Howdy folks! Welcome back to my blog series on aquaponics. In last the last post, titled “Not Quite Free” I discussed the idea that aquaponics isn’t really “free”. One of those things that I do try to keep in mind is that any my success in aquaponics is a result of “standing on the shoulders of giants”. Which is why I’m writing these blog posts. I’m hoping to help other folks who are interested in this remarkable sustainable farming technology. I’ve made some mistakes and I’ve tested some theories, and I want to make sure that anything I’ve learned or proven or tripped over winds up out in the collective record of the Internet community.
Charcoal vs Biochar – The Important Detail
So as I’ve mentioned previously, the “growth engine” in any aquaponics system is the beneficial bacteria (nitrobacter (1)) that handled the conversion of (fish waste product, fish-toxic) ammonia to nitrite (fish-toxic) to nitrate (fish-safe, plant food). They live as a “bio-slime” coating over the surfaces of the tanks, pipes and anything that is wet at gets a supply of oxygen.
The biggest single reservoir of nitrobacter in an aquaponics system is the plant grow bed media. The more surface area in the volume of the grow beds, the better the total system will be at converting fish waste to plant food, and purifying the returning water to the fish.
Volcanic rock, expanded clay or expanded shale, or biochar all have very high “Biological Surface Areas” (BSA) measurements. Another thing you need to consider is the “Void Space Ratio” or VSR. Large pieces of gravel have a greater amount of gaps between them than smaller pieces. Sand has very little VSR because it packs together so tightly.
VSR dictates how easily water will flow through the media. In an ebb-and-flow system, this is crucial. Sand has a remarkably high BSA, and a remarkably poor VSR. So, it’s a great filter, but a lousy grow media.
Fist-sized rocks have a great VSR, but a comparatively low BSA. So, it’s a great drain material, but not a great grow media.
20mm to 30mm (~0.7 to 1.1 inch) pieces of biochar, on the other hand, have a nearly perfect combination of BSA and VSR. That’s great news! So, what’s biochar?
charcoal produced from plant matter and stored in the soil as a means of removing carbon dioxide from the atmosphere.
In more detail, when you heat biomass in absence of oxygen to about 400–500 °C (752–932 °F) , all of the stuff that would burn just turns to vapour and “boils off” (called pyrolysis (2)). That leaves a remarkably capable soil amendment and water purifier behind; biochar.
Now, you might say “that sounds like charcoal to me,” which is exactly what I said. So, I bought a bag of rock-maple charcoal “chunks”. I knew that the formed briquettes are often “doped” with stuff to make them light easier and burn better; stuff that tends to be toxic to fish. So, I went for the “chunk” charcoal.
The grow beds are 20 cm (8 in) deep, in total. The bottom is 10 cm / 4 in 20mm to 30mm (~0.7 to 1.1 inch) pieces of charcoal, and the top is 10 mm / 0.4 in diameter river pebbles. While the pebbles have a high BSA, the VSR is low; they pack together well. On the other hand, they are great for the plants to grow in, and easy to “farm” in terms of planting, harvesting, etc.
The charcoal “foundation layer” ensures that deep in the bottom of the grow bed, there is lots of air and water movement. The pebbles do a great job of providing the plants with a place to grow. Win-win, right?
Well, “ish”. Yeah, “ish“.
As I’ve remarked earlier in my blog, this has had a noticeable side effect in pH management. In a system with as relatively small a set of grow beds as my own, the system pH just doesn’t acidify fast enough to compensate for the alkaline effects of the charcoal. So, I routinely float around pH 7.5, instead of the 6.5 I’d rather see.
I’ve even poured (diluted) sulfuric acid into the sump. No real effect.
The other issue I noticed is that the water coming out of the grow beds often has an almost “oily” look to the surface. Now, because of the “dropped water aeration” I use at each step of the process, I’ve never had any appreciable oil film on the surface of the fish tank. It seems to get broken up and then broken down pretty quickly, and doesn’t seem to be bothering the fish. They’ve tripled in size since they went into the tank.
However, this leads me to the principle difference between biochar and charcoal. Charcoal is meant to be burnt. Biochar is meant to be buried. That means that when they make charcoal, they stop the pyrolysis process “early”, leaving a mixture of volatile organic compounds (VOCs) — mostly natural oils — left in the charcoal. That way, you don’t need a blowtorch to get the stuff to burn. A bit of naphtha or kerosene will do it.
For biochar, all of the VOCs are burnt off, leaving nothing but the “activated” carbon behind. So, lesson learned: there is a difference between charcoal and bio-char. Bluntly, I got lucky that the difference didn’t kill the fish or the nitrobacter in my system.
While I will recommend using a bio-char and sand mixture if you want to use an external filtration system for “polishing” the water, it’s not required. I may try making some “is really biochar” myself and using it as grow media in my next system. I’ll keep you posted.
As always, comments and questions are welcome!
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