Charcoal as a media

[Paul Letby]

Hey guys, I'm going to open this up for debate, as things are getting a little off topic on my personal system discussion.  There have been questions as to the safety of charcoal as a media, and I'm sure there are questions out there regarding it's environmental impact.  So let's let fly with the questions and hash this out.  I'm definitely not the expert here, and I am hoping George will weigh in on the issues. 

 

Personally I decided to use charcoal based on the fact that I have free access to wood, and needed a cheap and light media for use in a GB supported by a light stand that's not meant to support a lot of weight.  I know that activated charcoal is used in aquaria as a filter media, so I'm pretty sure it's safe.  That said we all know that many things from tropical fishkeeping don't transfer to aquaponics very well.

[Vlad Jovanovic]

That's probably a pretty good assumption Bob. Most commercial charcoal operations probably aren't concerned with syngas production, but rather getting the most mass out of their feed stock (which happens at even lower temps than are typically used for making bio-char and syngas combos).

The less heat = less syngas, but more bio-char mass

The more heat = more syngas, but less bio-char mass

I think most charcoal is made at 300C to 380C, (as opposed to low-temp bio-char 400-550C).

Where's the pH at currently? What did it start out as?

Bob Campbell said:

  The large chucks of charcoal that I bought at Cash and Carry were most likely made at low temperature, but I'm pleased with the durability of this charcoalt.  

[Chris Carr]

Although "activated" is not what we are after here I just thought it was interesting to mention you can achieve high surface area at lower temps using a chemical "activation" process which I believe is used for some commercial production of activated carbon.  http://en.wikipedia.org/wiki/Activated_carbon#Production

I had intensions of making charcoal but I lucked out and we get 30kg bags of it delivered to the door for $3 each here . The in laws use it regularly for cooking anyway so already have 100kg of it piled up in the back, oddly enough in AquaFeed bags. Maybe it's a sign...

Vlad Jovanovic said:

Hi there Jesse. If I've understood the topic at hand correctly, the SSA (specific surface area) of bio-char created at "low-ish" temps (circa 400C) like Jon's apparently is, isn't really analogous to activated charcoal/carbon (while circa 900C temps would be). Process temperature seems to be the main factor governing surface area in such endevours. I bet a nice 'low' temp bio-char is much more suitable as an AP media than a 'high' temp activated carbon scenario...or as a 'control release' of fertilizer nutrients in soil...

  

[Vlad Jovanovic]

Hehe, yeah I'd definitely take that as one of those signs...Good deal too. 30Kg for $30..Wow!

Even the SSA of low-temp bio-char is still nothin' to sneeze at SSA=120 m2 m-3 (Day et al.,2005)...Compare that to medium gravel (25 mm diameter), SSA=69 m2 m-3 ; (Crites, et al., 2006)

Might not cut it as a chemical filter, but for an AP medium that's pretty darn good! (Some would say, 'awesome')... 

[Jon Parr]

Jesse, I used the standard fire bricks because, well, I didn't know any better. Fire brick is what was recommended by the masonry supply house, and is what I saw in all the YouTube vids. I presume it will suffice, but I'll look into the refractory bricks you spoke of, and always looking for good advice, thanks.

Char is not ordinary media. The carbon is very active material, whether or not it was prepared as "activated carbon" or not. The cellular structure left behind from the wood, and the carbon's action as nutrient sponge allow nutrients to be stored, and retrieved as needed by bacterial action and roots. This special relationship is unique to char. It is not something that goes away when bioslime coats everything. That's when it really gets started. It's not necessary to AP, of course, but it makes sense and works well.

Hi or low temp? Good question. I've read convincing arguments for both hi OR low, and convincing arguments to use both hi AND low temp char. The reality is that an indirect retort produces both. The material near the edges gets hot, the middle not so hot (not even charred if you pull the plug too quickly).

Either way, char is very durable in AP. It's not dirty or sooty, it doesn't crush under use any more than other media, and is a joy to work in. Most of all, folks, it's the most local media most of us have access to. And it doesn't consume loads of petroleum like hydroton, expanded shale, and slate, expanded glass, etc. That expanded glass stuff is a joke, btw. It's labeled as the lightest media (translated: it's as sharp as razor blades and crushes to dust in your fist), and all feel-good green and environmental because it's recycled. News flash! Glass is already being recycled into, you guessed it, more glass.

[Chris Carr]

Jon, If anything "activated" will just be more brittle and float easier would it not? also depending on the size of the pours and the size of the bioslime, perhaps any added surface area will become less effective, or not any more effective at all compared to low temp. Was only pointing out temp wasnt the only factor in creating two different products.

Vlad. That was 1..2...$3, not 30. Wasnt a typo  Everything in Vietnam cheap if domestically sourced, expensive if imported.

[Vlad Jovanovic]

 hehe...

[Vlad Jovanovic]

Sorry Chris..I actually did get that. MINE was a typo...3 bucks and not 30 is how my brain understood you...Now if my fingers would do the same...

Chris Carr said:

Jon, If anything "activated" will just be more brittle and float easier would it not? also depending on the size of the pours and the size of the bioslime, perhaps any added surface area will become less effective, or not any more effective at all compared to low temp. Was only pointing out temp wasnt the only factor in creating two different products.

Vlad. That was 1..2...$3, not 30. Wasnt a typo  Everything in Vietnam cheap if domestically sourced, expensive if imported.

[Jon Parr]

Right-O Chris, Right-O.

Vlad, I need to PM you, but I'm not at home, and this forum won't let me send a PM from my cel. It used to, probably will again, but for the moment either my phone or this site are glitched. So, shoot me a note at [e-mail] when you get a minute.

Thanks

[Jesse Hull]

I would think that charred wood would certainly raise the pH.  Obviously more so if you hadn't rinsed it (wood ash = potassium source).  I'm not privy to the details, and I don't think the test was even completed, but I do know of experiments started locally where using it as an alkaline pH adjuster was actually one of the targeted goals.

Do you suspect, or has it been anyone else's experience that the pH of the charcoal media will drop over time?

Soaking it in an acid bath might work to neutralize it.  It certainly does for rockwool.

Bob Campbell said:

The pH was off the scale to begin with and climbs to over 9.  I try to keep it near 6 and adjust it every other day..    When I first began, I rinsed the charcoal 4 times and the water was clear so I went no further.  It might have helped if I had rinsed it more.  Next time I think I'll try that, and also soak it in a very strong acid bath before planting.

Vlad Jovanovic said:

...

Where's the pH at currently? What did it start out as?

Bob Campbell said:

  The large chucks of charcoal that I bought at Cash and Carry were most likely made at low temperature, but I'm pleased with the durability of this charcoal..  

[Jesse Hull]

Activated charcoal has more 'pitting', increasing the surface area.  Eventually this surface will become covered with bacterial slime.  The pitted surface gets smoothed out and you have less surface area exposed to the aerated ammonia & nitrite containing water.  Aerobes require oxygen, so as the slime gets thicker only the bacterial colonies nearer the outer surface are having much of an impact on nitrification.

This is one reason why mechanical biofilters, where the media used also has a strategically increased surface area, must be agitated and purged regularly.  The bio slime will eventually work against the system.

As you go through planting and harvesting of whatever media you choose, the media gets 'worked' and bio slime is shed to clear the surface and make way for more colonies to form.

Chris Carr said:

Jon, If anything "activated" will just be more brittle and float easier would it not? also depending on the size of the pours and the size of the bioslime, perhaps any added surface area will become less effective, or not any more effective at all compared to low temp. Was only pointing out temp wasnt the only factor in creating two different products.

Vlad. That was 1..2...$3, not 30. Wasnt a typo  Everything in Vietnam cheap if domestically sourced, expensive if imported.

[Jesse Hull]

In my other life, I was a ceramic chemist and studio owner (www.jessehull.com).  I built many kilns while in school and in the ceramic art world.  

The denser and more vitreous the material, the greater the risk of thermal shock, micro cracks, and eventual fracturing.

Jon Parr said:

Jesse, I used the standard fire bricks because, well, I didn't know any better. Fire brick is what was recommended by the masonry supply house, and is what I saw in all the YouTube vids. I presume it will suffice, but I'll look into the refractory bricks you spoke of, and always looking for good advice, thanks.

Char is not ordinary media. The carbon is very active material, whether or not it was prepared as "activated carbon" or not. The cellular structure left behind from the wood, and the carbon's action as nutrient sponge allow nutrients to be stored, and retrieved as needed by bacterial action and roots. This special relationship is unique to char. It is not something that goes away when bioslime coats everything. That's when it really gets started. It's not necessary to AP, of course, but it makes sense and works well.

Hi or low temp? Good question. I've read convincing arguments for both hi OR low, and convincing arguments to use both hi AND low temp char. The reality is that an indirect retort produces both. The material near the edges gets hot, the middle not so hot (not even charred if you pull the plug too quickly).

Either way, char is very durable in AP. It's not dirty or sooty, it doesn't crush under use any more than other media, and is a joy to work in. Most of all, folks, it's the most local media most of us have access to. And it doesn't consume loads of petroleum like hydroton, expanded shale, and slate, expanded glass, etc. That expanded glass stuff is a joke, btw. It's labeled as the lightest media (translated: it's as sharp as razor blades and crushes to dust in your fist), and all feel-good green and environmental because it's recycled. News flash! Glass is already being recycled into, you guessed it, more glass.

[TCLynx]

Hum, I think to large extent the pH action related to charcoal is going to be dependent on

1-did ashes get into the system at first (remember that leached hardwood ashes are how we used to make old fashion potash lye or potassium hydroxide which will raise pH)  Wood ashes are used in the garden often to raise pH and add potassium to the soil.

2-what kind of wood was it and how completely are the previous constituents of that wood burned out (or how much pure carbon is left compared to other stuff)

I know of some one who actually used a lump of charcoal to reduce pH.  He did an experiment where he took a jar of water and put a lump of charcoal in it and left it till the pH dropped below 6 and then he pulled the charcoal out and put a lump of calcium carbonate it it till the pH got up over 7.6 or 8 (I don't remember which) and he alternated this way for a while just to see what would happen until he got board with it (probably a week or so.)  He wasn't sure the type of wood, probably gum.