Salt Bridges

I tried three batches of salt bridges before managing to have a stable one made.

The right protocol:

  • Mix 200 ml water, 5 g of agar and 75g of salt in a cooking pan.
  • Bring it to a boil while mixing regularly.
  • Let it boil for a few minutes (not too much, otherwise you will loose your water)
  • Pour the boiling mix in appropriate containers
    • Reused glass jars (with an opening having a size close to the widest diameter of the pot)
    • Glass petri dishes
  • Let them cool down a bit
  • Place them in the fridge for a few hours

What not to do:

  • Use hot water from a kettle and mix everything in a cold pot
    • This induces a very uneven mix, where salt and agar are not well dissolved
  • Let it sit in the pot beore pouring it on top of the substrate
    • Warm liquid may harm the microbes in the substrate
    • Most of the agar and salt will sediment at the bottom of the pot, and your mix will not become a gelatin
  • Do not forget that the fridge step is important !
  • I highly suspect gelatin to be inefficient for this process. My successful batch was made from agar-agar from a chemical provider. I think other sources may be fine, but I have not managed to use them properly yet.

Mud, Tall

Mud samping location - Sint-Jans-Molenbeek, Belgium

Design details:

  • The mud sample was taken from the waterland at the bottom of Scheutbos Park in Sint-Jans-Molenbeek.
  • Its approximate volume is 50 cl.
  • On top is a highly concentrated Petri-dish-sized salt bridge.
  • On top is filtered water.
  • Electrodes are made of copper from dead batteries. They have an approximate surface area of 100cm2 each.
  • Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

  • Tension is unsensitive to oxygen presence, and is in the 300mV range.
  • Current is in the 1-2mA range.
  • Power is therefore in the mW range.

Longer-term results (12 days & more):

  • Tension is stable at 110 mV, but drops when measuring current (and rises up afterwards).
  • Current is stable at 107 microA, but drops after measuring tension (and rises up afterwards).
  • Overall power is therefore around 11 mW.

Mud, Small

Design details:

  • The mud sample was taken from the waterland at the bottom of Scheutbos Park in Sint-Jans-Molenbeek.
  • Its approximate volume is 3 cl.
  • On top is a salt bridge the size of the jar.
  • On top is filtered water.
  • Electrodes are made of copper from dead batteries. They have an approximate surface area of 10-20cm2 each.
  • Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

  • Tension is sensitive to oxygen presence, and is in the 100mV range when activated, 30 mV at rest.
  • Current is sensitive to oxygen presence, and is very low - about 0.3 mV when activated, dropping to 0.1 mV at rest.
  • Power is therefore in the microW range.

Longer-term results (12 days & more):

  • Tension is stable at 20 mV.
  • Current is stable at 1 microA.
  • Overall power is therefore around 20 microW.

Vegetable Waste & Compost Tea

Design details:

  • The substrate is made of vegetable scraps (potatoes, carrotes, zucchinis) covered with compost tea (from FoAM's vermicompost production), and inoculated with a tea spoon of mud from Scheutbos Park.
  • Its approximate volume is 7 cl.
  • On top is a salt bridge the size of the jar (broken during the manipulations).
  • On top is filtered water, which was quickly populated by compost tea from below the salt bridge.
  • Electrodes are made of copper from dead batteries. They have an approximate surface area of 10-20cm2 each.
  • Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

  • Tension is sensitive to oxygen presence, and is in the 200mV range when activated, 80 mV at rest.
  • Current is sensitive to oxygen presence, and is very low - about 0.4 mV when activated, dropping to 0.1 mV at rest.
  • Power is therefore in the microW range.

Longer-term results (12 days & more):

  • Tension is stable at 120 mV.
  • Current is stable at 135 microA.
  • Overall power is therefore around 16 mW.

Conclusion

  • Electrode surface area seems an important feature.
  • Most of the mud in the Tall-Mud design has been sitting here for a week, which may have an influence on community development.
  • Considering electrode area and overall volume, the compost tea design seems very productive.

Further Designs

  • Scale-up the vegetable+compost tea design.
  • Look for carbon electrodes, which seem more appropriate
  • Replace salt bridge with paper