Backyard Aquaponics
A place to discuss aquaponics

Your first reason doesn't really apply in a GB based AP system because the surface area is so much more than you need even if you were to use marbles (relatively very little surface area per/m3).

The second reason may help a bit and I'd love to know by how much but it probably wouldn't be much since trapped air would help to extend the time a system could stay flooded and constant flooded systems seem to do so well?

I don't fully agree, the surface area is not always biger than you need in a grow bed. You design the grow bed to have a good surface area in relation with the quantity of fish and the quantity and quality of food you gonna feed them. If you use a good media you can increase a bit the quantity of fish/food or at list having a kind of security even if it´s not good to have a big extra surface because it can leave Space for bad bacteria.

There is never too much surface area. There is media that is too small not allowing the water to flow, but when it comes to biofilters, surface area is king. Bad bacteria could possibly also grow, but the beneficial bacteria will out compete the bad bacteria quite quickly.

It is true that the surface area is not always bigger than it needs to be but when its not other things will be going wrong as well like GBs clogging with too many solids. In a well designed system there is always more surface area than needed to take on the role of bio-filtration because of the large volumes of media needed to store the solids while they are processed.

You design a bio-filter based on surface area. GBs are a different beast, because the solids are never removed (phyiscally) the primary function of a GB is solids storage so they can be removed over time as they are mineralised. Since this is the case it is not the micropores in scoria or hydroton that give them an advantage over other granular medias because it is the spaces between granules that are important where the solids get stored as they are being broken down over time.

Stuart Chignell:
''the primary function of a GB is solids storage so they can be removed over time as they are mineralised.''

The two primary functions of a GB are:
a support for bacteria (transformation of NH4+ to NO2- and NO3-)
a support for vegetables

you can ad a mecanic filter before your grow bed and it will work. it is true that a Growbed and a bio filter are different because the Growbed can't be washed with a simple back wash but the principle is the same in both systems. even if it is true that there is an accumulation of organic matters (fish waist, roots, dead bacteria...) I definitly see an interest in the use of porous materials as scoria and expensed clay balls to increase the surface area.

Due to increased tension levels lately (on other threads) I want to make it crystal clear that this is meant to be a debate about ideas and a disscussion of principles. Obviously you think my statement is wrong and I have no problem with that what so ever. Equally I think your statement is wrong and I hope that you don't have a problem with that or the way I express it....



If you like we can get into the aquacultural engineering calculations but for now discussion only.

A very, very, important nay essential role of GBs in systems without any other growing or filtration components is bio-filtration where the surface area of the media provides a habitat for the nitrifying bacteria to convert the TAN to nitrates. Without this function being performed the fish would die. In other words as I said an essential function.

Having said that it is still not the primary function. If the bio-filtration function of GBs was their primary function then we could size them much smaller than we do and we could size them much smaller still by using media with a higher surface area like scoria, hydroton or even better yet k1 biomedia.

Consider this the claim of the manufacturer of the K1 media:



Taking the example of the k1 media and assuming a feeding rate 3% body weight per day a bio-filter containing 125L of K1 should be able to support a 1000L IBC stocked with 20.1kg of fish. Now if the the manufacturer is being a little "excited" about their product maybe we should double the size just to be safe but we would still end up with a GB/bio-filter that was only 250L to 1000L of FT or a 1:4 ratio.

I hope people would agree that this is way undersized for a GB system where the solids are not removed from the system. Of late (last few years) there are a number of ratios that are touted about the internet and this forum and while many of them suggest that you can have less GBs than the tried and true 2:1 ratio (GBs:FT) none of them go so far to say that you can have a ratio so biased towards the FT as 1:4 (GBs:FT), well not with any credibility at least.

The reason you will see some people on this forum still advocating for such relatively large GBs is because you need the volume of gravel to store the solids while they are turned from organic insoluble wastes into inorganic soluble nutrients. Also it has been found that when GBs are constructed to be 30cm deep they tend to provide enough growing area to keep the nutrients in check.

You could convincingly argue on this basis that the GBs primary functions were equally solids storage and crop production components. Indeed in systems where the GBs are installed with sufficient volume to match the fish feeding rate and are 300mm deep it would be hard to argue against this. However, there are a number of systems which we havn't heard much of lately that wanted to explore the concept of the volume of gravel being important not the volume of gravel that was 300mm deep.

These systems used gravel beds with depths of greater than 300mm to test how the GBs would work when the gravel was deeper. Some of them had GBs as deep as a 1m. Without the volume of gravel relative to the FT being increased this would decrease the square meters available for planting. These systems where designed to include other plant production components such as DWC or NFT to boost plant production and keep the nutrients in check. Lots of reasons for wanting to design and build a system this way but one of the inarguable features of such a system was that the primary function of the deep GBs was solids storage while they were processed within the beds.

You can have a system which incorporates GBs and DWC or NFT and if you where to isolate the system from the GB it would still function with the other plant components operating as the bio-filter. You could say that this is a completely different example and you would be right expect that the system could continue to operate for quite sometime before the solids became a problem because there was no component that was capable of processing them. In such a system the GB may be performing as a bio-filter but it certainly isn't its primary function also its role of plant production is secondary because if the bed was not planted the system would still perform for quite some time even if the DWC or NFT was not large enough to grow enough plants to keep the nutrients in check. A solution to this could easily be the addition of more DWC or NFT or more simply planting the GB but still the plant production function of the GB is secondary.

My system was one of these but unfortunatley I still haven't got around to building the DWC component, Outback Ozzies was another, I think TCLynx may have some deep GBs in one of her many systems. There are probably others but I can't think of any off the top of my head.



You are absolutely right there is interest but my contention is that this interest is misplaced and misinformed. A couple of years ago a few people took this design principle to heart without being fully educated and reduced their GB volume (by reducing the depth) according to the media they filled the bed with. The result was dead fish which was then one of the examples used to justify the belief that it was essential to remove solids from AP systems and not allow them into the GBs.

The first person I saw to use clay balls in their systems was EB and the biggest reason he did and does is because commercially it is so much easier because they are so light. While materially they cost more any savings made by using gravel are lost to his customers because of the increase labour costs. I believe that if EB was in Victoria and WA then he probably would not use clayballs but scoria because scoria being so light and so cheap would win against clayballs. Again both have a higher surface area than many other gravels but it would be the light weight of the material that would be the deciding factor not its surface area.

Stuart, Thanks for the reply, sorry if I didn't put the good form in my last messages, I respect your point of vue and my only aim is to exchange the opinions, there is absolutly not tension from my side. English is not my mother tongue so pardon me if I can be direct sometimes. I understand that for you the advantage of porous materials are more the light weight than the surface area because of the obstruction of microporous holes by solids. I also agree with this phenomenon but I still thinking that even if the solids are obstrusting some of the microholes it's not a big deal and porous materials still more efficient than regular gravels. Furthermore porous materials can handle more solids than gravels. However I reconize that the function of solid storage is important in a GB but I designed systems with vortex mecanic filters before the GB and in this case the quantity of solids sent to the GB via the water flow is very low. In thoose particular cases the quantity of solids in the GB is due to the growth of bacteria and roots more than anything else. Finally I completly agree to say that the volume of growbed media has to be a little oversized and it's not because we use porous materials that we can grow double quantity of fish.

Stuart you should write a book!

Do you have any articles or studies to support this?

It may be true, it would be logical, but there are so few studies of GBs in an AP context or even a non AP context that are applicable that we can refer to. I don't know of any that have trialed different media where the solids are kept within the GBs.

Of course you have said you are removing the solids in that case the GBs are not processing the solids at all so of course it can't be their primary function.

Haha.

Since I thought we were talking about GB based systems I just thought I'd check what the original question was...

So far off topic as usual (couldn't find the eye roll emoticom).

I don't have an article to support the view that Jonathan put forth, but I do have a theory. With more surface area, there is more bioslime. With more bioslime, the more particles get held. Look at Colours pool conversion thread. She tried everything to get her algae in check. UV lights, barley extract, sand filters. What finally worked for her was dumping the sand in the sand filter and replacing it with scoria. While it is not still perfectly clear, I suspect that once she has a second bathtub filled with scoria inline on her system, she will be much clearer. Never underestimate the power of bioslime.

Thanks Slowboat.

I think I could write a chapter or two or some articles but not a book. In particular my horticulture knowledge/skills are not what they would need to be.

There are multiple ways of interpreting that. Sand has a high surface area to volume ratio (compared to gravel anyway) as does scoria but scoria has more voids between granules.

My theory and observations is that all the pores of scoria and clayballs get blocked pretty quickly by bio slime growth. Unless organic compounds are somehow transported from the outside of a clay ball through the bio-slime to the inside of a ball I don't see how much is going on in there to process the solids or the ammonia. Much like corals are dead on the inside, all the activity is on the outside

Having said that I wouldn't be surprised if more complicated things were going on inside scoria or clayballs. I'd love to see a real trial that compared the different options.

You say blocked by bioslime, but bioslime is the whole point. It is the fact that bioslime inhabits those pores that makes them desirable. It is where all the action is from nitrification to solids filtering. It is not like we are trying to put solids into the pores. The tackiness of the bioslime grabs the finer particles, then the bacteria go to work on said solids to release the nutrients that were previously locked in them.

The tackiness of bio-slime only traps particles large or small if there is water moving past the sticky surface. As a piece of scoria becomes covered in bio-slime the small pores can get covered over. With larger pores they remain and still increase the surface area but in the case of something like clay balls because the pores are smaller and can get "filled in" or covered over that much quicker. the majority of the nitrifying will be occurring on the outside of the granules.