Treating hard water with Lime

By Hubert Hanghofer

Extracted from the HBD archives, 1997

As has been noted by A.J. deLange, slaked lime removes hydrogencarbonates:

Ca(OH)2 + Ca(HCO3)2 -> 2CaCO3 (precipitate) + 2H2O

The process works well with Ca based temporary hardness but doesn't remove MgCO3 because of its solubility. MgCO3 requires additional Ca(OH)2 to precipitate: MgCO3 + Ca(OH)2 -> CaCO3 + Mg(OH)2

The method is widespread among Austrian / Bavarian brewers (geology is based on limestone, water high in temporary hardness). It's allowed according to Bavarian Reinheitsgebot and even according to the very rigorous bioland brewing guidlines (...those green clean bio beers).

...Now let's try a simplified approach:

  1. Set up a water treatment cask that holds the whole volume of brewing water (mashing, sparging).
  2. Fill in half the water, add all Ca(OH)2 necessary for the *whole volume*. Thus some of the Mg will precipitate, too.

    Calculation of the necessary amount Ca(OH)2:

    grams / Litre = Alkalinity (ppm CaCO3) x 0,74 / 1000

    grams / Litre = Alkalinity (mMol/L) x 74 / 1000

    grams / US gallon = (g/L) x 3,785

    If you have to use CaO (burnt lime), multiply the grams Ca(OH)2 by 0.757 to get the grams CaO, that have to be slaked with water prior to adding (be careful, much heat may develop, both CaO and Hydroxide are caustic).

  3. If needed, add water salts to adjust water chemistry.
  4. Gradually rise the volume by adding more water. Keep rising the lime by stirring every 10 minutes or so to aid the reaction and help convert the initially very fine precipitate to a coarser one.
  5. *IMPORTANT: Don't rely on the calculated amount. Keep an eye on the pH. If it's <8 you're done, otherwise add more water!!!*
  6. Remove the stirring paddle and let the precipitate settle out for at least 12 hours. So the whole work is done on the day before brewing.
  7. Rack off the water into your kettle.... Low turbidity is allowed, but you should take care to leave the sediment behind.

I'm water chemist and could make more efforts, but use this primitive approach in my brewing for years and am very pleased with the results. I'm able to produce Pilseners with my tapwater (alkalinity 300ppm CaCO3), adjusting residual Kolbach alkalinity to (*measured*) negative values.

Notes:
pH-Control (5) is the key factor for this simplification! It's like a titration of slaked lime with tap-water's HCO3. The drop in pH is significant, so pH test sticks can be used (I use them at home). I recommend however, to add 5-10 vol% more tapwater after reaching pH<8.

After step (2) Mg(OH)2 will precipitate, but gradually re-carbonate and dissolve when you rise the volume and lower pH. So the split treatment as described by A.J. deLange should be used, if water contains more than 20 mg Mg/L. After racking off into the secondary (...water treatment cask!) proceed from (3).