Backyard Aquaponics
A place to discuss aquaponics

Yep - members here have found that in the most part F&D works better than CF for a series of reasons stated in threads elsewhere.

Agreed, although my system... the one this thread talks about, is continually flooded and looks to be working well, I have doubts whether it will be 1. A long term method eg. Runs well for extended periods and 2. Scaleable to support huge numbers and size of fish where the size of the solids increases and oxygen demand is higher.

These are the two things I wish to discover after maybe 6 months to a year of trialling my current small system.

I think we should re-define and post in the glossary continuously flooded and continuous flow, might cause some confusion.

I take continuously flooded to mean a bed that is always flooded and as more water flows in the same amount flows out.

I take continuous fllow ( the one i tried) as an unflooded bed where water is distributed (sortof evenly) over the top and tricles through the media and exits at the bottom( may be one side or across the bottom).

That is the way I see it too Steve. Sorry for the confusion

LOL, which way? LOL

Now who is confusing people. The third from the top..... :~

So continuously flooded is pretty much (recirculating) dwc with medium, continuous flow is a top fed drip/trickle system.
Flood and drain logically falls somewhere between the two.
It`s interesting that continuously flooded appears to do somewhat better than continuous flow.
I used to use trickle towers on my aquariums and they were quite effective at reducing nitrates.
The continuous flow (trickle) works in exactly the same way so perhaps the plants aren`t getting so much of the nitrate?

The continuously flooded bed behaves much like a submerged biofilter and loses less nitrate to the air leaving more available for the plants.

Flood and drain is a bit of dilemma, something between the two extremes. I`ve noticed many say the that FD system seems to "kick in" at a certain point, which could be the point when enough solids/biofilm have accumulated making it closer to a continuously flooded than continuous flow system?

It could be that simply the buildup of nutrient rich sludge causes the performance increase but there may well be other factors involved.

Any thoughts?

I don't think its just a f&D type that has the delayed "kick in"

Of course, all systems will have the mandatory 4 week minimum startup time due to the nitrogen cycle establishing, but also was it dr.rackoy? (some one correct me with the right bloke) while doing his trials on AP vs simulated inorganic salts that found the AP performed better after an "aging" period. And i'm fairly sure he was using nft with solids filtering?

Thanks Steve,
I`ll have to look around for that.
Maybe all systems perform equally well after some tipping or balance point is reached.

I read somewhere that an essentially submerged media system alledgedly performed (20%) better than one that had access to virtually unlimited supplies of O2.
It seemed reasonable to assume that there may be less chance of anything escaping to the atmosphere in the submerged system as the majority would be reabsorbed into the water column.

I like the idea of a trickle fed format, but if it doesn`t perform as well as FD then its probably not worth the effort.

A pity really as you wouldn`t need a completely watertight bed for it except at the bottom perhaps, and it would certainly be a simple/cheap way of adding extra temporary (seasonal) growing space in cooler climates.

Hi Hex,

welcome aboard.

We went over this a fair bit recently mainly due to my incessant questions.

To summarise:

The 20% improvement in productivity of Flood and Flow over Flood and Drain was recorded by Leonard Wilson. See here:
http://attra.ncat.org/attra-pub/aquaponic.html

His research is the only research that I know of where productivity comparisons have been made between F&F and F&D.

The preference for F&D systems by members of this forum seems to be a result of shared experiences of what has worked (good evidence but not in the formal documentable sense).

The continous flow as a system component is actually widely used in peoples vertical towers and I believe some of the horizontal PVC growing systems are packed with substrate and have a contious flow (Would these be CF or NFT? Is it still NFT even if the pipe is packed with substrate?).

Stuart

Hi all my system has 4 differentthings [cant think of the word] water from the fish tank goes to flood and drain beds also fish water flows threw the strawbery tower and lemon tree for 3 min every hr filtered water from the grow beds then goes threw the nift continuse then to the dwc yabby pond and floating raft in my whole system all the water is one big loop and with the experiments i have done there is very little or no difference in the growing would put up pictures but the camera has carked it but there all in my thread

Hi Stuart
If the flow is more than a few mm deep then probably not. It wouldn`t be a "film", but more likely SWC perhaps?

Nothing beats practical experience and as FD is the most popular system it must be a good one, whether the data supports it doesn`t really matter... on paper, bumble bees can`t fly

I was just interested in finding any likely looking reasons why a continuously flooded (but flowing) system might do better than one that had copious availability of O2.

Taking aeroponics as the most extreme version of this type (high O2), alledgedly having the fastest growth characteristics, it would point to the FD or trickle/drip system being the more optimum choice.

The contradiction is that a trickle tower used in a pond/aquarium setting will reduce the nitrates to very negliable levels without using plants.. it also uses high O2.

If you put plants in this type of setup they could be competing against a system that can lose nitrates very effectively to the air (70-100ppm reduction per day is very possible) ..the plants would probably not do so well.
It does appear to closely resemble the FD or trickle/drip system at least until solids or biofilm accumulate sufficiently to reduce air infiltration in the bed?

It could be one explaination (if it is actually the case) why a continuously flooded bed with sufficient quantities of DO will apparently do better than FD or trickle flow.
It`s reasonable to assume that more of the nutrients would be held or reabsorbed into the water column rather than lost to direct contact with atmospheric air.

Just an academic exercise
Any system that grows fish and plants is a good one

Hi FF
Just goes to show that all the data in the world doesn`t make a jotts worth of difference, when it comes to practical real world testing LOL

hex, you've lost me man. Nitrates are a dissolved salt, hence can not to my knowledge be "lost to the air" in the way that a dissolved gas can.

They can be reduced all the way back to N2gas and oxygen, by a bacteria that reverses the original nitrification process, but very specific anoxic conditions are needed for this to happen.

The growth media is not oly concerned with growing plants, but solids entrapment. with wilsons CF, they needed to clean their filter screens twice daily, and the removal of solids excludes growing of many plants i think.

Hi Steve,
That`s exactly what i thought.
Here`s my train of thought which might help clarify my conclusions.

When i read the claim that a constantly flooded bed could be 20% more productive than one open to the air i tried to find out why.

It wasn`t down to having more 02.. as there would probably be less, so i figured it may be something to do with the food supply and went looking for info to support the theory.

Trickle towers reduce nitrates in an aerobic environment and function in a similar way to FD beds, so perhaps the FD is behaving like a trickle tower and dumping some nitrates..? Constant flood was lacking the same air contact so logically that could be the reason for the supposed improvement.

Next thing was to figure how a trickle tower loses nitrates..turns out there isn`t a definitive answer, but here`s a reasonable one:

"The way trickle tower filters, or shower filters, or well aerated submerged media filters get rid of nitrates is to adequately degas the variety of nitrogen oxides that are formed from nitrate and nitrite conversion. That is why the media needs to have good air contact, the nitrogen oxides must escape into the air for the nitrogen to leave the water. If the nitrate or nitrite conversion occurs in submerged media that is not well aerated, the nitrogen oxides which are the product of bioconversion get dissolved in the water which react to make more nitrites and nitrates.

There is a chemical equilibrium between the chemicals nitrate, nitrite, and nitrogen oxides in the water. This equilibrium is promoted by the bioconversion bacteria. However, to drive the equilibrium toward less nitrate (and nitrite which is in chemical equilibrium with nitrate), the nitrogen oxides must be removed by the aeration process of degassing the products of biofiltration. It is no wonder that directly degassing these nitrogen oxides by having the bacteria in contact with air is a lot more efficient than even strong aeration of submerged media."


So taking that onboard at face value, a system that`s open to the air may be losing something that a submerged one isn`t.. perhaps adding some weight to the constant flood claim.

Last part of the puzzle is do nitrosomas actually produce these gases?

"Nitric oxide, nitrous oxide and dinitrogen are well-documented gaseous products of the metabolic activity of Nitrosomonas."

"Nitrosomonas europaea has an aerobic metabolism.
Evidence suggests that ammonia-oxidizing bacteria contribute significantly to the global production of nitrous oxide (produced by the reduction of nitrite) "

Nothing totally conclusive but one possible reason why a constant flood might have some advantage over a FD.. if that really is the case, which in FF`s mixed system it apparently isn`t

Sorry for the long winded post, hopefully you find it interesting reading.

Hex, just so I am getting what you say correctly. Copious amounts of O2, as in F&D, will reduce the nitrates in the system due to gas discharge?