Biochar as a Biological Filter Media?

4 posts in this topic

I have been searching the web for information on using biochar as a biological filter media. There are dozens of posts for Koi ponds that refer to charcoal as a chemical filter, but not biological.

Here is my line of thinking:

  • I can easily produce bichar on my farm with my retort in large quantities
  • Biochar works great as a grow bed media
  • It's close to neutral buoyancy
  • It contains trace wood vinegar elements that may help micororganisms (this is true for microorganisms in the ground... not sure if the same holds true for AP). At any rate, wood vinegar exists only when you prematurely stop the pyrolysis. I wouldn't chance it though... Would burn to completion.
  • It has a massive surface area
  • It can be reused as biochar (in the traditional manner) of putting it in the ground after service in the AP system

Please note that I am not referring to "activated carbon," nor am I referring to it in a powdered form. I'm thinking of pieces around 5mm. It seems that activated charcoal, used as a chemical filter, only lasts a few weeks before needing to be replaced (not convenient). But if it was used to help facilitate a colony of nitrifying bacteria, I don't see why this wouldn't work. Again, charcoal chunks have already proved to be successful in grow beds.

My aquaponic set-up is a bit unique. Here is the basic system (not yet assembled):

  • I recently acquired 120 5-gallon food-grade buckets for an amazing 50 cents (US) a piece
  • I'll use 24 of these for long-term food storage, and the other 96 as miniature grow beds
  • I'm only planting large plants, or plants with extensive root systems in the buckets (i.e. tomatoes, peppers, etc.)
  • Smaller plants/veggies will go in a traditional gravel grow bed, and at least 1 DWC for lettuce
  • Each bucket will be coated with marine-grade epoxy, and wrapped with twine (for added rigidity, UV protection, insulation, and aesthetic reasons)
  • With the bucket system, the grow bed media will be 50-70% reduced (so, less of a home for bacteria) as they would if I planted them in a traditional bed.
  • 12 buckets will be plumbed together to create one "unit"
  • A simple 2" bulkhead fitting will act as a drain (no stand pipe). I'm going big here so that the 12 units can freely equalize their respective water levels before the outside siphon kicks in
  • A plastic screen, formed into a cone shape will act as a media guard for the drains. (still worried that smaller worms may still get sucked away... any suggestions?)
  • The 12 buckets will connect to a larger PVC pipe, which in turn will connect to an Affnan-style swivel valve siphon
  • There will be a total of 8 units, with 12 buckets in each unit
  • 5,000 gallon, above-ground pool as the fish tank
  • Sump, vortex filter, and biofilter (see below)

I'm planning on pre-filtering the solids with a

, before sending the water to the biofilter. I need some fish solids to get through in order to provide food for the worms in the buckets. I just don't want a massive amount of solids. A 50-60% reduction would be ideal (at least this is my thinking). For the DWC, I'll route the plumbing to an extra mechanical filter for that bed alone.

Here is my tentative plan of attack for the biofilter:

  • I'll produce the biochar from hardwood in my double 55-gallon steel retort
  • I'll crush the char using a type of mortar and stone technique in another steel drum
  • I'll screen the biochar through a 1/8 inch sieve/sifter to remove any powder
  • I should be left with 4-7mm size pieces
  • I'm using a 275 gallon IBC tote with 4 large air stones (discs) at the bottom
  • I'll fill the tote to 80% biochar (yes, it's a lot of char and ill take several burns to get this amount)
  • I'll top it off with well water

In short, I don't want to go spend hundreds of dollars on fancy media... I am not against using sponges, PVC shavings, shredded bottle caps, or any other low cost alternative if biochar doesn't work... But that is the big question. Why wouldn't it?

Any feedback, words of wisdom, constructive criticism, etc. is greatly appreciated.

Thank you!

- BigDaddy

Edited by BigDaddyOffGrid
spelling (see edit history)

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Welcome to APHQ.

Charcoal could be used in a grow bed.....but I question how successfully it would support plants. It appears to be used most frequently in connection with managing water quality. Having said that, you are growing in buckets so support may be less of an issue.

Your proposed system sounds very comprehensive.


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Hi Gary,

Thanks for the response... The buckets will not contain charcoal, just the biofilter. Even with the support of the side walls of a bucket, I have a feeling that some of these veggies would still tend to tip over. I'll be using 3/4 gravel for the buckets. I am mainly interested in using charcoal for the biofilter because I can readily make it (low cost, high labor), it's somewhat buoyant, and has great surface area. Plus, it's a super cool idea! (only if it works)

I guess I'm just waiting for somebody to throw some facts at me on why this is a crazy idea... Although secretly, I'm hoping people will say the same thing I'm thinking -- It should work.


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OK.....perhaps I should have read your post a little more carefully.

I'm with you.....I think it's cool to be able to make your own bio-char.


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