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

 
Posts: 25
Location: San Cristóbal, Chiapas, Mexico
4
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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!
 
pollinator
Posts: 121
Location: Treasure Coast, Fl
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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.
 
pollinator
Posts: 1438
Location: Bendigo , Australia
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Is the house too big for the land area you have?
 
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