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I have seen a few posts of people having problems in using their well water because of high pH as the result of very high alkalinity.  One solution that I have used on a batch basis is to use muriatic acid (32% HCl acid).  The acid will shift carbonate and bicarbonate over to gaseous CO2.  The CO2 can then be stripped using aeration (12 to 24 hours depending on how vigorous the aeration is).  For each cubic meter (1000 liters or 264 gallons) you can use 2.3 ml of acid for each ppm of alkalinity reduction.  For example if you had a 1000 liter tank of water with an alkalinity of 300 and you wanted to get alkalinity down to 50, then you would need 250x2.3=575 ml of muriatic acid.  Never go for complete elimination as the pH of the water will not recover and will be too low for safe use within your system.  I recommend not targeting less than 50 ppm alkalinity for this procedure.  At first, after the acid is added, the water will have a very low pH (low 4s), then rather quickly with aeration the pH will begin to rise as CO2 is blown off.  If aeration is vigorous, the time for pH to rise back up to about 6 can be just a few hours.  The rate in the rise of pH slows as pH rises and CO2 concentration drops.  The water is ready to use after pH has recovered to above 7.  This is the indicator that CO2 has been reduced to levels safe for the system.  This procedure is for outdoors or in a very well ventilated space.

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Good info David... but I think it needs a couple of caveats...

 

While the figures are correct for the % concentration posted... differing concentrations will alter the figures quire considerably.... check your acid concentration....

 

Secondly... IMO... don't adopt this approach if you are stocked with fish...

If you are... then proceed much more cautiously... only altering pH by a maximum of 0.4 points at any one time....

I think this procedure is meant to be done in a separate tank from the fish system.  Where you treat your water and let it stabilize before adding it into the AP system.  Using a separate adjustment tank to prep the water for top up is what I would recommend to anyone who needs to do this on occasion.

However, keep in mind that regularly doing this is in many places likely to liberate lots of calcium into the water which may cause issues with potassium getting precipitated out resulting in potassium deficiencies in the plants.

I've been doing just what David described on a way smaller and fish-less scale and was wondering...in addition to the release of the above mentioned CO2, is it realistic that you could also release plant usable Ca2+ in an amount that could cause problems with the uptake of other cations? K+ or Mg2+ (with Mg2+ being more likely effected than K+)? Or would the amounts of Ca need to be ridiculously high to cause such a problem (or K and Mg to be really low) for something  like that to happen?

I ask because recently I've decided to stop using 2/3rd's distilled and 1/3rd (HCL treated) well water, because I don't want to buy distilled water anymore. Seemed silly to use inputs like humonia, wormcastings from the bin, and wood ashes, finely ground egg shells, (all "free") etc...while buying litres and litres of distilled at the store? So, was wondering about the Ca release (which I look at as a good thing, but it seems as though Ca, K and Mg co-exist in a somewhat 'delicate' balance with one another, K I can always somewhat remedy (I think) by adding a bit of home made lye, then riding the pH equivalent of the Elvis roller coaster. But I don't know of a 'natrual' easy source of Mg?

The procedure is definitely something to be done on a batch basis offline and separate from the plants and fish.  Adding a little bit of acid into a system with fish and plants will not get CO2 high enough to effectively strip and if it did get high enought the impacts on fish would not be good.

The process will liberate Calcium.  Alkaline waters are usually hard waters.  However the well water being used would have to be very high for calcium to be an issue (over 300 ppm alkalinity).  I have only done this for water going to fish.  Although I have not done it to a system with plants, it should be OK. 

If you are using rain water and well water is not being used because of alkalinity the above is a good back up plan for having makeup water for a system during dry times.

My well water is hard but I don't think of it as super outrageously so though I don't know the ppm or hardness numbers.

Anyway, I have seen that seasons where I was getting lots of rain water and I did not need to top up with well water, my system pH would drop and I could alternate using potassium bicarbonate and calcium carbonate to buffer the pH back up and keep all in balance.  However, in a non rainy season when I'm stuck topping up with well water the pH will stay up around 7.6 because of all the calcium carbonate in my well water.  There is enough calcium in that well water that the potassium starts to become a problem if I'm stuck using well water for a whole season.

In my big system this is painfully obvious since that system has shells as media and the potassium precipitation is visible and the potassium deficiency signs are grossly obvious.  That system is getting it's media changed out though it did struggle on for several years until I saw how much better the system with the other media did in relation.  Even then, the well water is able to mess with the potassium and I fear simply using acid should only be an occasional emergency stop gap option rather than the regular practice.

Potassium precipitation from a solubility standpoint under the conditions that would exist in an aquaponic system seems odd.  Are you sure that the precipitate is potassium and not phosphorous based?

TCLynx said:

My well water is hard but I don't think of it as super outrageously so though I don't know the ppm or hardness numbers.

Anyway, I have seen that seasons where I was getting lots of rain water and I did not need to top up with well water, my system pH would drop and I could alternate using potassium bicarbonate and calcium carbonate to buffer the pH back up and keep all in balance.  However, in a non rainy season when I'm stuck topping up with well water the pH will stay up around 7.6 because of all the calcium carbonate in my well water.  There is enough calcium in that well water that the potassium starts to become a problem if I'm stuck using well water for a whole season.

In my big system this is painfully obvious since that system has shells as media and the potassium precipitation is visible and the potassium deficiency signs are grossly obvious.  That system is getting it's media changed out though it did struggle on for several years until I saw how much better the system with the other media did in relation.  Even then, the well water is able to mess with the potassium and I fear simply using acid should only be an occasional emergency stop gap option rather than the regular practice.

No I'm not sure.  I'm basing this on tidbits I've picked up from others online with better understanding of chemistry than I.

If you can describe the difference in the signs of potassium deficiency compared to phosphorus deficiency we might be able to hone in on the situation.

TC, even if you were getting visible phosphate (Calcium Phosphate, and Calcium Sulfate, being the most common in soil-less culture)  precipitates, you wouldn't necessarily be experiencing phosphate deficiencies in your plants. So looking for signs of phosphate deficiencies may not prove helpful in identifying what it is that is precipitating out of that system. An excess of phosphorous seems almost always more realistic of a condition than a deficiency. You may have in some of your hydro set ups from earlier noticed a grayish-white-ish sludge collecting in your media (if you had media), that is usually Calcium Phosphate, (other elements like Zn, Mg, even Fe with often stick to its surface too...free nutes if managed properly :) ... 

Excess Calcium will make itself felt by messing with Potassium (as well as Magnesium) uptake in plants. that might be why you're your seeing those K deficiencies even if the precipitates are phosphorous based.

This is just my 2 cents, and is all based on 'classical' hydro situations. But I don't see why it would be outlandish to apply to an AP setting :)

Ok, Sounds logical Vlad.

I believe the excess calcium is causing me potassium potassium problems (to the point that we are changing out the media in the big system to get rid of the shells.)  And the excess calcium and phosphorus precipitating as calcium phosphate on my tank walls could certainly be the case.  I definitely know the high pH locks out iron on me and I'm getting sick of needing the chelated iron all the time.

My hard well water doesn't help much but I think if I can ever collect more rain I might be able to balance nicely between the rain and well water (worked fairly well last summer on the 300 gallon system that doesn't have any shells.)  But I'm starting to wonder if we will ever get more than a sprinkling of rain up here on the hill again,

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