Given its central role in the success of iAVs, sand is dear to our hearts and is something we’ll be talking about – a lot.
iAVs-suitable sand must be three things:
- It must be inert – there should be no chemical reaction when the sand comes in contact with water.
- It must be free of silt or clay.
- It must drain effectively.
The sand that best satisfies this criteria is crystalline (shar) quartz – silicon dioxide. Granite – and rounded sand – will work, too.
Sandstone and beach sand will usually contain substances that make it unsuitable for iAVs purposes. Similarly, flat and flaky sand is to be avoided.
For comparison, common table salt is 0.1 to 0.3 mm and standard cane sugar is 0.2 to 1.4 mm (w/ 70%+>0.4).
Obviously, the above has been generalized and simplified. Every sand source on Earth is unique.
Now, that was easier than reading 10 to 20 pages of mind-numbing verbiage, was it not?
NOTE. In the USA, sand used for quality concrete and/or masonry is called “washed builder’s sand”.
This sand has no salts, carbonates, clays or silt fraction. These substances can impair the function of a biofilter.
Some quarries will also custom crush and screen to specification if desired and can also wash (more than once if requested) – for a fee. Delivery costs are almost always greater than the price of the sand itself. Dry sand weights are approximately 2,850 lbs/cu yd (1.5 MT/cu m). Do NOT over load your vehicle.
In Montana, ‘good’ sand is $20 to $30/cu yd (depending on quantity) and delivery (8-10 yard loads) costs from $0.30 to $0.50 per mile. Your smilage WILL vary.
“Sand” refers to a specific range of particle sizes. All of the particles shown in the pictures below (highly magnified) are considered to be sand. [UPDATED for clarity: The first two images on the left hand side (black background) are of various materials (composition) but are NOT silica (silicon dioxide) (SiO2) sand. The image to the right appears to be a crushed granite and/or quartz. Shown here merely to convey that sand is a term for a specific range of particle sizes, and is not indicative of the particular material it could be composed of. When we refer to sand for use in an iAVs biofilter/sand bed, we mean inert silica (SiO2) sand – and perhaps some volcanic glasses as viable.
(click on image to enlarge)
stainless steel mesh screens https://www.twpinc.com/catalog/category/view/id/80/?weave_type=26
Keck Sand Shaker– NO affiliation – have not used
” An accurate mechanical sieve kit designed to provide reliable grain size analysis! 20 stainless steel screens including one each of the following U.S. Sieve Sizes: 4, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 60, 100, 120, 140, 200, 230 and 270.”
MANY online sources. US$124 on Amazon
Mesh scale unit conversion table
Mesh and screen sources (registration required)
(click image to enlarge)
Best I can source around here without getting into a special order blend ….and significantly more money…. is 6/20 (3.35/.841 mm) coarse blasting sand. I’m hoping this size range will be acceptable since it’s costing $86/ton, and according to the supplier’s calculations I will be needing approx. 4 tons for a 10x4x1.5 ft bed. By my calculations I come up with needing 3 tons, but I guess we’ll see since I pay only for the sand I carry out, and I’ll be getting the sand in stages so as to not over tax my trailer. Anyway, that’s where I am on my conversion to iAVS so far. Really anxious to get this done and operational before the season progresses too much further along.
Hi Gary, I got my sand for my Nanniode project.. it is river sand though. Can you point to the document where Dr. McMurtry’s elemental analysis of his source sand during ratio studies is mentioned? Sorry I could not find one.. still searching.
VKN….Great news! Now, undertake whatever testing is necessary to establish its suitability in terms of turbidity, hydraulic conductivity, and pore space volume. Most important of all (given your problems elsewhere) establish that it is inert – that it does not produce any chemical reaction in water. The simplest way to do that is to mix the sand with a dilute acid solution. If it produces bubbles, it’s got something in it.
My only other concern is that of pollutants. Many rivers are polluted with chemicals and organic substances so you need to ensure that your proposed sand is free of such pollutants…in your own interests.
The elemental analysis of the sand in Mark’s system has no relevance to your situation. Suffice to say, he established that the sand was inert.
Dr. McMurtry never talked about elemental analysis of source sand in his ratio studies or elsewhere so that is new for me. It could be that I have not seen that publication yet.
The sand that I chose is from a remote river. I do not think there would be chemical pollutants in them but yes there could be organic substances. I doubt or has no clue on how to make several truck loads of river sand sterile or devoid of any organic substances. However, I am trying to locate if there are any sand/soil testing lab for elemental/particle analysis. I hope to have good luck!
One of my local suppliers sources their sand on the coast – my guess is that it’ll contain too much shell and possibly sandstone. I have the info sheets they provide and I thought at one point there was more detailed sand info here in the iAVs site – but now I can’t find it in order to compare the specs with your recommendations. Can you point me toward it please, Gary? Thanks!
Also, apparently the sand I mentioned is rounded…
maybe this was what I saw some weeks back… http://iavs.info/commercial/sand-vs-gravel-etc-as-a-biofilter-media/ . But if there’s something I’m missing, please let me know!
sorry for all the comments 🙂
Wendy…We’ve written several articles on sand (including this Sand Selection Guide). You’ll find more information about the use of sand in iAVs in “Sand Bio-filter Construction and Operation – Part 2”
Wendy…..you can’t go too far wrong if you remember that iAVs sand is quartz (silicon dioxide) in the size range 0.6 – 1.2mm….free from clay or silt. If the sand is washed and graded, then that implies that it’s of consistent size (within a stipulated range) and that it is free of impurities (like clay/silt).
Any sand which contains a carbonate source will not be suitable.
Thanks Gary, I’m just trying to read two companies’ different ways of describing what they sell. Locally available bagged sands are described variously as “silica, crystalline, quartz” (Sakrete All-Purpose Sand, for US based folks with access to that) vs. a percentage-based breakdown of a sand that is described as having Calcium (CaO) at .96% and Sphericity/Roundedness of .5-.6 (.5-.6 WHAT, they don’t say) (Cemex Lapis Lustre dried sand). My LHS has 1,000 pound sacks of sand but nobody was available the other day to tell me what it was composed of.
This doesn’t warrant a reply, Gary – I’m not asking you to research this or tell me what to buy and I think you probably have enough on your plate. I just wanted to follow up with details of where I’ve gotten so far.
Good day Gary and Dr. McMurtry! I am in the process of testing several samples of granite sand, mined from nearby quarries for our latest Nanniode iAVs project. I found pH of these sand ranges between 8.2 and 8.65. The difference in pH of these samples is relatively small. The pH of source water is 7.2. Do I need to send these sand for mineral content analysis such as P, Fe, Ca, Al, Mg, etc.? Is there a direct relationship between mineral content and pH? Does the pH of sand media affect system water? I have not seen you talking about sand pH as a parameter in this sand selection guide or in other pages. Dr. McMurtry, can you please elaborate a bit on this? I look forward to hearing from you.
VKN…the sand specified for use in iAVs is granite sand – and granite sand is inert. That is, it comprises nothing but silicon dioxide…..and, in normal circumstances, it will not produce any sort of chemical reaction.
pH is a measure of how acidic/basic water is. Sand has no pH.
One way of testing whether sand has carbonate in it is to, first of all, establish the pH of your source water. You then mix silicon dioxide (eg….granite sand) in with the water – and then test the water again. If the the test reveals a pH over that of your source water, that means that the sand has some sort of carbonate impurity in it.
Assuming that your tests are accurate, and that your source water is actually 7.4 and, after stirring in the sand, the reading is pH 8.2 (as you’ve indicated with the Turquoise Farm project) then you have a problem.
A pH reading of 8.2 is way Way WAY too high for anything to do with aquaponics. Even 7.4 is way too high in terms of the availability of some plant nutrients. For the record, a difference of pH 8.2 and 8.65 is not relatively small. Remember that the pH scale is logarithmic.
We see little point in testing sand for metal content since any metals that are bound up in the crystalline structure of the sand will neve become water soluble; much less be available to the plants.
Strange sand things. Although granite is very widespread, not every rock that is named so is a true granite. (google sandatlas.org)
Let us look at for now to focus on my present scenario at Nanniode. I have been using granite stones from those mentioned quarries of 1/2 to 3/4 inches as media but never had a pH problem in my Aquaponics systems. They were inert as gravel stones. The pH were high in the beginning but found to be more or consistent for years in the range of 7.2 and 7.4. You said, sand has no pH. I am confused why the sand samples taken from the same quarries have changed my source water pH of 7.4 to the range of 8.2 and 8.65? By carbonate impurity, do you mean carbonate hardness? With a constant supply of NH3 and the nitrification process, won’t it naturally come down and make the water more acidic? Thanks for taking the time to answer. I appreciate it.
VKN….carbonate hardness would be an issue if you adjusted the pH of your source water only to find that it subsequently bounced back up to the original pH…..but that doesn’t explain why your system water is increasing from 7.4 to 8.2 – 8.65.
I can only repeat our former recommendation….that you arrange a full professional elemental anasysis of your source water and the sand that you propose to use. When I refer to “professional” I mean from an appriately accredited testing laboratory. Such tests are relatively inexpensive…..particularly when the test cost is balanced out against the prospective risk that not doing so invokes.
Another attempt to clarify the so-called sand pH issue (also applies to soils)
pH is a measure of the Hydrogen ion concentration (H+). Minerals, including those of sand sized particles, do NOT include ANY ‘free’ (reactive, unbound) Hydrogen. pH is a measure of the H+ concentration in an aqueous solution PERIOD – meaning the water and any non-inert compounds that can dissolve in and/or go into solution in water.
The sand that I/we recommend is Silicon dioxide (SiO2). SiO2 does not contain ANY H+ whatsoever – by definition. There is NO H+ there to be measured, so pH is entirely meaningless (BS) in this context. Certain other common minerals/rock – and most glasses (e.g. volcanic) – are also inert (do not dissolve/solubilize in water). However, many other common and potential contaminants do/will effect pH – including many clays, and most salts (which includes some chemical fertilizer forms).
There can be and may well be other material(s) (minerals, compounds) in a sand you have accessed other than the SiO2. A very common example of this is Calcium carbonate(s) – which IS water soluble – and it will progressively dissolved in water – and it WILL effect (raise) the pH (lower free H+ concentrations) of the water. Common sources of CaCO3 in sand is sourced from limestone and or limestone bearing strata (rocks) and MANY beach sands formed of/including crushed shells of marine organisms. Most sandstone will also contain contaminants or some if not many types. River sands and virtually always a mixture of various parent materials (rock types) and also a VAST range of other potential pollutants, both chemical and biological). River sand can be extremely risky and is therefore strongly advised against for all sorts of reasons (too much to describe here).
We/you do NOT want ‘high’ pH (above 7.0). Plants and ‘soil’ microbes function best in the general range of pH 5.5 to 6.8 which is slightly acidic. Within this tolerance range, most plants and beneficial soil organisms perform best around pH 6.4 (slightly dependent on specific species).
The pH of your water can/will also influence the botanical availability of many plant essential nutrients – of particular concern being Phosphorus (P). Plants have difficulty assimilating adequate Phosphorous above pH 6.8 to 7.0 (regardless of how much is there) – as also with certain essential metals and Boron. But I digress.
Sand composed of quartz, aka silica, aka Silicon dioxide has NO pH in and of itself. It cannot be measured because it isnt there. HOWEVER, certain contaminants – IF in a sand you sourced – can and will effect the pH of the water that comes into contact with these contaminants. SiO2 itself is chemically inert. It does not chemically react with water in any way -ever. It therefore can and will not change pH of your water. IF your pH is being effected when coming into contact with your media, then there is something else in that media which is not SiO2. Commonly, the ‘other’ material will be a form of carbonate, which will raise pH above 7.0, often far above. If the amount of carbonate is relatively low, aiding an acid such as Phosphoric or Sulfuric acid can neutralize the carbonate (base) and render it non reactive. This may require more than one application of diluted acid. If after a few application of acid, you do not see an improvement (if pH continues to bounce back up/rise) then your media almost certainly has too much carbonate (or potentially another mineral(s) producing the same result. In which case, find a source of sand without these contaminants.
BTW, I NEVER had the slightest problem with contaminants effecting pH with ANY sand I ever used. Perhaps I am ‘lucky’ (I don’t feel I am). Other people are having significant problems sourcing a sand that is inert. This is regrettable but FAR beyond my control. All my/our efforts ‘discussing’ sand are solely meant for you benefit. If I could change reality (e.g. chemistry), then I’d be a god. I assure you that I am a mere fallible mortal. If you aren’t a fallible mortal, perhaps you’d be willing to perform some magic and make ‘perfect’ sand instantly available to everyone who wants some. Without apology, I cannot.
So, I suppose I’ve rambled on enough for now. Bottomline, sand and soils do NOT have pH. Water (and solutes in the water) does. Silicon dioxide is chemically inert – does NOT change water pH. If your water pH is changing when coming into contact with your sand, there are contaminants in it. If your water pH keeps rising, there is most likely a carbonate material in your sand. If a small amount this can be relatively easily dealt with (if you can access a weak acid). A larger amount however will likely be too challenging to correct for. In that case, find a different ‘sand’ source and then test it carefully before purchasing/transporting. In fact, the best advice is to test everything for yourself and do NOT take any vendors word for anything. All they want is your money and are likely to not understand any question you present and whether they understand your concerns or not will virtually always tell you what they believe you want to hear. Obviously I would wish this to be otherwise, but I have zero influence to modify anyone’s behavior. Caveat emptor (Buyer beware).
PS There was considerably more to my ‘rant’, but somehow it got lost (trashed) by WordPress (so I claim). Not repeating it now.
The sand sold at Home Depot or local hardware stores (Sakrete Natural Play Sand) … so this sand will probably not work well?
Richard Lui….I don’t know what the specs of play sand are so I’d probably suggest that you go to a specialist sand supplier (or your local quarry) and give them the specifications that we’ve provided. It will almost certainly work out a lot cheaper, too.
This is a very helpful explanation, in easy to understand terms, of how to select the proper sand. I am planning on building a couple of sand beds here in Oklahoma in the next month. A couple of questions:
1. The washed mason’s sand in our area is made from river rock, and if I understand your chart, this would not be an ideal selection.
2. Is this due to the pH effect that the sand will have on the AP water?
3. I can purchase decomposed granite from a local supplier, and I can self-grade and use the fines and 2mm + pieces as the base on my greenhouse floor. Since this first sand bed will only have about 1cu yd of sand, I think I can do this in a few hours of shoveling.
Mark & Gary….thanks for putting this website together…..mh
1. Alluvial sands will tend to be rounded, smooth, polished much like dune sands (both are tumbled smooth by friction). It will not have as much SSA as a sharp sand and also somewhat different porosity and hydraulic conductivity characteristics. It could ‘work’ – certainly better than gravel – but I have NO experience with it. Desert sand in the Sahel ‘worked’ okay – better than not – but any fines at all will fill the voids between larger grains and substantial reduce percolation/drainage. Therefore, smooth sand particle size need be larger overall. and of relatively consistent grain size to mitigate against sorting (aka nesting, settling)
2. not pH factor (assuming no carbonates in the river sand,which is questionable). A) shape factor B) could introduce potential pathogens
3.There ‘must be’ a sand and gravel pit (quarry) somewhere nearby. Someone sells to area concrete plants and masons, then its almost certain to be ‘sharp’ (crushed), screened and washed at least once. If the result of their standard screening/washing process isn’t adequate, most quarry will have the ability to redo to whatever spec is called for. Talking them into that for a 1 yard sale is almost as likely as my growing younger.
Reread your 1st point/question. Now I understand(?) that they collect river rock and then crush it and screen/wash for mason’s sand. If that is the case. then it should be ‘sharp’. Then the question becomes, what is the parent (source) rock? If its granite (quartz and feldspar) it should be fine. If its from sedimentary rock, then not good.