Edible wall

This page is under construction!

Objectives and context

Motivation: upgrading a multi-functional “living wall”

Our general motivation is to produce food on-site, while valorising excreta (human waste) and combining several advantages. In this project, our mission is to upgrade existing “living walls” to combine several functions and characteristics:

• Grow edible plants (or medicinal, scented… instead of just decorative exotic plants)
• Fertilise the plants with human source-separated urine only (and avoid using any chemical fertiliser)
• Purify indoor air, which passes through the soil and root system
• Use material that is renewable, locally available, … (as much as possible)
• Have a beautiful planted wall, that increases well-being in offices, inside buildings, or in the city

Among these functions, we want to focus in priority on valorising human urine to grow edible plants. Yerk?! Yes, you read it correctly. And you should get convinced of the many advantages of the reuse of urine for agriculture by reading a coming page…

System description and constraints

Soil substitute
The vertical “modules” consists of panels of a kind of compressed roock wool: Grodan (although we might change this in the future if we find another renewable material with the same properties). The plants are planted in holes, and their roots then develop through this 'matrix' and at the back of the panels. Specificity: very little real natural soil, lightweight.

Fertigation (= irrigation + fertilisation)
Drip irrigation, with a nutritive solution made of tap water and currently a chemical fertiliser, which we’ll replace by urine. Similarly to a hydroponic system, the solution must contain all nutrients needed by the plants, in a readily available form, and in the appropriate concentration for continuous fertigation (as opposed to punctual fertilisation).
This means that the nutrients concentration shouldn’t be too high, because 1. Too high an ammonium concentration is toxic to plant [refs] 2. Excessive fertilisation also causes damages, such as nitrates accumulating in leaves [refs].

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Plants for vertical indoor garden
[…]

Specific objectives

Challenge for urine fertigation in a semi-hydroponic system

Obviously we want to avoid odours, especially because this version of living walls is designed for inside buildings. But we also need to find the ideal urine dilution also to avoid excessive nutrients (see just above) and the toxicity of ammonium.

What's more, most plants prefer nitrate-nitrogen to ammonia-nitrogen as a nitrogen source, and these plants are easily damaged by excessive ammonium nutrition. (Shinohara et al, 2008) [enter ref] This is why most hydroponic solutions contain nitrates, not ammonium [ref needed]. However stored urine contains nitrogen in the form of ammonium, which must therefore be converted to nitrate. This requires the presence of nitrifying bacteria, and sufficient oxygenation. It is generally the case in soil… but how about in our living walls, in soil substitue? Is it a suitable medium to host active nitryfiers? For this reason, we first want to test the 'nitrification in modules'.

Questions for experiments

1. Nitrification in modules - Does nitrification of urine take place in our soil substitutes? Is it quick enough? How it it influenced by the following factors?
   1. Inoculation (none / with nitrifying solution)
   2. Dilution factor (between 1:10-1:50)
   3. aeration properties of the panels (free surface / covered with a plastic sheet / active aeration)?
   4. Different soils, i.e. different source of microorganisms (none? / special for germination, sterilised / garden compost / field, without chemicals? / inocculated with an nitrifying culture)
2. What is the ideal dilution rate?
   1. Limit to prevent odours
   2. Appropriate dilution to reach at least the same performance as the chemical solution (or compare between dilutions) – in these modules
3. How do different edible plants grow under these conditions? Which are the most adapted?
   1. First selection: common herbs that grow well
   2. More interesting other plants: 2nd run, or first just in pots vertically – pre-test the dilutions

Could we start this last point in parallel during the nitrification experiments, or if it takes very long to build the structure, or in collaboration with Vertikal Farms ? (TBD)

Urine collection & storage

See the page DIY waterless urinals (This may serve also to other projects!)