Backyard Aquaponics
A place to discuss aquaponics

Hi and welcome stewart you are welcome any time just email to arrange a time i will let the others answer the tec side

30cm is the optimum depth for the gravel to allow room for root growth and bacteria colonisation - more depth would not be a problem in growing plants however:
gravel is heavy - more gravel = more weight on the growbed stands
1 to 2 metres would be a waste IMHO



excess gravel is not a problem, the ratios are a guide for max stocking/minimum grow bed - greater growbed medium or less fish weight just has the system working well within its comfort level.

There are other factors which could be taken into account like:
more gravel means more fish water needed to flood growbed
aeration maybe less
longer to fill GBs may have a slight impact on ammonia to nitrate conversion......and so on

I've got one bed at 40cm and one at about 55cm. They have only just been planted, so we'll have to wait and see how the system goes.
Oh, and of course I've got 6 vertical half drums, 40cm too. The fruit trees are in them.

Thats actually the reason I was asking. Looking at EB's recyled system where he has several layers of activity got me thinking. If I can have all the gravel on the bottom of the system dug into the ground. Then above ground in the super structure have stuff growing in shallow gravel (lettuce maybe) or via NFT. Means I can get away with a less sturdy structure (read easier, cheaper).

Why we are on the topic why is 30cm considered the optimum

Also that green house I mentioned that I bought is actually only th metal frame any ideas where a good place to get the plastic from would be?

Stuart.

Stuart,

this company at Bendigo sells greenhouse film

I think the 30cm is so that the bacteria have enough but not too much depth of water over them, ensuring optimum oxygenation and water exchange

the reason we've got deeper beds is partly for stability for the fruit trees, and partly cos its what we got with the tanks

i agree with LKB's responses. (we need to have a "tick" smily)

First off I'm not really sure of the whats what on a site like this, this being the first forum I've joined but where should I be asking my questions posting my responses about my first system?

Any way further research has been done.

I don't know who it was but who ever posted the link for the SRAC site thank you very much. Especially SRAC 454.

The system that they detail has a biofilter for nitrification and then all the hydroponic side of things is done via floating raft.

So the answer to my question would seem to be that the gravel (biofilter) can be as deep as you like as long as the water column moving through the gravel dosn't have any calm/still spots. As long as this occurs all the gravel will be recieving oxygenated nutrient rich water and will support a healthy growth of nitrifying bacteria.

Does my interpretation make sense?

IF so then systems could be designed where all or at least a portion of the biofilter is contained in the one spot. Possibly an advantage for systems trying to take advantage of several different heights within a green house.

Having said that is there an advantage to having the plants rooted in the biofilter? I ask this because I've been reading about grow beds colinized by worms and the complex ecosystem that develops in a mature aquaponics system. Does separating the growing of the plants and the processing of nutrients into different containers eliminate some of these complex interatctions and hence their benefits? For example do any of the possible



SRAC 454 page 10

bind to the medium of the biofilter? If that were to happen then separating the biofilter and growing beds would not be a good idea.

Does anyone know?

Stuart

Stuart, I've split this off to a new spot, continue the discussion here

Stuart, i think a definative answer is a long way off

IMO there'd be no problem in locating the gravel in once section, apart that if you are using any type of NFT or DWC then the supply to this should be taken after the biofilter as it also acts as a solids trap thus preventing solids from accumulating on the plant roots.

The 300mm depth and volume per fish per tank volume that has been stated has shown to

a) provide enough biofiltration (you could probably get away with much less for this function)

b) provide good solids trapping

c) is large enough in both volume and area to allow an equilibrium to be estabilshed between solids trapping and solids breakdown by micro flora/fauna.

(C) IS THE TRICK. It allows us to retain ALL solids which provide P and K allowing us to grow fruiting vegetables versus "leafy greens only"

Sorry to ask this because it has been covered before, I know this because I read it some where here.

I've spent hours looking for the ratio between water, gravel quantities and grow bed square meterage.

Anyone help?

Stuart

Stuart, If what you mean is volume of water compared to volume of grow bed then I think from memory it's about 40 - 50% water volume to grow bed capacity... if you're using gravel as your medium slightly higher for other media.

You can work out how much water is displaced by getting a clear 2L jug, fill to 1L with water. Add gravel to the 1L mark. The altered volume is the displacement.

or does he mean the fish to tank to growbed ratios? maybe link to the current thread on this? J? you're so much better at linking than me

1kg fish : 100 liters fish tank : 200 liters growbed
1 pound fish : 2 gallons fish tank : 4 gallons growbed.

A 600 ltr grow bed filled with 20mm drainage gravel takes 187 ltrs of water to fill right to the top, but in a normal cycle the water fills to within 20mm of the top of the gravel and therefore only about 130 ltrs of water is pumped into a 600 ltr grow bed.
The grow bed I am talking about is one of mine and can be seen here
Grow beds

Muzza