mhaigh

Sand Bed Details

119 posts in this topic

Thanks. That clears up the math quite nicely for me.  Basically, less free space between particles means more surface area. But there does seem to be a trade off of that extra SA.

 

Unfortunately, the reality is that these small particles trap solids much more efficiently and rapidly foul with accumulated biosolids, leading to anerobic conditions and lower dissolved oxygen (DO) concentrations that negate the benefits of small particle size.  This low hydraulic conductivity and small pore size (low void space/void fraction) makes small-particle media inappropriate for most biologically active systems with active cycling.  To avoid this problem, larger particle sizes are commonly used (17 mm crushed granite or ¾ inch crushed granite) having higher void ratios (and resulting high hydraulic conductivity) so that solids impact percolation less.  However, even though these crushed aggregates have significantly higher SSA than non-angular and non-crushed aggregates, SSA is still comparatively low, resulting in reduced overall system Biological Surface Area (BSA or total surface area of system measured in m2).

 

So I guess a happy medium between SA and filtration needs to be reached so that the filtration does not trap and clog too much too quickly.  

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#1 is absolutely correct , that's just basic chemistry. It's considered a Given.

 

I don't mean to insult you as I already know you know this, but others may find this helpful.  http://www.chemguide.co.uk/physical/basicrates/surfacearea.html

 

It will be interesting to figure out the differences in the case of #4.  I've broken growbeds down ranging from small 3' x 2' to 8' x 4' close to 50 times with running times of a few weeks to a year and have seen it in the bottom, perhaps bottom third is more accurate though.  I do utilize scoria mainly and hydroton in only one or two beds before abandoning it.

 

Another use of sand with similar function and results. It's just a generic overview, but gives the idea. I new that somthing good would come from working at the wastewater treatment plant. 

http://aglifesciences.tamu.edu/baen/wp-content/uploads/sites/24/2017/01/L-5229.-Sand-Filter.pdf

 

Edited by ande
fix sand-filter pdf link (see edit history)

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#1 is absolutely correct , that's just basic chemistry. It's considered a Given.

 

I don't mean to insult you as I already know you know this, but others may find this helpful.  http://www.chemguide.co.uk/physical/basicrates/surfacearea.html

 

In terms of nitrification.. I absolutely agree...

 

But you wrote

 

1. Vastly greater amount of surface area with sand vs other media. Greater surface area = faster solids breakdown and greater fish load.

 

I don't believe that claim to be necessarily true... if it was.. then surely we would be discussing optimal (actual) grow bed surface area, rather than the oft discussed optimal depth... and/or the optimal grow bed surface area in conjunction with the SA of the media, and perhaps even the actual media size, in relation to void spaces between media sizes

 

Paul's question is related to such questions.. no disputing the huge SA of sand as a media (and increased nitrification potential).. and Mark's data indicates the need for a specific size (and type) of sand media....

 

But the question remains (validly).. as to whether void space between any selected media is more, or less, optimal.. in relation to solids breakdown...

 

And the link you posted to sand filtration in waste water treatment clearly discusses the points that Paul and yourself raised...

 

It’s important that the sand particles all be about the same size.

 

If the grain sizes vary greatly, the smaller ones will fill in the spaces between the larger particles, making it easier for the system to clog.

 

The larger the grain size, the faster the wastewater moves through the sand and the more wastewater that can be filtered.

 

Small media slow the water movement and increase the chance of clogging.

 

The grain size also affects how deep the solid particles penetrate the filter and how clean the final effluent is

Edited by RupertofOZ (see edit history)
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In fact (and I'll photograph it tomorrow)... I'm currently pulling down and cleaning a hydroton grow bed (previously heavily stocked system.. and long term tomato root growth)... that's been running "constant flood" for 2 years...

 

That shows most of the solids accumulation in the middle third of the media... and I've seen similar results in any "gravel bed" that I've decommissioned... probably even more so... ;)

post-3396-0-76745100-1406869240_thumb.jp

post-3396-0-43342300-1406869251_thumb.jp

post-3396-0-15726400-1406869255_thumb.jp

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According to the link I posted above:

an intermittent recirculating free access sand filter can have loading rates up to 15 gpsfd. To reduce the size of the sand filter, the designer may use a free access sand filter with a higher loading rate, but the higher rate generally means more maintenance requirements. For this system, a three-bedroom home (240 gallons a day) with a sand filter loading rate of 10 gpsfd would have a 24-square-foot sand filter (6 feet by 4 feet)

 

Notice the 15 gpsfd vs the 1.2 gpsfd of the buried sand bed.   The buried sand bed functions more like a continuous flow GB where the the intermittent is closer to the flood and drain GB.

 

I can't tell for sure from you photo's, but it appears that the solids accumulation is primarily at the root that have broken off in the GB.  Regardless of whether it is the middle or bottom. I think we can both agree it would be easier to scrape and/or replace the very top layer of sand, vs shoveling out 1/3 to all of growbed to clean out solids.

 

As to your comment about grain size, I believe that is why Mark was very specific on size of sand and grade and the need to have fines and clay removed.  He had also stated that he had not done any studies on maximum loading of the system. That is something that will need to be investigated.  Though there might be some studies in the similar field of wastewater treatment.

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Hi,

 

My two cents for what it is worth.  I know what I "fish" out of my RFF and it is no small amount each week.  I cannot imagine not having a filter of some sort working with any media. Why would sand be the exception?  It will clog the works.  I know it is eventually broken down by bacteria and all, but that would have to be sme really fast super bacteria to keep pace with the waste.

 

Here's my take on what's happening with the sand beds:

  • The water flow is intermittent......the furrows are flooded and then the pump stops while the water percolates down through the sand.  Water only flows during the daytime - and then only for relatively short periods compared to other flood and drain regimes - not overnight.
  • The particulate matter is trapped on the surface of the sand.  Only dissolved solids make it any distance into the sand because of its density.   There is little impact of the dissolved oxygen in the system because the decomposition of the solid wastes is largely happening on the surface.  Clogging at the surface does occur - which is why the beds are "scraped" as part of the regular maintenance regime.  This disturbs the surface and allows the water to percolate into the sand.
  • The biological action that is occurring on the surface is accelerated by the formation of algae.  I'm guessing that you have large amounts of heterotrophic activity....combined with the photosynthesis of the algae.....happening on the surface.  During the daytime, the algae is producing large amounts of oxygen to support oxidisation.  At night, where the algae would be demanding oxygen, there no water being pumped onto the beds.  At night, there's no shortage of oxygen at the top of the beds since they are not being watered - everything is just lying on top of the sand....or within the top few millimetres of the top.
  • Bio-filters of any type work best when they are not loaded up with solids (a fact that seems to escape the basic flood and drain crowd) and since only dissolved solids find their way deep into the beds, nitrification should be no issue.  Somewhere in the stuff that Mark gave us is a report of a relatively high (single pass) removal of ammonia so this seems to bear this out.

As someone who advocates for the removal (and external processing) of solids, I'd probably continue with the use of filtration.....not because the sand beds are not adequate when it comes to solids removal and nitrification.....but in the interests of optimising the conditions in the fish tank and to enable the isolation of fish production from plant production should it be necessary.

 

Gary

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Hi

I think the sand media bed, have a lot of simmilarieties to recharging groundwater aquafiers.

Well explained in this doc/PDF on slow sand filtration (chapter 6)  http://www.who.int/entity/water_sanitation_health/publications/ssf6.pdf?ua=1  from here    http://www.who.int/water_sanitation_health/publications/ssf/en/ 

I use the method in restoring old (abandond) hand dug wells, and I'm still working on one for my AP hybrid, but for polishing the water, the plants will be anual and set in soil (earthan bed).

 

cheers 

Edited by ande (see edit history)

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Good response Gary. Although after doing some research I most likely will not use sand as there is no Quartz or granite sand in the Islands. There is lava sand (2 types) that would be ideal but that sand is not native to Oahu, just  on some of the Hawaiian islands and would most likely be expensive as well. So I guess I use what I have which is Hydroton and Lava rocks.  But why not develop a similar "sand type bed" using lava rock?  It retains the water very well and it has a better surface area than gravel, but obviously less than sand.

I would have to run the input PVC along the length of the bed channels or use 1/4 Rain Bird plastic sprinkler line, to get a good saturation.  All other particulars would be followed and tweaked as needed. Necessity is after all......yada yada yada..

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Hi Strider,

 

But why not develop a similar "sand type bed" using lava rock?  It retains the water very well and it has a better surface area than gravel, but obviously less than sand.

 

If you use upstream filtration in conjunction with your lava rock, there's no issue at all, that I can see.

 

Without the upstream filtration, however, you're not going to get the same solids screening effect......or the same single pass nitrification effect.....as the sand......and you'll need to run your pumps continuously.

 

Gary

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I'll be doing a very small scale trial of iAVS as soon as I get the new greenhouse finished. I owe Dr. McMurtry a great deal of gratitude for reviving my interest in it.

Some of the things I really like about furrowed sand beds, and the iAVS method in general:

- Greater nutrient capture/utilization. Not just nitrogenous compounds, but all the other essential, and frequently overlooked nutrients required by terrestrial vascular plants... it's pretty rare for any system to be terribly deficient in N, and if it is, it's simple enough to fix that. It's the "other stuff" (trace elements and micro nutrients) that are often deficient, and iAVS is focused on fully retaining them to the greatest extent possible.

- Relative simplicity in build/assembly. Plumbing, pump and other requirements are drastically reduced in iAVS, as demonstrated by Mark.

- Reduced costs of build/assembly. Again, reduced plumbing, pump and other requirements, which potentially saves a lot of money.

- Reduced energy requirements (possibly significantly reduced) relative to most other AP "methods".

- I'm also well acquainted with the benefits of sand as a biological filtration media in an aquaculture context, and have designed, built and used sand filters for most of my life. It just works, and works very, very well.

Some things I still wonder about, and that also compel me to try iAVS out myself:

- How well will the sand beds continue to perform with a significant amount of solids over a prolonged period of time (mainly, will bio floc and sludge accumulation present any problems over time)?

- How well will the furrows stand up to flood/drain cycles over time?

- How will I keep the sand where it's supposed to be and not travelling where it shouldn't be?

- How will maintenance and labor requirements compare to other methods?

- Will biological oxygen demands be a limiting factor in any way regarding sand bed performance?

- I advocate against running any AP system without pre-filtering the growbeds. I like to challenge my own biases/beliefs, and this will certainly do that, and it seems to me that the iAVS method has excellent potential to be the "one kind" of AP system that may not require the requisite pre-filter to run in an optimal fashion. Then again, it may also benefit from pre-filter too (filtering solids and dosing back the nutrients in a controlled fashion). Who knows. I'll try both, and compare side by side.

Should be fun and extremely educational for me. I'm looking forward to it.

Thanks again for sparking my interest Mark! Take care of yourself, and I hope to see you around again. You're alright in my book. ;)

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Hi,

 

In post #86, I said........

 

Clogging at the surface does occur - which is why the beds are "scraped" as part of the regular maintenance regime.  This disturbs the surface and allows the water to percolate into the sand.

 

Mark has advised me that disturbing the sand surface (to break up algae) was actually only necessary following the removal of one crop and the establishment of the next.....to the point where it shaded the surface of the sand.

 

He also tried to overload sand bio-filters - but failed to do so within the timeframe that he had available to him.

 

Kellen, I agree with everything you've said about the opportunity that Mark's appearance has brought to us.

 

Gary

Edited by GaryD (see edit history)

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I have to say that McMurtry's responses toward the end here are quite unfortunate since I initially assumed he was a reasonable, intellectual fellow and someone whom I wanted to read, that is until I read how he chose to react later into this thread, which is not very reasonable nor intellectual. Aggravation as justification in of it self for rash reaction is only allowed and utilized by children. I am getting quite tired of this peculiar attitude that seems to rear its ugly head particularly in hobby forums. The interaction persuades me to think less of the product, which I really do not want to think less of it.

 

 

Abe,

 

If you open up some of the documents that Kellen has put up (the American Grower one comes to mind), you'll see drawings of Mark's system.

 

Gary

 

 

Ugh, I am having a tough time finding these documents you are referencing. I tried a search for "American Grower" and looked at Kellen's post history, but couldn't find anything. I might have missed something.

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Hi,

 

In post #86, I said........

 

 

Mark has advised me that disturbing the sand surface (to break up algae) was actually only necessary following the removal of one crop and the establishment of the next.....to the point where it shaded the surface of the sand.

 

He also tried to overload sand bio-filters - but failed to do so within the timeframe that he had available to him.

 

Kellen, I agree with everything you've said about the opportunity that Mark's appearance has brought to us.

 

Gary

 

 

Thus far, this reminds me of undergravel filtration, that is typically used in aquariums and even by some ornamental ponds. Except it has a twist to it so to be applied to aquaponics.

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According to the link I posted above:

an intermittent recirculating free access sand filter can have loading rates up to 15 gpsfd. To reduce the size of the sand filter, the designer may use a free access sand filter with a higher loading rate, but the higher rate generally means more maintenance requirements. For this system, a three-bedroom home (240 gallons a day) with a sand filter loading rate of 10 gpsfd would have a 24-square-foot sand filter (6 feet by 4 feet)

 

Notice the 15 gpsfd vs the 1.2 gpsfd of the buried sand bed.   The buried sand bed functions more like a continuous flow GB where the the intermittent is closer to the flood and drain GB.

 

I can't tell for sure from you photo's, but it appears that the solids accumulation is primarily at the root that have broken off in the GB.  Regardless of whether it is the middle or bottom. I think we can both agree it would be easier to scrape and/or replace the very top layer of sand, vs shoveling out 1/3 to all of growbed to clean out solids.

 

As to your comment about grain size, I believe that is why Mark was very specific on size of sand and grade and the need to have fines and clay removed.  He had also stated that he had not done any studies on maximum loading of the system. That is something that will need to be investigated.  Though there might be some studies in the similar field of wastewater treatment.

 

 

Yeah, back when McMurtry's name was first mentioned to me sometime ago, it sounded quite interesting and I remember making the statement that I bet a specific size of sand is required to make this type of grow bed methodology operational.

 

This truely sounds quite strangely familiar with Dr. Ron Shimek's work on deep sand bed filters for coral reef tanks, except for the obvious aquaponic twist.

 

 

Is there anyone, or can provide references, that is presently relying on this type of grow bed for a significant amount of time?

 

Anyone that has an illistration reference of a root ball after growing in this type of grow bed?

Edited by crsublette (see edit history)

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 Complete drainage in 15-20 minutes from pump cycling off.  Entire media atmosphere exchanged (refreshed) with each cycle.  Leave drained (gas saturated) for approx 1.5 to 2 hours - still plenty of water (and O2) to keep 'everyone happy' even at 40C+ with full sun. 

 

 

The sand's particle field capacity, that is moisture retention ability, is huge. Even though there is a complete drain of free available water within 15~20 minutes, there would still be a quite large significant amount of water remaining due to the sand's field capacity.

 

Actually, this is starting to sound like a basic flood and drain version of a soil wicking bed, except it is sand rather than soil.

 

 

More I think about it... The less I am inclined to try it...

 

Ugh... More questions, fewer answers... More assumptions, fewer clarifications...

 

 

Need to see someone upclose with it as a grow bed currently functioning, possibly a video or some type of series of illustrations.

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Charles,

 

I first referred to Dr Mark McMurtry (and his part in the history of aquaponics) on this forum in 2007.  When I look back on those words, they seem woefully inadequate in light of what we've learned recently.

 

Yeah, back when McMurtry's name was first mentioned to me sometime ago, it sounded quite interesting and I remember making the statement that I bet a specific size of sand is required to make this type of grow bed methodology operational.

 

Yes......I think that conversation started back in February.....with this post.

 

You're right about the sand particle size......but that's only part of the story.  The iAVs system is flooded from the top......so that the water flows along the furrows.  Once the predetermined flood cycle ends, the pump shuts down and the water percolates down through the sand.....(and this is just one point of difference with the basic flood and drain system) leaving the solids trapped on/near the surface.

 

More I think about it... The less I am inclined to try it...

Ugh... More questions, fewer answers... More assumptions, fewer clarifications...

Need to see someone upclose with it as a grow bed currently functioning, possibly a video or some type of series of illustrations.

 

Sometimes, you can intellectualise things to death.  There's no substitute for experience......and context......and that can only come from trying yourself.  

 

The more I learn about it, the more I am inclined to try it.  I find myself wondering what aquaponics might have really looked like had iAVs remained free of the distortions wrought be the basic flood and drain system.

 

 

Gary

Edited by GaryD (see edit history)

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Hi C

Nice to see you back on the forum

 

 

Ugh, I am having a tough time finding these documents you are referencing. I tried a search for "American Grower" and looked at Kellen's post history, but couldn't find anything. I might have missed something.

 

Look at Kellens posts from 15 july and onwards here http://aquaponicsnation.com/forums/forum/21-useful-information/

You have to download the PDF docs

 

cheers

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Hi C

Nice to see you back on the forum

 

 

Look at Kellens posts from 15 july and onwards here http://aquaponicsnation.com/forums/forum/21-useful-information/

You have to download the PDF docs

 

cheers

 

 

Yep yep, just found it. Finished reading the thread, Raft vs Flood/Drain, and it is quite impressive.

Edited by crsublette (see edit history)

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1) You're right about the sand particle size......but that's only part of the story.

 

According to what Dr. McMurtry has written, the sand particle size is much more than "only part" of the story. Sure, the construction and operations is mutually exclusive with the sand particle size, but, from what I understood, the sand particle size is the primary actor in his research and the entire system.  From what I understand, as discussed at post#44 and post#124 in the Raft vs Flood/Drain thread, if there is any deviation at all with the sand, then there may likely be problems resulting in a crash; however, in contrast to the construction and operations, I could see how there could be deviations yet still not lead to a system crash.

 

I might be placing too much emphasis on the sand, but, if I am correctly interpreting Dr. McMurty's text, the sand's grade and quality sounds to me to be #1, incredibly important, as it is most important in these type of filters in other contexts.

 

 

2) Sometimes, you can intellectualise things to death.

 

My motto is "plan for the worst and hope for the best". Much like a canary in a coal mine. The canary is going in hoping for the best, but the entire purpose of the canary is to plan for the worst possible scenario. My father has taught me this and this is the reason my family owned farm land, and god willing for good health, will qualify for the cintennial agriculture register here in 20 years; nowadays, quite a rare event to have a farm last 3~4 generations or longer. The funny thing is... We got started all due to my grandfather buying land to start farming during the Dust Bowl while everyone was running away from farming the area, that is when this area was turning into the sahara desert for a significant period of time.

 

Thus, I am keeping a skeptical open mind about Dr. McMurtry's research.

 

 

 

3) There's no substitute for experience......and context......and that can only come from trying yourself. 

 

Sure there is a partial substitute for experience, that would be wisdom, except wisdom alone is not sufficient. Where would we be if it were not for the wisdom being past down through generations... We would end up repeating quite unecessary and costly experiences. I think Dr. McMurtry attempt here is sharing his wisdom so we have a somewhat easier stance to further add our experiences to the literature.

 

 

4) I find myself wondering what aquaponics might have really looked like had iAVs remained free of the distortions wrought be the basic flood and drain system.

 

Unfortunately, the problem I find too common is that too many always want to implement wisdom lacking experiences. However, in this context, there is not such thing as "always".

 

Thus, I imagine there might be good reasons for some of the deviation or reluctance in accepting it wholeheartedly, unless 100% of all the deviators and skeptics are just simply ignorant fools that lack enough brain cells to rub 2 pennies together in the business context.

 

 

 

This is why I need more information rather than just accepting the good doctors research whole cloth. It is wisdom that leads folk to ask for second and sometime's third opinions, especially when it involves doctors and university tested research.

 

 

Is there anyone, or can provide references, that is presently relying on this type of grow bed for a significant amount of time?

 

Anyone that has an illistration reference of a root ball after growing in this type of sand grow bed?

 

Testimonials upclose with it as a grow bed currently functioning, possibly a video or some type of series of illustrations?

 

 

 

Charles,

 

I first referred to Dr Mark McMurtry (and his part in the history of aquaponics) on this forum in 2007.  When I look back on those words, they seem woefully inadequate in light of what we've learned recently.

 

 

Yes......I think that conversation started back in February.....with this post.

 

1) You're right about the sand particle size......but that's only part of the story.  The iAVs system is flooded from the top......so that the water flows along the furrows.  Once the predetermined flood cycle ends, the pump shuts down and the water percolates down through the sand.....(and this is just one point of difference with the basic flood and drain system) leaving the solids trapped on/near the surface.

 

 

2) Sometimes, you can intellectualise things to death.  3) There's no substitute for experience......and context......and that can only come from trying yourself.  

 

The more I learn about it, the more I am inclined to try it.  4) I find myself wondering what aquaponics might have really looked like had iAVs remained free of the distortions wrought be the basic flood and drain system.

 

 

Gary

Edited by crsublette (see edit history)

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Allow me this grace and take this into consideration before there is reaction to what I wrote. I am not writing to dispute Dr. McMurtry's research. I am not writing to bait Dr. McMurtry. I am not writting to suggest Dr. McMurtry's iAVs sand system does not "work". However, I am quite disappointed that I am compelled to write this disclaimer.

 

"It works" by univserity research and doctorates and other select greenhouses is not the correct positioning for me to be persuased; since, as I see it, Dr. McMurtry has already done the work and research to clearly show "it works" and I am looking forward to reading the further PDF files. However, "it works" is not the correct persuasion to convince me to adopt it whole hog because.... "It works" is the reason why legacy technology still remains thus inhibiting software innovation. "It works" is why there is thousands of acres wasted on the most costly, in both environmental and economical respects, form of modern renewable energy that is influencing some eastern European countries to experience a new form of modern energy poverty. "It works" is the reason why so many still use DWCs, or whatever other methodology, as their system rather than Dr. McMurtry's iAVs sand bed system.

 

 

I think Mr. Van Der Werf's offer of wanting to collaborate with Dr. McMurtry to be quite exciting, as mentioned in post#114 in thread Raft vs Flood/drain, but, due to recent interactions in this thread between the two, I have the impression that fruition of the arrangement might have been spoiled due to unfortunate chosen reactions. I hope cooler heads prevail so for the benefit of this quite fascinating facet of agriculture, that is integrated aquaculture (i.e., aquaponics).

 

 

So to start creating benchmarking mechanisms, I am simply curious as to whom, that is (rather than a handful) over an appropriate large sampling of hobbyist and organizational test plots sharing results and testimonials, has taken the leap of faith to replicate Dr. McMurtry's iAVs and achieved the same level of university research success at various scaling and various environments within a comfortable variance.

 

 

My impression is that Dr. McMurtry's iAVs sand system has been around for a while so I am assuming ample time has been allocated to allow folk to try it; so therefore, my query for more public information from folk that are "in the fields" utilizing Dr. McMurtry's iAVs sand bed system.

 

This type of request does not require decades of information gathering.

 

 

For the backyard hobbyist, I view the iAVs system as something fun to do. However, since I can not yet afford a greenhouse, I can see how the system might be easily fouled by turbulent weather; so, it might be a system reserved for the greenhouse, which is quite fine.

 

 

 

I am quite excited to read about Dr. McMurtry's iAVs sand system. I always enjoy learning a different way of doing things.

 

Welp, that's all I have to say and not going to stir up anymore trouble. I'll leave the responses to their own devices.

 

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As a new back yard hobbiest I do not tend to get to "scientific about why it works, just that it works...Sorta like the American Indians putting 3 fish around their corn stalks. They had no idea why it worked, only that the corn grew faster.

I look at the Profs's work in sand culture as a type of flood and drain. I know flood and drain works for a variety of media and have no rerason to believe it won't work for sand. My only concern is how well it works with sand. And if not for the fact of my current location I would already be testing it.

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According to what Dr. McMurtry has written, the sand particle size is much more than "only part" of the story. Sure, the construction and operations is mutually exclusive with the sand particle size, but, from what I understood, the sand particle size is the primary actor in his research and the entire system.  From what I understand, as discussed at post#44 and post#124 in the Raft vs Flood/Drain thread, if there is any deviation at all with the sand, then there may likely be problems resulting in a crash; however, in contrast to the construction and operations, I could see how there could be deviations yet still not lead to a system crash.

 

I might be placing too much emphasis on the sand, but, if I am correctly interpreting Dr. McMurty's text, the sand's grade and quality sounds to me to be #1, incredibly important, as it is most important in these type of filters in other contexts.

 

Not at all.  The emphasis should be on the sand type.  This concerns me quite a bit because the system was designed and in Marks mind for poor areas to build basic "wetlands" to grow food inexpensively.  However, I live in a place with more sand than you can imagine and yet finding the right type of sand here would be like a needle in a haystack.  There is sweet sand (the good stuff for growing in) and there is salt sand which is not good for growing in.

 

I have seen organic farms using sweet sand beds enriched with compost to help it hold water and yet they go through water like no tomorrow.  Despite claims sand filters hold moisture really well.  Screening that sand to the appropriate size would be a nightmare in poor communities, if they had access to the right sand.  Who would determine the right sand?....

 

Either way, I remain unconvinced the sand would hold up to feed loads with any other type of fish that required better water quality than catfish or tilapia.

 

Further to that, my concern is the statements the water cascades back to the fish tank, the water in the bed is oxygen rich... but the pump only runs during the day...  The logic of that escapes me.  

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Further to that, my concern is the statements the water cascades back to the fish tank, the water in the bed is oxygen rich... but the pump only runs during the day...  The logic of that escapes me.  

 

Me too

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Ok well found at least some information on the loading rate of the sand bed.

 

I can not say if the different sized systems were optimized for fish production or the feed rates were to suit the plant species grown.

 

Attached below is a capture out of this document:
http://aquaponicsnation.com/forums/topic/8952-water-quality-maintenance-and-mineral-assimilation-by-plants-influence-growth-of-hybrid-tilapia-in-culture-with-vegetable-crops/

 

Seems the maximum density of fish at harvest was 16.73kg/m3 with a great feed conversion ratio of 1.27 using a fish tank volume to growbed volume of 1:2.25 (fish:growbed)

 

Experiment 1 - Average density 15.99kg/m3 with 1.37 FCR and average harvest weight of 216 grams each

Experiment 2 - Average density 10.30kg/m3 with 2.16 FCR and average harvest weight of 444 grams each

Experiment 3 - Average density 13.89kg/m3 with 2.76 FCR and average harvest weight of 692 grams each

 

No info as to how hard they pushed the fish system.  The feed conversion appears to get worse regardless of the grow bed size with increase in fish size (even with decrease in density), though in each experiment (notably experiment 2) the feed conversion appears to improve with an increase in growbed size.

 

Experiment 1 and 2 would raise questions on its sustainability at a larger scale.

 

My assessment based on that set of data, every 1000 liters of water you may need at least 2500 liters of sand in the growbed to maintain a density and subsequent feed loading of 15kg of fish per 1000 liters.  Others report similar results with larger void space media like expanded clay and 20mm gravel.

Edited by Paul Van der Werf (see edit history)
crsublette likes this

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