Backyard Aquaponics
A place to discuss aquaponics

I currently have two beds running that are constant flow. I am having success and am in the planing stages of my next bigger system. My question is, it seems on this site and others fill and drain systems are favored over any other (constant flow, raft, etc.), why is that?

Hi TheP,
Simply because you can use Gravity to great advantage with GB or FT above each other, but most importantly, by continuously running a smaller pump to serve the system you save on both energy costs and maintenance on the pump. Continuous duty pumps are designed for just that, and Flood & Drain using auto siphons for the timing cycle is far more energy efficient vthan using timers to switch a pump on and off etc. I'm sure you'll get this gist of this by redaing and searching the forums for more qualitative information. You can still use F&D on DWC or Raft in an NFT system, it is just a matter of getting rid of the cause for another point of mechanical failure!

And the main reason is: it gives air time to plant roots and Bacteria between flood cycles, thus making better use of the available media surface area.

Cheers IanK

Thanks Ian. I see what your saying about the air getting to the roots and the bacteria. My pump is constantly running though, I have set up the drains in the GB so that I can controle the water level in the GB. I would say I have 6cm of water 3-4cm of wet expanded clay and 1-2cm of dry expanded clay on top. The beds are fed with water from fish tank on one end then drains from the other back into the tank.

Flood and drain gives greater water aeration and more time for the plant roots to get air. Agree
A smaller pump using less electricity to do the same job ,, sorry but that's wrong.

Hi Chappo,
I would have thought that by using say a 100W submersible centrifugal pump continuously would have been more 'efficient' than using a 200W pump of a similar type, timed to switch on and off for half the time, to deliver the same amount of water, all other things being equal. Wouldn't the intermittent start-up and switch-off on the larger pump use more power than if it were continuous running? I thought that for single phase (which is all I have) an electric motor uses a 'surge' of power for starting, then uses less than that for running? The other 'efficiency' I would allude to is that of the maintenance of bits and pieces in the pump and motor. I thought that a continuous duty rated pump and motor operated better and for longer than if it were switched on and off long term. I will take your advice as being correct on these matters as I'm just another bloody Architect who is also trained in Mechanical Engineering. I did my Mech. Eng. apprenticeship at Ebsray Pumps (only Positive displacement pumps) at Brookvale in Sydney. Maybe Centrifugals are different for power use? Thanks for your input, I look forward to your good advices.

Cheers IanK

Ian , I get what you are trying to say but pumps/electric motors become more efficient ( in general ) with additional size.
Example Rio HF12 35 watts lifts 1,200 litres of water to 1.2 M height in half an hour.
Rio HF 8 20 watts for a full hour to lift 1,020 litres.
Efficiency tends to increase with size.

The current surge at start up is negligable , not in size but length of time. In a water pump application probably less than half a second at around six times normal motor current. So in a half hour run peroid that extra draw represents less than 0.3%, not worth considering when calculating running efficiencies.
I do agree that machines in general tend to wear and / or fail at start up ,, but a motor starting 24 times a day would not be overly stressfull.
I'm just a Technical manager , but i did my apprenticeship in Electrical fitter/mechanic A-grade Electrician , 2 years post trade electronics , Engineering in Electrical and Electronics ( certificate level) and 28 years experience hands on. But i could be wrong:)

"I currently have two beds running that are constant flow. I am having success and am in the planing stages of my next bigger system. My question is, it seems on this site and others fill and drain systems are favored over any other (constant flow, raft, etc.), why is that?"

Because you get a clip round the ear if you suggest anything else; very rarely based on rational discussion

There is a difference between constant flow and constantly flooded.

My system is flood and drain and constant flow since the flood and drain is achieved by auto siphons. The other option is to run a pump on a timer cycle and stand pipes to manage flood and drain but that then would leave my fish tanks without much or any flow or aeration during the pump off times which I don't like at least for my type of fish and system.

As you are doing, you have constant flow with a pretty constant flood level in your grow beds. This can work but system balances will be different than in a system with flood and drain grow beds. I think Creative1 might have a constantly flooded system that is working well so might be able to elaborate on how that system works.

The Advantages of flood and drain media filled grow beds are multiple and all in a package.
They act as solids filters
They act as bio-filters
They provide plant space with support for large plants.
They provide space for composting worms to help deal with the solids making more nutrients for plants
They do all these things without any added aeration or separate bio or solids filtration devices.

Constantly flooded media beds may need extra aeration or be more prone to root rot problems with the plants if not careful. If the aeration is not good enough, the bacteria my have trouble keeping up with a heavily stocked system and there could also be problems with solids clogging things up. Aeration becomes very important for a constantly flooded system.

If you want to do DWC (raft) or NFT they are options but as with constantly flooded set ups, aeration becomes very important again. Also, DWC and NFT don't take care of the filtering in and of themselves (well some bio-filtering takes place on all wet surfaces that are protected from light that get enough aeration but some additional bio-filtering is likely needed.) Primary need in DWC and NFT becomes solids filtering and this does need to be dealt with separately and before the water goes to the DWC or NFT. This is how most commercial set ups do it. I'm actually doing some NFT pipes that are fed with water after it comes out of the grow beds. It seems to be working great for basil. I may also try some DWC at some point. The main drawbacks with these more "hydroponic" methods is that they require "net pots" and extra pipes and transplanting and supplemental plant support for large plants or root pruning in small pipes and large plants etc etc. I think these methods are far more appropriate to a production model commercial set up that employs a person to maintain it every day. These methods are far less appropriate to a backyard APer that might go away for a week long holida.

Most of us tinker with many of the "other than Flood and Drain" methods but only as add on bits attached to the BYAP model of Flood and Drain media beds as the primary method.

Hi TheP.... have you measured your nitrite levels at all...

I have a theory that constantly flooded systems are on a knife edge of imbalance... often reflected by either a really prolonged nitrite spike... before the system cycles... but just as easily spikes back again....

Or, in fact never really cycles at all, with the nitrite reading always present.... in the order of 0.25-0.5.... and spiking easily to 1.0....

Lynx: I hear what you are saying and I think I'll study the BYAP model for my next system. Thanks for the info.

Rupert: I my Nitrite spike lasted for a couple of days, and my readings now are right around 0.25 and have been for a couple weeks.
I havnt really had any problems with the fish but that could be just because I'm only using Gold Fish right now. I do feel like my system has found a "balance"? my water tests have been consistant for a couple of weeks, but the Nitrites have never gone to 0 they bottmed out and stayed at 0.25. Current water readings are Amonia 0, Nitrite 0.25, Nitrate 10, P.H. 7.4. Advice? Comments?

Pretty much what I expected... and IMO... that's where your nitrite reading will stay... running a continuous flooded system...

What type of problems do you anticipate I will encounter with that level of nitrite?

As you up your feed for another type of fish, you my see higher nitrite spikes which could had long term effects on your fish health. The more aeration you can provide to the flooded beds, the better chance you have of lowering your nitrite levels.

Chappo,

Thanks mate, I appreciate your patient explanation and I hope others take note of this advice as well!
I may just have to rethink the sizing of my pump I was ready to purchase.
Thanks again,

Cheers IanK

In what way is nitrite uptake limited by constant flow systems?
Is it a lack of oxygen that does not promote nitrifying bacteria?
What are your opinions on a wet/dry (trickle filter) to alleviate this problem (and deal with initial ammonia and nitrate filtering)?