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pH, CO2, Alkalinity and Water Hardness in Aquaponics

pH, CO2, Alkalinity and Water Hardness in Aquaponics

I’ve been struggling lately trying to get my water pH numbers a bit lower in my aquaponic system and it hasn’t been easy. The water in my area of south Florida is off the charts in alkalinity and hardness. It could be because the state is built on old coral beds.  My well water comes out at pH 8 – 8.3. It is so hard that any metal hose fittings immediately weld themselves to the hose bib. Be that as it may, I have resolved to remedy this problem as I can see the change in growth rate of the plant life in my system. This, I think, is due to the iron being locked out from use by the plants at that pH.

Initially my approach was to add acid until I saw a drop in pH. Luckily, while reading up on water quality, I read that this approach would indeed bring down the pH, but it would most likely due it in a precipitous crash when the acid finally overcame the buffering ability of all that calcium carbonate and that would most likely kill my fish. So now my plan is to get the alkalinity and hardness in better shape then adjust the pH.

So here are a few facts that I’ve uncovered in my research on this subject:

  •  pH is a scale of  the negative logarithm of the molar hydrogen ion concentration of water. If it is below 7 it is acidic if it is above 7 it is basic, if it’s 7 it’s neutral. It runs from 0, very acid, to 14, very basic
  • Each number on the pH scale represents an exponential increase in the number of hydrogen ions. So 8 is ten times the amount of ions that 7 is, etc.
  • Fish like a pH of around 7 – 8. Below 4 or above 11 they die.
  • Plants like a pH of around 5.5 – 6.5.
  • The higher the pH the greater the amount of ammonia, the lower the pH the more becomes the less toxic form of ammonium.
  • Alkalinity increases pH. High alkalinity = high pH = high ammonia toxicity
  • Alkalinity represents the amount of base in the water. These bases includes berates, phosphates, hydroxides, carbonates and bicarbonates. The level of alkalinity measures the amount of acid that the water can absorb before the pH changes. This is called buffering. Calcium carbonate is the most common occurring base.
  • Total alkalinity is represented as mg/L or as parts per million, ppm.  A. Good range for aquaponics is around 75 – 200 mg/L CaCO3.
  • Water hardness is a measure of the positively charged ions, generally calcium, Ca2+, magnesium, Mg2+, and or iron, Fe2+. The positive charge makes these molecules very reactive which can be seen in the form of lime scale.  It is measured as  mg/L CaCO3
  • Because hardness is measured the same as alkalinity it is often confused with it. Depending on the ions present in the hardness it is possible to have water that is of high alkalinity and low hardness or vice versa.
  • The calcium and magnesium present in the water are beneficial to fish health helping maintain the levels sodium and potassium in the blood, and are critical in bone and scale growth. A good level is 100 – 250 mg/L CaCO3.
  • In areas with low CaCO3 concentrations, soft water, ( the northern US) agricultural limestone can be added. At pH greater than 8 agricultural gypsum or food grade calcium chloride should be used, as it will dissolve better.

So with this information in hand it looks like the way forward is to obtain a better source of stock water than what is available coming from my aquifer. Once I get the hardness under control I should be able to nudge the pH to a better level.

The first and least expensive method is too divert the rain water from my house and greenhouse roofs into a cistern. As it rains here every day for 7 months out of the year this is a great choice.  Rainwater tends to be relatively pure but a little acidic from picking up atmospheric CO2. The water can then be blended with the well water to produce water that has a normal alkalinity and hardness but isn’t stripped of all the salts needed for a healthy system.

The next method of obtaining water treatment is to bite the bullet and put in a reverse osmosis unit.  For about $1800 I can get a unit that produces up to 6000 gallons a day. Pricey but a lot of water when you need it, so some savings on a large cistern.

The last option is to install some solar distiller panels. These panels produce about 4 gallons a day for each 2′ x4′ panel. The best part is that they can easily be made at home, cost about $6 each and require no power or upkeep once they are tied into your sprinkler system.

My plan at this time is to start with the rainwater catchment installation then to install enough solar distiller panels to offset the rainwater during the dry season. I will post the construction photos of these builds.

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