Water Butt Bio-reactor

Making liquid fertilizer from comfrey leaves is a widely accepted process in the organic gardening world. The problem with the traditional method is that it makes a very smelly liquid that is unpleasant to use. My device does away with the smell and makes lots of rich dark plant feed for very little effort once it is set up.

  • Take a common barrel shaped 200L plastic water butt with a tap at the bottom. Cut it horizontally in half, I used an angle grinder but a sabre saw or a hand saw will do.
  • On the top half, drill 3mm holes around the inner rim about 30mm apart all the way round. This will provide anchorage for some galvanised wire mesh to be ‘sewn’ onto it using either galvanised or plastic coated gardening wire.
  • Cut a piece of galvanised wire mesh about 50mm bigger than the opening in the top of the butt.
  • Using garden wire, stitch the galvanised wire mesh to the underside of the inner rim of the butt using the 3mm holes.
  • Take a bag of lump-wood charcoal that hasn’t been treated to make it easy to light on a BBQ or make your own. Wet the charcoal to prevent dust and crush it with a lump hammer. You are aiming at around <10mm bits.
  • Using a garden sieve, sieve the charcoal into the bottom half of the water butt. Put a few big bits of charcoal as barricades to stop the small charcoal blocking the tap.
  • Invert the top of the water butt and insert it upside-down into the bottom half. It should sit nicely in the bottom half, rather like an egg in an egg cup with the mesh well above the charcoal.
  • Fill to the top with lush comfrey leaves or other lush vegetation.
Water butt bio-reactor

The level of leaves will fall quickly as they break down. As they are held up by the mesh, they break down aerobically. The resulting liquor drips down onto the charcoal which begins to absorb the chemicals and becomes biochar. As time goes on the liquor levels build up as you add more leaves and rain gets in the top. You can tell the process is working because turning on the tap will release a dark brown almost odourless liquid. Periodically draining off the liquor into storage containers to keep the level in the bio-reactor below the mesh. The extracted liquor should be diluted about ten times before use as a plant feed.

The break down of the leaves, I think, creates butyric acid and sulphurous smells but the presence of charcoal, alkaline with a large negatively charged surface area, reacts with the acid and bonds to the various volatile aromatic products. At some point in the autumn when the supply of fresh foliage dwindles, I will dismantle the bio-reactor and replace the biochar with fresh charge of charcoal ready for next year. The biochar will then be used as a soil amendment. The soil is heavy clay and the aim is that the biochar will help aerate, drain and improve the soil structure. Plants should be able to access the nutrients and water bonded to the surface of the biochar.

It will be interesting to assay the chemical make-up of the liquor produced. How concentrated is it? Does the composition vary depending on the feedstock? What is the pH of the liquor? What variation in composition is observed between having the charcoal filtering and not? Initial tests with a soil testing kit on the liquor show a pH of about 7.5, very little nitrogen but plenty of phosphorus and potassium, making quite a good tomato feed. I will have to capture some of the liquid prior to it coming into contact with the charcoal. My expectation is a more acid liquid and maybe more nutrients since the charcoal should adsorb a fair amount of the chemicals, at least initially.