Sand as a medium in a flood and drain bed?

[Kobus Jooste]

I have taken an interest in using sand as a media in one of my grow beds again.  Only thing is that my beds cycle permanently on flood and drain auto siphons.  I am worried that the sand will pose a considerable problem in continuing this type of water circulation.  One option is to put the beds on timed cycle and just use an overflow standpipe with drilled base (overflow if filling rapidly, draining when water is cut).  I'm not too keen on this, as I am intent on keeping the "no aerator" idea going with the use of cascades in the unit.

 

The next option is to have a small amount of gravel in the bottom of the bed and a screen between the gravel and the sand.  This will likely develop management issues in the medium and long term.  A final option is just to have a fine mesh strainer on the guard pipe, but I am not sure if this will allow enough water through to efficiently operate the siphon.

 

Any thoughts on / experience with this issue?  

[TCLynx]

I agree that you probably want to leave the system on a constant pumping set up (at least for the rest of your beds,) but I also think you will need a "timed" inflow to this sand bed because I don't think you will manage to get a fast enough flow through the sand to manage a siphon.

 

I think you might want to look into using a small "header" tank to "flush" into your new sand bed.  This would allow you to probably keep the same inflow (as long as you can manage a little extra height for a header tank near your sand bed) rate all around roughly but give you a fast infow into the sand bed and allow the sand bed to drain at it's own rate with a standpipe.  The means of "flushing" the header tank could be siphon or flush valve or even FLOUT, as they are all pretty easy in a wide open flush tank.

You may still have issues with the screen around the drain of the sand bed getting clogged so you may want to design a multi layer sand guard that you can swap out layers for cleaning.

 

Hope one of these ideas gives you some inspiration to help you figure out a way to work this.

[Harold Sukhbir]

Hi Kobus,
We had discussions sometime ago about the use of sand as AP media here
http://aquaponicscommunity.com/forum/topics/4778851:Topic:28202
Clogging and flowrate will be some of main issues to address. I've been playing around with the idea of a timed flow bed with horizontal multi holed drain pipes at the base, just about covered with a layer of smooth gravel, and covering this a bed liner made with nylon type cloth fabric. I think sand offers many benefits in terms of electricity use, nitrification, ease of root growth( which might lead to faster plant maturity ). Someone just has to get serious about establishing various system trials using sand. It's from these we'll get conclusive!

[Kobus Jooste]

I saw most of the latter bit of your sand discussion Harold, and now went back to recap on all the other bits.  I am interested in sand for two reasons - It has an enormous surface area for the same bed compared to gravel, which means that if it is used in combination with other media, it could greatly improve the biological filtration capacity of the unit.  Secondly, my system is in need of the extra solid media bed space.  I had the idea to begin with that it would not be easy to integrate the sand into an always on, fast cycling water line, hence the brain picking with this topic. 

 

Here is my scheme:

1. in the set-up I am going to add for the NFT line will be a relatively clean line of water in a 20 liter drum, from where the filter boxed pump will feed clean water to the NFT.  I can put another pump (which I already have) on a timer in this 20 liter supply drum, and feed the sand bed on its own.

2. Downgrade the main pump as it is not having to feed the whole greenhouse any more.  With the main pump reduced in capacity and two smaller pumps on timer, a three pump hybrid will run on less power than the current 1 pump configuration.

3. I'm going for large sand grains - swimming pool filter sand basically.  That, coupled with relatively clean water and a longer timed rotation will be the essence of the experiment.

 

I also think the sand is a good alternative for a wicking bed in trying to grow root crops - the sand will give way nicely and should be able to allow carrots, onions etc to grow.  I will try leaf and root crops to begin with.  Timer on a rotation of 15 minutes on 1 hour off, or what do you guys think? 

[Harold Sukhbir]

Hi Kobus,

Good point of the use of silica sand with it's PH neutral. The coarser grain(Heavier), coupled with a low pressure drain should be a good combination to allow a larger mesh size with fairly quick cycles. I've tried clay sand for growing seedlings which works quite well but requires controlled watering to prevent damping off. With this type of sand your trial should easily be successful!

[Kobus Jooste]

Let us hope it pans out.  I can also have the same thing set up later on in a 300 liter aquarium system that I wanted to have as a simple gravel bed.  Perhaps it should be sand too, with different flooding cycles.  I'm planning to make a lot of changes during my winter........

Harold Sukhbir said:

Hi Kobus,

Good point of the use of silica sand with it's PH neutral. The coarser grain(Heavier), coupled with a low pressure drain should be a good combination to allow a larger mesh size with fairly quick cycles. I've tried clay sand for growing seedlings which works quite well but requires controlled watering to prevent damping off. With this type of sand your trial should easily be successful!

[TCLynx]

Seems to me that you would definitely want a fairly long period between floods for sand beds.  Seems like it would probably be a good situation to use something like an indexing valve for.

[David Waite]

Harold  dont use screen around your drain as it will become a cleaning nightmare. Use 1/8 inch peagravel around your siphon 6 inches out at least. This will work much better as a filter because the worms will clean it. Highly recommend worms to help keep the sand from packing in. Sand will go to 80 percent compaction with just water. Good luck I hope it works.

[Harold Sukhbir]

Hi David,

There are varying sizes to pool filter sand. I think the one that Kobus wants to use is really tiny tiny gravel with clean water so it won't be as big a problem as regular sand. Here is a picture............

http://www.google.tt/imgres?imgurl=http://www.members.shaw.ca/kuhli...

[Kobus Jooste]

TCLynx: It could have been a nice thing, but I will have two problems.  First up, to have a timed cycle away from the main gravel beds, I will have it on a different pump, and secondly, to have it on a different cleaner line it will operate out of a smaller sump being set up for the NFT line.  There will not be enough water volume to allow the kind of pump that will be required to drive the cam on an AP indexing valve.  Further, there will only be two lines out of the clean line - one for the NFT and one for the sand bed.  Getting an indexing valve for two low flow lines will cost more (it will be an import) than getting two timers and one pump. 

TCLynx said:

Seems to me that you would definitely want a fairly long period between floods for sand beds.  Seems like it would probably be a good situation to use something like an indexing valve for.

[Kobus Jooste]

I will consider it.  I was thinking about having the same kind of drains for my outflow as what you get in pool filters - basically a pipe with a lot of fine gaps cut in them (I will use a band saw blade) placed in a grid on the bottom of the bed and then protected by some gravel.  The gravel layer I was thinking of would be around 10 - 12 mm deep as the bed is not all that deep and do not want to loose too much "sand space".  What do you think?

David Waite said:

Harold  dont use screen around your drain as it will become a cleaning nightmare. Use 1/8 inch peagravel around your siphon 6 inches out at least. This will work much better as a filter because the worms will clean it. Highly recommend worms to help keep the sand from packing in. Sand will go to 80 percent compaction with just water. Good luck I hope it works.

[Kobus Jooste]

I have no idea what coin that it thus it does not help me with scale - our pool filter sand range between 1 and 5 mm.  Possibly the finer ones are running a risk of going right through, as a band saw blade will probably leave a 1 mm gap in the drain pipes.

Harold Sukhbir said:

Hi David,

There are varying sizes to pool filter sand. I think the one that Kobus wants to use is really tiny tiny gravel with clean water so it won't be as big a problem as regular sand. Here is a picture............

http://www.google.tt/imgres?imgurl=http://www.members.shaw.ca/kuhli...

[David Waite]

Kobus sorry for the name screw up earlier. Sand really wont move much under low flows once settled in to your grow bed. I think the larger gravel keeping it away from your drain is a must. I like the band saw idea. I just dont know how they are going  to handle the fish poo over the long run. I have skill saw cuts in mine and I had a hard time getting enough water to flow in my affnan syphons. Take pics and keep us posted. Very curious to see how it goes for you.