure using my anarchist brother in law's recipe but it ended up smelling worse than the shit. We're planning to have another go though. Having said that we have a to-do list of like 150 items and that one is not in the top ten...

A friend of mine had deep bedding for her three hens (and priapic rooster) and it was very impressive. Also not in the top ten for us though.
All the best!!
Emilia