Backyard Aquaponics
A place to discuss aquaponics

In a gravel bed, it floods, drains and leaves a thin water film.
Usually the water flooding and drain time is much less than the time for which it is left with the thin film of water (usually a ratio of 15min : 45 min).
Suppose we assume the main processing by the biofilm is done for the 45min, then we see that for every flood and drain cycle, the volume of water actually processed is the volume of the thin water film!!

What do you think?
Is this the actual need for the large growbed to fishtank volume ratio?
Is this the reason why additional filters help greatly when the volume ratio of growbeds:fishtank is very less than 2:1?
Is this why sand might help so much, (except for solids processing), especially as fluidized sand bed filters or something?(sand - 1000:1 surface to volume ratio. But the problem is about maintaining aerobic conditions.)
Is this why continuous flow is said to be way better than F&D, if you take care of the plant roots? (I read some forum members saying this on a continuous flow vs F&D thread).

Nope Gokul, you've got it all wrong....

The growbed media is what houses the nitrosomas bacteria that process the nutrients into nitrates which are available for plant uptake.....

The bio-film is basically an algae growth byproduct that builds up on the walls of the tank/sump/pipes etc... and in the bottom of the growbeds (as you say) because of a thin layer of water left behind after a drain.....

The layer in the bottom of the growbed also probably contains some semi or unprocessed solids material.... that's why worms like it and many encourage worms....

Most people DON'T use additional filters... unless they have a particular need like a seperate isolation tank, temporary shortage of growbed space, lack of plants etc....

Some have experimented with RSG filters and fluidised filters as a way of bringing down excess nitrates....

UV filters, solids skimmers etc have been discussed.... mainly because the people have used them in either aquaculture or aquarium setups..... and intend using them as a SUPPLEMENT to their growbeds or in isolation.

IMHO... there not needed in an AP system... the flood and drain growbeds work just fine....

Is flood and drain better than continuous flow..... well yes, if you want to do more than floating raft applications with lettuce basil etc...

Because of the depth and methology of flood and drain it lets us grow just about anything, becuase it gives us the depth for plant support... great oxygenation and large bio-filter capacity...

Continuous flow systems usually involve pre-requisite solids filters, skimmers and either very high flow rates or supplementary oxygen.....

Flood and drain works beautifully...

Oh it seems there is a new post before I could submit this! I will reply to the above post next.

But here is the post I typed with so much effort:

Maybe we shouldn't underestimate the volume of the thin water film though! The surface area is quite huge.
What is the volume of the film? Let's see. (In the end it turns out to be 0.02 of the growbed volume eeks!)

Let us take an example and study it (I'm going to use hotch potch guesswork numbers here, hopefully it won't be too far from the real thing):

We assume pea gravel because it is mostly spherical and things are easy to calculate.

For 10mm pea gravel (don't know if it's available, but we have 10mm gravel, it'll be close), the surface area to volume ratio of each gravel bead is S_TO_V = (4 PI r^2)/(4/3 PI r^3)
(^ means raised to the power of). 'r' is the radius of the gravel bead.


Let the displacement ratio be D; Usually D =1/3, that is water volume needed to flood the growbed is 1/3 the growbed volume.

The volume of gravel in a growbed of volume V is thus V - DV = V (1 - D)
The total surface area of the gravel growbed is approximately (neglecting gravel touching each other and the walls of the growbed itself) is S = V (1 - D) * S_TO_V


Let the thickness of the water film be t = 0.1mm ( I don't know what it could be - really wild guesswork).
The volume of the water film left in the growbed after drain is therefore


For this case, t = 0.1mm, D = 1/3, S_TO_V = 3.
So
Vt = 0.01 cm * V * (1 - 1/3) * (3 cm squared per cubic cm)
= 0.01 * 2/3 * 3 * v
=0.02


I think this is small!
For a growbed volume of 100 litres, only 2 litres is processed in each cycle!

Note that this is complete guesswork. But the real numbers are not going to be far.

The nitrosomonas are first are in suspension in the water coloumn (start of cycling). Then they attach to surfaces. They form a slimy matrix in which they live, on every possible surface where conditions (oxygen, ammonia, nitrites) are available. This is the biofilm.


Don't get me wrong, I dislike too much complexity when KISS is possible. I dislike unneccesary high tech (although shortly I will be an electronics engineer!). I would like nature to do the work as much as possible, we shall only be facilitators to get nature to be able to work.
I love flood and drain the best especially because of it's oxygenation ability and energy efficiency. I am an energy efficiency freak. I am also a don't-leave-a-large-footprint-on-the-environment freak.

And I am a peaceful kind person who likes to see everyone happy so I will put this smiley ->

gokul, your assumptionon pea gravel being round in order to calculate surface area is way off the mark.

What is the measurement around Australia's coast-line. Now what is the measurement if we get more accurate and start measuring in and out of every bay/river. Nowwhat id we measure around evry rock in every bay / river and coast,,NOW what if we measure around every grain of sand on the coast and every river/bay,,,youget my drift?
The pea gravel has holes,dents . and minute gaps, these are all areas for bacteria to grow.

Not trying to poo your thinking.

I guess , in the real world,guys on herethat have played with fish/grow bed ratio's that they have not calculatedthe actual surfaceareas but have , over years of experimenting,concluded that the amount ofgravel in a bed twice the size of the fish tank has enough surface area to do the task required.

So, following the KISS principle,,on this point,stop calculating and start believing:)

Don't get mewrong,I love when people think and question the staus quo.

[quote="Gokul"]
The nitrosomonas are first are in suspension in the water coloumn (start of cycling). Then they attach to surfaces. They form a slimy matrix in which they live, on every possible surface where conditions (oxygen, ammonia, nitrites) are available. This is the biofilm.[/qoute]

Gokul, it may be true to some extent that initially the nitrosomas may be in suspension....

But personally I believe it's more likely to be the nitrobacter... the nitrite processing bacteria that's in suspension....

It's my belief that in order to survive, thrive and reproduce the nitrosomas bacteria NEED a surface to attach to.....

As Chappo says the surface area of pea gravel, hydroton etc is definiely NOT smooth and in fact the surface area is vast..... certainly sufficient for our purposes..... sand has huge area, but doesn't oxygenate because the small grains pack so tightly together...

Sand filters are usually anerobic .... unless fluidised....

I still maintain that the GREATER volume of nitrosomas processing is down IN the growbed media.... not in the thin film on the bottom...

Ganted this will no doubt do some processing... but as you have so brilliantly calculated the volume is insignificant.... it's the 98% of the other stuff... the growbed media, that does the work

Rope - I generally agree with what you have said, however I think your interpretation of continuous flow is wrong. Continuous flow when referred to in AP is not normally NFT or DWC, but rather a method where the water flows into the grow-bed at the top - through a grid - and out the bottom through a drain, without flooding the bed. This has been said to be more efficient than F&D. We don't like it as much because of issues with getting even distribution of water and the need to have a space inefficient grid.

Every time I've discussed "continuous flow" in that way... it's inevitably been .... "no, no, like UVI"

Yep, regardless... If it was "countinuous flow in the sense you indicate (and that's the correct definition - UVI's system is DWC) then Gokuls' point is still basically invalid (IMHO) regarding the thin bio-film volume doing all the work.... in fact it wouldn't apply at all in a continuous system.

Which is why I guess he leans towards the argument that the nitrification is taking place in the water...... still not so IMHO....

And yep to it's in-efficiency compared to flood and drain.... in fact is anyone running continuous flow.... or at least have they been for a sustained period of time with success?

The most efficient method to run a grow-bed in terms of biofiltration, IMHO, is continuously flooded. Problem is, this will not suit most plants. I currently have my gravel bed running in this way, through necessity (I've not plumbed it all properly yet), and have definitelly noticed that it's conversion capabilities are heaps higher than my F&D (not unexpected given that water is contacting the gravel all the time - not just while flooded).

BTW - it should be noted that quite a lot of us have a much more rapid F&D cycle than 15/45, particularly those of is using autosiphons. I haven't timed mine lately, but I expect it would be about 7/4. Of course when doing it like this, the flood time is higher than the drain time, the opposite of a timer/float based system.

I'm NOT going to repeat the THREE functions of the growbeds AGAIN.

food for thought, nitrifying bacteria also colonise within a medium (unless its totally not porus) there have been studies on the degredation of limestone buildings due to nitrifying bacteria acidic byproducts eating away the mineral. this happens BELOW the surface.

So multiply your figures by even 1mm multiplied by the radius of the bacteria and you will arrive a a vastly different figure. and thats not for very porus media.

take scoria for an example and you can probably multiply the new figure by 100 again.

then take into account the liquid retention of the media over the 1 hr and you get a higher figure again.

dont re-invent the wheel. read. learn. then discuss.

Good point... suppose we often take things as granted, just becoz it's the way most do it...

You could say that most of us run a semi-continuously flooded system then ...

1

oh dear I've upset Chappo. I was afraid someone would surely be upset by those horrid calculations... but I thought a few would like it. Didn't mean to put half of you off by these numbers upon numbers.
You can skip that stuff, only noting the main idea that the volume of the water film could be small.


No, I am not saying that nitrification happens by bacteria in the thin water film. I am saying that it happens mainly due to the bacteria physically and permanently attached to surfaces forming a biofilm, and that after draining, these bacteria on the biofilm can process only the water that is stuck to the biofilm, that is, to the surfaces of the gravel.
Note that there are two 'films' stuck on surfaces here : the biofilm with the nice bacteria and the water film stuck on top of the biofilm (after draining).

Yes I meant veggie boy's continuous flow.

AHHH, I was not put off. Geeze I've got a bad habit of typing in a way that makes people think I'm angry,.....,,definately not angry.

Extra smiles all round.

Oh ok, my arguement is for those who do 1hr cycles or something similar, maybe for plants that need airy root zones?. Oh forget that, maybe there's enough air even in the 7/4 cycles?

Yes, yes, I know... But what I mean is that the maximum amount of water that the biofilter in the growbeds can process per F&D cycle is the volume of water that is needed to flood the growbeds.
With an example, what I am saying is that if you have a 100 litre growbed with 1/3rd, that is around 30litres of water to flood it, in F&D, in every cycle, at most only 30 litres of water can be processed. This is usually much smaller after most of the 30 litres drains away (leaving only a water film on the surfaces).