michka:research:microbial_fuel

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 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:

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.

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):

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):

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.