Edible wall

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Introduction

At ESTEE, we (Alexandra Florin and Michka Mélo) work on a project aiming at feeding living green walls from urine to produce food. We will first aim at feeding hydroponics green walls with urine as a nutrients source. We will then identify which edible and/or useful plants are suitable to grow in such an environment.

Our very first step will be to grow nitrifying bacteria (or nitrifyers) to convert urine into nitrate. Such nitrifying bacteria are very important in aquaponics, where we grow fish and plants in the same system, the waste from the fish providing nutrients to plants for their growth. However, fish waste need (at least partly) processing by bacteria to be edible for plants. Such bacteria include nitrifyers, which will transform ammonium from fish waste into nitrate, edible by plants.

Our first “Nitrification in Aquarium” (NA) experiments (NA1, NA2 & NA3) will therefore be adaptations of the first “cycling” step of aquaponics. We will start from an aquarium, an air pump and drippers and some pond water. We will feed the system with urine everyday, as they do with ammonium in this protocol.

From urine to nitrate : how does it work ?

Urea is hydrolysed into ammonium (NH4+) by bacteria containing an enzyme called urease. Ammonium is transformed into nitrite (NO2-) by a first class of nitrifyers, which we will call nitrifyers I. Nitrite is transformed into nitrate (NO3) by a second class of nitrifyers, which we will call nitrifyers II. The transformation of urine into nitrate is called nitrification.

Urea hydrolysis

Urease-containing bacteria are ubiquitous, and urea hydrolysis happens naturally in any traditional toilet system.

When urine is hydrolysed into ammonium, its pH gets quite high (around 8.5). Ammonium is part of an acido-alkaline with ammonia (NH3), which pKa value is 9. Ammonia is a gas. If stored in an open container, ammonia will fly out until pH of hydrolyzed urine reaches 9, causing nitrogen losses and reduced efficiency in converting urine to nitrate. Urine should therefore be stored in a tightly sealed container.

Hydrolysing urine is also a way to sterilize it, as high pH will kill pathogens.

High pH also induces precipitation and slight degradation of minerals.

Nitrifying bacteria

Both nitrifyers I & II are sensitive to nitrite (NO2), which is toxic for them and inhibits the nitrification reaction. We therefore have to feed the nitrifyer culture with small amounts of urine, because a too large amount at once may lead to a high production of nitrite by nitrifyers I, and lead to poisoning of both nitrifyers I & II.

Nitrifyers I have a tendency to make the culture more acidic, and nitrifyers II are inhibited by low pH conditions (below 6). Higher pH conditions (around 8) are therefore more favorable for nitrification [1].

Nitrifyers are sensitive to light [2].

Urea, ammonium & evaporation

Urea is not volatile. Ammonium (in the form of ammonia) is volatile.

Experiments

So far, we performed three experiments, all having a dedicated report page and detailed lab journal page.

Reports for NA1, NA2 and NA3 include introduction, material and methods, main results, discussion and conclusions.

Detailed lab journal for NA1, NA2 and NA3 include day-by-day report of measurements and actions taken.

A summary of the measurements in the form of an Excel sheet can be found here.

Conclusion

NA2 led to a nitrate concentration of 160 mgN/l, which is in the range of the concentration used in hydroponics to feed plants. NA1 and NA3 failed to reach a nitrate concentration above 20 mgN/l.

It is yet unclear why NA2 was a success while NA1 and NA3 failed, further investigation needs to be performed to have a better understanding of the dynamics at stake.

Next steps

To simplify the complexity of the investigation, urea hydrolysis should be performed before feeding urine to our system. Its dynamics should be observed by frequent pH measurement, to observe if it happens within days or within hours.

Ammonium content in urine should be measured before being fed to the bioreactor.

Ammonium/nitrite/nitrate concentrations in the bioreactor should be monitored as frequently as possible after feeding, to have a better understanding of the possible evaporation issue. A second experiment may compare the potential evaporation rate in different aeration rate & bioreactor volume combinations, to test if increased aeration induces increased evaporation.

A protocol should be designed to try to discriminate between failed nitrification, nitrate consumption, or even possibly ammonium consumption by competing bacteria.

The need for physical support to host nitrifyers should also be investigated and tested.

During experiment & experimental set up design, it should be kept in mind that:

  • It is wiser to protect the bioreactor from light
  • Low aeration did not seem to help
  • Feeding very large quantities of urine to the system did not seem to help
  • Performing urea hydrolysis before feeding urine to the system will strongly help investigation

Open questions

If you have any elements which would help us solve these open questions, you are most welcome to edit this section. Please add references when possible.

  • Is nitrifying bacteria growth inhibited by lack of physical support (f.i. clay beads) to host them ?
  • What are the appropriate pH, light & oxygen conditions for a quick urea hydrolysis by urease-contaning bacteria ?
  • Which bacteria may compete with nitrifyers for ammonium in the same culture conditions ?