Backyard Aquaponics
A place to discuss aquaponics

Aside from the obvious problem of it getting into the outer pipe & going down the standpipe / autosiphon / whatever when GBs drain , is there any other reason NOT to use sand as a growbed media ?

from looking at how effective fluidised bed filters ( or whatever theyre called ) are , I assume sand is able to hold a hell of alot more bacteria than gravel / hydroton . Or is that kind of filter only so effective because its constantly in contact with the water ?

Is there any reason bacteria would NOT prefer to be in flood & drain sand rather than flood & drain gravel ?

Im thinking I can make the outer pipe of my standpipe for my new system "sand-proof" with flywire , filter foam / sponge or something . Sponge would probably get clogged , but fine enough flywire / mesh . coupled with extremely coarse sand should stop any clogging problems . The way I see it - coarse sand surely has more surface area compared to the equivalent amount of gravel / other media type . Or is hydroton so amazingly awesome that its got more than sand ?

Im making the outer pipe for the standpipe tomorrow , so any reasons to / not to use sand would help me decide what to do .
Maybe sand would need mulch / a covering of some sort over top of it

If the sand is compacted into a large bulk it becomes anaerobic, and the bacteria can't survive.

what about something smaller than 5mm bluemetal ? would the same thing happen ?

If i was to do a bed with sand, I would be looking at way to keep solids out of the sand, where it can clog and produce anaerobic areas as tamo says. Maybe a section of sand surrounded by geotextile, surrounded by a more freely draining media like gravel that can filter solids without clogging.

Also, the general feeling I get from the forum is that the total surface area for bacteria is very unlikely to be the limiting factor - long-term solids buildup and surface area for plant growth is more of an issue. Are you planning a system where you will push the ratios of surface area to growbed volume beyond what is normal?

IMHO... the major reason for not using sand is that it is so fine that it compacts down with constant water and takes up the spaces between the sand particles that would otherwise fill with air (oxygen) during a drain cycle.

This is the MAJOR benefit of flood and drain systems.... and the types of media generally used...... the ability to provide plants with a highly oxygenated and porus growth media....

Think of clay.... almost impossible to grow anything in .... because it retains excessive moisture (i.e doesn't drain well) and can't provide sufficient air to allow oxygenation of the plant roots....

Personally I can't see ANY reason to utilise sand as a growing media.....

Sand filters are employed in wastewater applications BECAUSE whilst they drain (slowly) they are in a sense impervious.... thus trapping solid particles and other nasties suspended in the water..... ultimately due to lack of oxygenation they become anaerobic and stagnent unless allowed to drain and dry for significant periods.... or in some way turned over or replaced....

A fluid sand filter works BECAUSE the sand is in suspension and is constantly keep moving by the pump action, which also introduces a degree of oxygenation as a consequence.....

ah ok - I thought the sand in a sand filter was compacted , but thinking about it now thats a silly idea & I see that the inflow of water pushing it around gets the whole thing aerated well

oh well , just a thought . bluemetal it is then

Rupe, I agree. But...

Sand doesn't compact over time if it's made of evenly sized particles that aren't too irregular in shape. It can't. Think of a stack of billiard balls. You could jump up and down on them as long as you want, and nothing would happen. That's why sand (and rubble for the same reason) is used for trenches etc - because it attains maximum compaction very easily, and maintains its permeability afterwards.

In dirt gardening, sandy soils drain too freely and dry out too quickly for growing crops in dry areas. You need finer particles and organics that can bind together to form structures to retain moisture.

But you are right. This argument breaks down if waste particulates are introduced, becaude they fill in the gaps very easily and clog them up.

And you're right about fluid sand - it's completely different.

Sand is theoretically useable in AP, but gravel makes more sense to me. Sand would probably need a slower fill rate, and longer times between floods to allow the plants to take up the moisture and open up the soil voids for aeration. So the overall throughput of water would be much lower than with gravel, reducing the effectiveness of the bed for nitrate conversion.

would it work better if the sand was loaded up with heaps of worms ?

you could use large screened river sand as enough air should get into the medium between flooding.

this would take a long time and effort to fill the beds ,especially when you can just wash some gravel and chuck it in.

however sand may serve a purpose for root crops like potatoes, carrots ect.

i know trials are being done using coco peat for root crops but not sure about sand.

i will chuck some screened river sand and do a trial. i just happen to have some sitting around.

the good thing about the screening process is that you can use the fine screened sand for a solids filter. this would serve to prevent clogging in the larger particle sand growbed.

i believe sand has a part in the ap chain. its just a matter of finding its place.

if you do use sand in a filter it would be sensible to sterilize it first with chlorine. never know where the sand has come from.

i say give it a go and let us know the results. what have you got to loose?

True Steem..... bad choice of words..... was trying to convey the fact that as the media drains it essentially "compacts" down in the sense that as you say...



as opposed to other types of media..... thus by nature the voids between other media are larger and provide greater pockets of air/oxygen... whereas the sand particles settle closely together with only very small voids... due as you say to its "regular" size and shape.



About it in a nut shell I think Steem... if it were possible to use it at all, then this would probably be the only way ..... the corrollary though is it might be a fine line between slowing the drain enough to provide sufficient water retention and providing enough oxygenation draw down through the media to prevent root rot and/or anaerobic conditions developing....

As BF says... maybe we need some further experimentation...

Still think the tried and proven methods and media are best adopted by those building a new system.....

Experimentation needs to be controlled and measurable against known "working" parameters... if you get my drift

Well summarised Rupe
Who says you need a difference of opinion to have a decent argument?

As an aside, F&F's carrots in ap vermiculite looked fantastic, and my soil potted plants are responding very well to fish water. There are plenty of options...

Since sand drains slower than gravel, the flood-drain cycles would need to be much longer....hours or days depending on how soil-like the media became. Still, as long as it drains long enough, it should be aerobic. Perhaps one could have a filter barrel (flood/drain) with gravel or other media for ammonia>nitrate conversion and set up a sand bed that would flood/drain, but with a moisture sensor so it does not do it too often....

There are a few people out there who have tried sand in some grow beds. It is noted that the surface tends to stay moist all the time. I think Jaymee and Axil may have a grow bed with sand.

I was doing some experiments with sand as I was digging about 3 yards of it out of the ground where my pond/tank went. My sand is too fine to be very useful in a grow bed though I have set up the top tank over my flush tank with some sand on top of fly screen over top of gravel/perlite. The water level never gets near the surface of that sand but due to cappilary action, the surface is always wet and starting to turn green. It might make a useful way to root moisture loving difficult cuttings though I don't expect to do more with it.