Water blending: diluting with RO to hit a target

Why dilute water

Ions in water are not “tasty” in themselves, but they set the background: sulfates emphasise hop bitterness, chlorides add malt fullness, sodium softens the flavour in small amounts, and bicarbonate pushes the mash pH up. The trouble is that salts can only raise a concentration: when the water already holds more sulfate or sodium than the style needs, gypsum and table salt will not help — they only make it worse.

Blending is the reverse operation. A low-mineral water added to the source lowers every ion at once and in proportion, because the contributions of source and diluent simply add up under the rule of mixtures. Residual alkalinity falls along with them, and with it mash pH — which is why dilution is popular for pale beer, where dark malts cannot offset alkaline water.

There is one price to pay: the ions you want leave together with the ones you do not. Calcium, needed by the mash enzymes and the yeast, is diluted as well, and after blending it usually has to be returned with salts. Blending and salts are therefore a pair rather than rivals: first bring the mineral content down by dilution, then raise the ions you want to the target.

How we calculate

The calculation is the rule of mixtures. Every ion in the result is a weighted average: the diluent share times its concentration plus the source share times its own. When no diluent is given it counts as reverse osmosis with all ions at zero, and the formula collapses to multiplying the source profile by the remaining share — which is how most home RO systems behave, stripping more than ninety percent of the salts.

In fit mode the calculator looks for the smallest share at which every given target ion is reached, and never dilutes further than that: diluting more than necessary only throws away ions and flavour. If an ion stays above its target even at a full share, the problem cannot be solved with this water, and the calculator says so plainly: `solved: false` and a “target unreachable” notice.

Two edge cases are worth knowing. A share of zero means the target is already met and there is nothing to dilute. A share of one hundred percent means the water has been replaced by the diluent entirely.

Result_i = Source_i · (1 − d) + Diluent_i · d
V_source = V · (1 − d),   V_diluent = V · d
where d is the diluent share, 0…1; V is the water volume, L

Residual alkalinity and Kolbach

Water alkalinity neutralises malt acidity, while calcium and magnesium acidify the mash. The balance between the two forces is described by residual alkalinity after Kolbach: the contribution of calcium and magnesium, divided by empirical coefficients, is subtracted from the total alkalinity. The higher the residual alkalinity, the more the water lifts mash pH for the same grist.

For pale beer the aim is zero or a small positive value: then the malt itself sets the acidity. Dark malts are more acidic, so dark beer tolerates, and sometimes needs, more alkaline water. A negative residual alkalinity means the water acidifies the mash: for very pale malt that can be useful, for dark malt harmful.

Blending lowers residual alkalinity almost linearly: bicarbonate, calcium and magnesium are diluted alike, so the subtrahend and the minuend fall together. Water in which excess alkalinity comes with high hardness therefore often needs alkalinity added after blending rather than acid.

RA = Alk − (Ca / 3.5 + Mg / 7)
where Alk is alkalinity in mg/L as CaCO₃ (for bicarbonate, Alk ≈ HCO₃ · 0.82); Ca and Mg are in mg/L
1 mEq/L of alkalinity = 50 mg/L as CaCO₃

How to use it

• Enter the six ions from your water report — or leave the fields empty: an empty field means “I don’t know” and counts as zero.
• Choose the diluent: reverse osmosis (the default, all ions at zero), a preset profile from the reference list, or type the composition in yourself if you are blending with another water of your own.
• For a style with a known target, switch on fit-to-target mode and fill in at least one ion — bicarbonate is usually enough. The calculator returns the smallest share at which the target is reached.
• If the share is already known — say you have a twenty-litre canister of RO water — switch to manual mode, move the slider and see what comes out. The litres of source and diluent needed for the volume are shown next to it.
• After blending, return the ions you want with salts: calcium and sulfates are diluted along with the excess, and you are unlikely to still be on target for them.

What the calculator does not do

• It does not remove ions without dilution: salts only add, and boiling concentrates rather than removes.
• It does not design the diluent profile: it comes from the reference list or is typed in. A mixture of two waters is treated as one if you already know its composition.
• It does not calculate mash pH or select an acid: that needs the grist composition and its buffering capacity, and a separate tool handles it.
• It does not account for sparge water or losses: the calculation uses the volume you entered, not the whole batch.
• It does not replace a water analysis: the numbers in a water utility report are averaged over the season and the network, and an RO membrane changes its rejection over time — it is time to replace it when the mineral content starts to rise.

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